Almost all phase-change memory materials (PCM) contain chalcogen atoms, and their chemical bonds have been denoted both as 'electron-deficient' [sometimes referred to as 'metavalent'] and 'electron-rich' ['hypervalent', multicentre]. The latter involve lone-pair electrons. We have performed calculations that can discriminate unambiguously between these two classes of bond and have shown that PCM have electron-rich, 3c–4e ('hypervalent') bonds. Plots of charge transferred between (ET) and shared with (ES) neighbouring atoms cannot on their own distinguish between 'metavalent' and 'hypervalent' bonds, both of which involve single-electron bonds. PCM do not exhibit 'metavalent' bonding and are not electron-deficient; the bonding is electron-rich of the 'hypervalent' or multicentre type.
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Journal of Physics: Condensed Matter covers the whole of condensed matter physics including soft matter, physics of chemical processes, and method development. Papers may report experimental, theoretical or computational studies.
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P C Müller et al 2024 J. Phys.: Condens. Matter 36 325706
Lilia Boeri et al 2022 J. Phys.: Condens. Matter 34 183002
Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization. This would not only open exciting avenues for fundamental research, but also pave the road for a wide range of technological applications, affecting strategic areas such as energy conservation and climate change. In this roadmap we have collected contributions from many of the main actors working on superconductivity, and asked them to share their personal viewpoint on the field. The hope is that this article will serve not only as an instantaneous picture of the status of research, but also as a true roadmap defining the main long-term theoretical and experimental challenges that lie ahead. Interestingly, although the current research in superconductor design is dominated by conventional (phonon-mediated) superconductors, there seems to be a widespread consensus that achieving A-SC may require different pairing mechanisms.
In memoriam, to Neil Ashcroft, who inspired us all.
Anjan Barman et al 2021 J. Phys.: Condens. Matter 33 413001
Magnonics is a budding research field in nanomagnetism and nanoscience that addresses the use of spin waves (magnons) to transmit, store, and process information. The rapid advancements of this field during last one decade in terms of upsurge in research papers, review articles, citations, proposals of devices as well as introduction of new sub-topics prompted us to present the first roadmap on magnonics. This is a collection of 22 sections written by leading experts in this field who review and discuss the current status besides presenting their vision of future perspectives. Today, the principal challenges in applied magnonics are the excitation of sub-100 nm wavelength magnons, their manipulation on the nanoscale and the creation of sub-micrometre devices using low-Gilbert damping magnetic materials and its interconnections to standard electronics. To this end, magnonics offers lower energy consumption, easier integrability and compatibility with CMOS structure, reprogrammability, shorter wavelength, smaller device features, anisotropic properties, negative group velocity, non-reciprocity and efficient tunability by various external stimuli to name a few. Hence, despite being a young research field, magnonics has come a long way since its early inception. This roadmap asserts a milestone for future emerging research directions in magnonics, and hopefully, it will inspire a series of exciting new articles on the same topic in the coming years.
Paolo Giannozzi et al 2009 J. Phys.: Condens. Matter 21 395502
QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
Giovanni Pizzi et al 2020 J. Phys.: Condens. Matter 32 165902
Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spinor-valued Wannier functions, more accurate methods to interpolate quantities in the Brillouin zone); improved usability (improved plotting routines, integration with high-throughput automation frameworks), as well as the implementation of modern software engineering practices (unit testing, continuous integration, and automatic source-code documentation). These new features, capabilities, and code development model aim to further sustain and expand the community uptake and range of applicability, that nowadays spans complex and accurate dielectric, electronic, magnetic, optical, topological and transport properties of materials.
Søren Smidstrup et al 2020 J. Phys.: Condens. Matter 32 015901
QuantumATK is an integrated set of atomic-scale modelling tools developed since 2003 by professional software engineers in collaboration with academic researchers. While different aspects and individual modules of the platform have been previously presented, the purpose of this paper is to give a general overview of the platform. The QuantumATK simulation engines enable electronic-structure calculations using density functional theory or tight-binding model Hamiltonians, and also offers bonded or reactive empirical force fields in many different parametrizations. Density functional theory is implemented using either a plane-wave basis or expansion of electronic states in a linear combination of atomic orbitals. The platform includes a long list of advanced modules, including Green's-function methods for electron transport simulations and surface calculations, first-principles electron-phonon and electron-photon couplings, simulation of atomic-scale heat transport, ion dynamics, spintronics, optical properties of materials, static polarization, and more. Seamless integration of the different simulation engines into a common platform allows for easy combination of different simulation methods into complex workflows. Besides giving a general overview and presenting a number of implementation details not previously published, we also present four different application examples. These are calculations of the phonon-limited mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model simulation of lithium ion drift through a battery cathode in an external electric field, and electronic-structure calculations of the composition-dependent band gap of SiGe alloys.
P Giannozzi et al 2017 J. Phys.: Condens. Matter 29 465901
Quantum ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the-art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudopotential and projector-augmented-wave approaches. Quantum ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement their ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.
Atsushi Togo et al 2023 J. Phys.: Condens. Matter 35 353001
Scientific simulation codes are public property sustained by the community. Modern technology allows anyone to join scientific software projects, from anywhere, remotely via the internet. The phonopy and phono3py codes are widely used open-source phonon calculation codes. This review describes a collection of computational methods and techniques implemented in these codes and shows their implementation strategies as a whole, aiming to be useful for the community. Some of the techniques presented here are not limited to phonon calculations and may therefore be useful in other areas of condensed matter physics.
K Katsiev and H Idriss 2024 J. Phys.: Condens. Matter 36 325002
Ce4+ cations are commonly used as electron acceptors during the water oxidation to O2 reaction over Ir- and Ru-based catalysts. They can also be reduced to Ce3+ cations by excited electrons from the conduction band of an oxide semiconductor with a suitable energy level. In this work, we have studied their interaction with a rutile TiO2(110) single crystal upon band gap excitation by femtosecond transient absorption spectroscopy (TAS) in solution in the 350–900 nm range and up to 3.5 ns. Unlike excitation in the presence of water alone the addition of Ce4+ resulted in a clear ground-state bleaching (GSB) signal at the band gap energy of TiO2 (ca. 400 nm) with a time constant t = 4–5 ps. This indicated that the Ce4+ cations presence has quenched the e-h recombination rate when compared to water alone. In addition to GSB, two positive signals are observed and are attributed to trapped holes (in the visible region, 450–550 nm) and trapped electrons in the IR region (>700 nm). Contrary to expectation, the lifetime of the positive signal between 450 and 550 nm decreased with increasing concentrations of Ce4+. We attribute the decrease in the lifetime of this signal to electrostatic repulsion between Ce4+ at the surface of TiO2(110) and positively charged trapped holes. It was also found that at the very short time scale (<2–3 ps) the fast decaying TAS signal of excited electrons in the conduction band is suppressed because of the presence of Ce4+ cations. Results point out that the presence of Ce4+ cations increases the residence time (mobility) of excited electrons and holes at the conduction band and valence band energy levels (instead of being trapped). This might provide further explanations for the enhanced reaction rate of water oxidation to O2 in the presence of Ce4+ cations.
Vincenzo Amendola et al 2017 J. Phys.: Condens. Matter 29 203002
In the last two decades, plasmon resonance in gold nanoparticles (Au NPs) has been the subject of intense research efforts. Plasmon physics is intriguing and its precise modelling proved to be challenging. In fact, plasmons are highly responsive to a multitude of factors, either intrinsic to the Au NPs or from the environment, and recently the need emerged for the correction of standard electromagnetic approaches with quantum effects. Applications related to plasmon absorption and scattering in Au NPs are impressively numerous, ranging from sensing to photothermal effects to cell imaging. Also, plasmon-enhanced phenomena are highly interesting for multiple purposes, including, for instance, Raman spectroscopy of nearby analytes, catalysis, or sunlight energy conversion. In addition, plasmon excitation is involved in a series of advanced physical processes such as non-linear optics, optical trapping, magneto-plasmonics, and optical activity. Here, we provide the general overview of the field and the background for appropriate modelling of the physical phenomena. Then, we report on the current state of the art and most recent applications of plasmon resonance in Au NPs.
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Maxime Gidding et al 2024 J. Phys.: Condens. Matter 36 365801
All-optical schemes for switching magnetization offer a pathway towards the creation of more advanced data-storage technologies, both in terms of recording speed and energy-efficiency. It has previously been shown that picosecond-long optical pulses with central frequencies ranging between 12 and 30 THz are capable of driving magnetic switching in yttrium-iron-garnet films, provided that the excitation frequency matches the characteristic frequency of longitudinal optical phonons. Here, we explore how the phononic mechanism of magnetic switching in three distinct ferrimagnetic iron-garnet films evolves at optical frequencies below 10 THz, within the so-called terahertz gap. We find that at long wavelengths the magnetic switching rather correlates with phonon modes associated with the substrate. Our results show that the process of phononic switching of magnetization, previously discovered in the mid- to far-infrared spectral range, becomes much more complex at frequencies within the terahertz gap.
Suchandra Mukherjee et al 2024 J. Phys.: Condens. Matter 36 365701
Sb2Te3, a binary chalcogenide-based 3D topological insulator, attracts significant attention for its exceptional thermoelectric performance. We report the vibrational properties of magnetically doped Sb2Te3 thermoelectric material. Ni doping induces defect/disorder in the system and plays a positive role in engineering the thermoelectric properties through tuning the vibrational phonon modes. Synchrotron powder x-ray diffraction study confirms good crystalline quality and single-phase nature of the synthesized samples. The change in structural parameters, including Biso and strain, further corroborate with structural disorder. Detailed modification of phonon modes with doping and temperature variation is analysed from temperature-dependent Raman spectroscopic measurement. Compressive lattice strain is observed from the blue shift of Raman peaks owing to Ni incorporation in Sb site. An attempt is made to extract the lattice thermal conductivity from total thermal conductivity estimated through optothermal Raman studies. Hall concentration data support the change in temperature-dependent resistivity and thermopower. Remarkable increase in thermopower is observed after Ni doping. Simulation of the Pisarenko model, indicating the convergence of the valence band, explains the observed enhancement of thermopower in Sb2−xNixTe3. The energy gap between the light and heavy valence band at Γ point is found to be 30 meV (for Sb2Te3), which is reduced to 3 meV (in Sb1.98Ni0.02Te3). A significant increase in thermoelectric power factor is obtained from 715 μWm−1K−2 for pristine Sb2Te3 to 2415 μWm−1K−2 for Ni-doped Sb2Te3 sample. Finally, the thermoelectric figure of merit, ZT is found to increase by four times in Sb1.98Ni0.02Te3 than that of its pristine counterpart.
Brandon Klein et al 2024 J. Phys.: Condens. Matter 36 365301
We study the low-frequency Raman active modes of twisted bilayer MoS2 for several twist angles using a force-field approach and a parametrized bond polarizability model. We show that twist angles near high-symmetry stacking configurations exhibit stacking frustration that leads to significant buckling of the moiré superlattice. We find that atomic relaxation due to the twist is of prime importance. The periodic displacement of the Mo atoms shows the realization of a soliton network, and in turn, leads to the emergence of a number of frequency modes not seen in the high-symmetry stacking systems. Some of the modes are only seen in the XZ Raman polarization setup while others are seen in the XY setup. The symmetry of the normal modes, and how this affects the Raman tensors is examined in detail.
Qiao Shi et al 2024 J. Phys.: Condens. Matter 36 365901
In this work, the hierarchical topology ring (HTR+) algorithm, an extension of the HTR algorithm, was developed for identifying gas hydrate types, cage structures, and grain boundaries (GBs) within polycrystalline structures. Utilizing molecular dynamics trajectories of polycrystalline hydrates, the accuracy of the HTR+ algorithm is validated in identifying sI, sII and sH hydrate types, hydrate grains, and GBs in multi-hydrate polycrystals, as well as clathrate cages at GBs. Additionally, during the hydrate nucleation and growth processes, clathrate cages, hydrate type, hydrate grains and ice structures are accurately recognized. Significantly, this algorithm demonstrates high efficiency, particularly for large hydrate systems. HTR+ algorithm emerges a powerful tool for identifying micro/mesoscopic structures of gas hydrates, enabling an in-depth understanding of the formation mechanisms and properties of gas hydrates.
Hong Du et al 2024 J. Phys.: Condens. Matter 36 365702
Layered materials with kagome lattice have attracted a lot of attention due to the presence of nontrivial topological bands and correlated electronic states with tunability. In this work, we investigate a unique van der Waals (vdW) material system, A2M3X4 (A = K, Rb, Cs; M = Ni, Pd; X = S, Se), where transition metal kagome lattices, chalcogen honeycomb lattices and alkali metal triangular lattices coexist simultaneously. A notable feature of this material is that each Ni/Pd atom is positioned in the center of four chalcogen atoms, forming a local square-planar environment. This crystal field environment results in a low spin state S= 0 of Ni2+/Pd2+. A systematic study of the crystal growth, crystal structure, magnetic and transport properties of two representative compounds, Rb2Ni3S4 and Cs2Ni3Se4, has been carried out on powder and single crystal samples. Both compounds exhibit nonmagnetic p-type semiconducting behavior, closely related to the particular chemical environment of Ni2+ ions and the alkali metal intercalated vdW structure. Additionally, Cs2Ni3Se4 undergoes an insulator-metal transition (IMT) in transport measurements under pressure up to 87.1 GPa without any structural phase transition, while Rb2Ni3S4 shows the tendency to be metalized.
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Volker Heine and Siyu Chen 2024 J. Phys.: Condens. Matter 36 353002
This theoretical discussion covers several effects of metallic bonding based on a simple formula. It comes from the first steps in the Moment Method for calculating the local electronic structure of a solid (such as at a surface or in a random alloy), and depends on the square root of the total coordination number C of near neighbours. Each atom is covalently bonded to its cluster of near neighbours as a whole. The properties of metals touched on include malleability, crystal structure and phase transitions, vacancy formation energy, surface catalysis, surface reconstruction, graphite stability, and some aspects of the benzene molecule seen as an atomic metal ring. In most of these, the 'saturation' type of curvature of the square root function plays a crucial role. A short historical survey indicates the development of the ideas from Bloch (1929 Z. Phys.52 555) to recent times.
Pengye Liu et al 2024 J. Phys.: Condens. Matter 36 353001
The successful prediction and confirmation of unprecedentedly high-temperature superconductivity in compressed hydrogen-rich hydrides signify a remarkable advancement in the continuous quest for attaining room-temperature superconductivity. The recent studies have established a broad scope for developing binary and ternary hydrides and illustrated correlation between specific hydrogen motifs and high-Tcs under high pressures. The analysis of the microscopic mechanism of superconductivity in hydrides suggests that the high electronic density of states at the Fermi level (EF), the large phonon energy scale of the vibration modes and the resulting enhanced electron-phonon coupling are crucial contributors towards the high-Tc phonon-mediated superconductors. The aim of our efforts is to tackle forthcoming challenges associated with elevating the Tc and reducing the stabilization pressures of hydrogen-based superconductors, and offer insights for the future discoveries of room-temperature superconductors. Our present Review offers an overview and analysis of the latest advancements in predicting and experimentally synthesizing various crystal structures, while also exploring strategies to enhance the superconductivity and reducing their stabilization pressures of hydrogen-rich hydrides.
Zi-Kui Liu 2024 J. Phys.: Condens. Matter 36 343003
Today's thermodynamics is largely based on the combined law for equilibrium systems and statistical mechanics derived by Gibbs in 1873 and 1901, respectively, while irreversible thermodynamics for nonequilibrium systems resides essentially on the Onsager Theorem as a separate branch of thermodynamics developed in 1930s. Between them, quantum mechanics was invented and quantitatively solved in terms of density functional theory (DFT) in 1960s. These three scientific domains operate based on different principles and are very much separated from each other. In analogy to the parable of the blind men and the elephant articulated by Perdew, they individually represent different portions of a complex system and thus are incomplete by themselves alone, resulting in the lack of quantitative agreement between their predictions and experimental observations. Over the last two decades, the author's group has developed a multiscale entropy approach (recently termed as zentropy theory) that integrates DFT-based quantum mechanics and Gibbs statistical mechanics and is capable of accurately predicting entropy and free energy of complex systems. Furthermore, in combination with the combined law for nonequilibrium systems presented by Hillert, the author developed the theory of cross phenomena beyond the phenomenological Onsager Theorem. The zentropy theory and theory of cross phenomena jointly provide quantitative predictive theories for systems from electronic to any observable scales as reviewed in the present work.
Shinichiro Akiyama et al 2024 J. Phys.: Condens. Matter 36 343002
We review the basic ideas of the tensor renormalization group method and show how they can be applied for lattice field theory models involving relativistic fermions and Grassmann variables in arbitrary dimensions. We discuss recent progress for entanglement filtering, loop optimization, bond-weighting techniques and matrix product decompositions for Grassmann tensor networks. The new methods are tested with two-dimensional Wilson–Majorana fermions and multi-flavor Gross–Neveu models. We show that the methods can also be applied to the fermionic Hubbard model in 1+1 and 2+1 dimensions.
Miao He et al 2024 J. Phys.: Condens. Matter 36 343001
Complex environments in advanced manufacturing usually involve ultrafast laser or ion irradiation which leads to rapid heating and cooling and drives grain boundaries (GBs) to non-equilibrium states, featuring distinct energetics and kinetic behaviors compared to conventional equilibrium or near-equilibrium GBs. In this topical review, we provide an overview of both recent experimental and computational studies on metastable GBs, i.e. their energetics, kinetic behaviors, and mechanical properties. In contrast to GBs at thermodynamic equilibrium, the inherent structure energy of metastable GBs exhibits a spectrum instead of single value for a particular misorientation, due to the existence of microstructural and chemical disorder. The potential energy landscape governs the energetic and kinetic behaviors of metastable GBs, including the ageing/rejuvenating mechanism and activation barrier distributions. The unique energetics and structural disorder of metastable GBs lead to unique mechanical properties and tunability of interface-rich nanocrystalline materials. We also discuss that, in addition to structural disorder, chemical complexity in multi-components alloys could also drive the GBs away from their ground states and, subsequently, significantly impact on the GBs-mediated deformation. And under some extreme conditions such as irradiation, structural disorders and chemical complexity may simultaneously present at interfaces, further enriching of metastability of GBs and their physical and mechanical behaviors. Finally, we discuss the machine learning techniques, which have been increasingly employed to predict and understand the complex behaviors of metastable GBs in recent years. We highlight the potential of data-driven approaches to revolutionize the study of disorder systems by efficiently extracting the relationship between structural features and material properties. We hope this topical review paper could shed light and stimulate the development of new GBs engineering strategies that allow more flexibility and tunability for the design of nano-structured materials.
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Zhou et al
Two-dimensional transition metal dichalcogenides (TMDs) lateral heterostructures exhibit excellent performance in electrics and optics. The electron transport of the heterostructures can be effectively regulated by ingenious design. In this study, we construct a monolayer MoSe2/WSe2 lateral heterostructure, covalently connecting monolayer MoSe2 and monolayer WSe2. Using the Extended Huckel Theory method, we explored current-voltage characteristics under varied conditions, including altering carrier density, atomic replacement and interface angles. Calculations demonstrate a significant electrical rectification ratio (ERR) ranging from 200 to 800. Additionally, Employing Density Functional Theory (DFT) with non-equilibrium Green's function (NEGF) method, we investigated electronic properties, attributing the rectification effect to electronic state distribution differences, asymmetric transmission coefficients, and band bending of projected local density of states (PLDOS). The expandability of the interfacial energy barrier enhances the rectification effect through adjustments in carrier concentration, atomic replacements, and interface size. However, these enhancements introduce challenges such as increased electron-boundary scattering and reduced ambipolarity, resulting in a lower ERR. This study provides valuable theoretical insights for optimizing 2D electronic diode devices, offering avenues for precise control of the rectification effect.
Liu et al
The two-dimensional transition metal carbide/nitride family (MXenes) has garnered significant attention due to their highly customizable surface functional groups. Leveraging modern material science techniques, the customizability of MXenes can be enhanced further through the construction of associated heterostructures. As indicated by recent research, the Mo2CTx/NiS heterostructure has emerged as a promising candidate exhibiting superior physical and chemical application potential. The geometrical structure of Mo2CTx/NiS heterostructure is modeled and 6 possible configurations are validated by Density Functional Theory simulations. The variation in functional groups leads to structural changes in Mo2CTx/NiS interfaces, primarily attributed to the competition between van der Waals and covalent interactions. The presence of different functional groups results in significant band fluctuations near the Fermi level for Ni and Mo atoms, influencing the role of atoms and electron's ability to escape near the interface. This, in turn, modulates the strength of covalent interactions at the MXenes/NiS interface and alters the ease of dissociation of the MXenes/NiS complex. Notably, the Mo2CO2/NiS(P6₃/mmc) heterostructure exhibits polymorphism, signifying that two atomic arrangements can stabilize the structure. The transition process between these polymorphs is also simulated, further indicating the modulation of the electronic level of properties by a sliding operation.
Wu et al
The exploration of the superconducting properties of antiferromagnetic parent compounds containing transition metals under pressure provides a unique idea for finding and designing superconducting materials with better performance. In this paper, the close relationship between the possible superconductivity and structure phase transition of the typical van der Waals layered material 1T-CrSe2 induced by pressure is studied by means of electrical transport and X-ray diffraction for the first time. We introduce the possibility of pressure-induced superconductivity at 20 GPa, with a critical Tc of approximately at 4 K. The superconductivity persists up to the highest measured pressure of 70 GPa, with a maximum Tc ~ 5 K at 24 GPa. We observed a structure phase transition from P-3m1 to C2/m space group in the range of 9.4-11.7 GPa. The results show that the structural phase transition leads to the metallization of 1T-CrSe2, and the further pressure effect makes the superconductivity appear in the new structure. The material undergoes a transition from a two-dimensional layered structure to a three-dimensional structure under pressure. This is the first time that possible superconductivity has been observed in 1T-CrSe2.
Pankratova et al
We report on the stabilization of ferromagnetic skyrmions in zero external magnetic fields, in
exchange-biased systems composed of ferromagnetic-antiferromagnetic (FM-AFM) bilayers. By per-
forming atomistic spin dynamics simulations, we study cases of compensated, uncompensated, and
partly uncompensated FM-AFM interfaces, and investigate the impact of important parameters such
as temperature, inter-plane exchange interaction, Dzyaloshinskii-Moria interaction, and magnetic
anisotropy on the skyrmions appearance and stability. The model with an uncompensated FM-AFM
interface leads to the stabilization of individual skyrmions and skyrmion lattices in the FM layer,
caused by the effective field from the AFM instead of an external magnetic field. Similarly, in the
case of a fully compensated FM-AFM interface, we show that FM skyrmions can be stabilized. We
also demonstrate that accounting of interface roughness leads to the stabilization of skyrmions both in
compensated and uncompensated interfaces. Moreover, in bilayers with a rough interface, skyrmions
in the FM layer are observed for a wide range of exchange interaction values through the FM-AFM
interface, and the chirality of the skyrmions depends critically on the exchange interaction.
Chang et al
In this article, we propose two methods for designing higher Chern number models from the topological defect perspective. Based on the fact that the Chern number is equal to a summation of the charges of meron defects, we show that the higher Chern number structures can be realized by either moving the positions of merons or increasing the amount of them. The combination of the two methods is also verified to be a viable approach. We shall construct several models and investigate their energy spectrum. More than one gapless state can be observed on the edges of these models. Expectedly, our theory promises to provide not only a simple approach to obtain the Chern number without computing any integrals, but also a practical technique for new material design.
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Open all abstracts, in this tab
Maxime Gidding et al 2024 J. Phys.: Condens. Matter 36 365801
All-optical schemes for switching magnetization offer a pathway towards the creation of more advanced data-storage technologies, both in terms of recording speed and energy-efficiency. It has previously been shown that picosecond-long optical pulses with central frequencies ranging between 12 and 30 THz are capable of driving magnetic switching in yttrium-iron-garnet films, provided that the excitation frequency matches the characteristic frequency of longitudinal optical phonons. Here, we explore how the phononic mechanism of magnetic switching in three distinct ferrimagnetic iron-garnet films evolves at optical frequencies below 10 THz, within the so-called terahertz gap. We find that at long wavelengths the magnetic switching rather correlates with phonon modes associated with the substrate. Our results show that the process of phononic switching of magnetization, previously discovered in the mid- to far-infrared spectral range, becomes much more complex at frequencies within the terahertz gap.
Brandon Klein et al 2024 J. Phys.: Condens. Matter 36 365301
We study the low-frequency Raman active modes of twisted bilayer MoS2 for several twist angles using a force-field approach and a parametrized bond polarizability model. We show that twist angles near high-symmetry stacking configurations exhibit stacking frustration that leads to significant buckling of the moiré superlattice. We find that atomic relaxation due to the twist is of prime importance. The periodic displacement of the Mo atoms shows the realization of a soliton network, and in turn, leads to the emergence of a number of frequency modes not seen in the high-symmetry stacking systems. Some of the modes are only seen in the XZ Raman polarization setup while others are seen in the XY setup. The symmetry of the normal modes, and how this affects the Raman tensors is examined in detail.
Michele Kotiuga and Karin M Rabe 2024 J. Phys.: Condens. Matter 36 355603
In ferroelectric switching, an applied electric field switches the system between two polar symmetry-equivalent states. In this work, we use first-principles calculations to explore the polar states of hydrogen-doped samarium nickelate (SNO) at a concentration of 1/4 hydrogen per Ni. The inherent tilt pattern of SNO and the presence of the interstitial hydrogen present an insurmountable energy barrier to switch these polar states to their symmetry-equivalent states under inversion. We find a sufficiently low barrier to move the localized electron to a neighboring NiO6 octahedron, a state unrelated by symmetry but equal in energy under a square epitaxial strain (a = b), resulting in a large change in polarization. We term this unconventional ferroelectric a 'fraternal-twin' ferroelectric.
E Osmic et al 2024 J. Phys.: Condens. Matter 36 355001
We have, in-situ, prepared and measured the temperature dependence of thermopower S(T) and resistance R(T) of Bi2Te3 topological insulator (TI) thin films in the amorphous and crystalline phase. Samples were prepared by sequential flash-evaporation at liquid 4He temperature. The S(T) in the amorphous phase is negative and much larger compared to other known amorphous materials, while in the crystalline phase it is also negative and behaves linearly with the temperature. The resistivity in the amorphous phase shows a semiconducting like behavior that changes to a linear metallic behavior after crystallization. S(T) an results in the crystalline phase are in good agreement with results obtained both in bulk and thin films reported in the literature. Linear behavior of the for T > 15 K indicates the typical metallic contribution from the surface states as observed in other TI novel materials. The low temperature conductivity T < 10 K exhibits logarithmic temperature dependent positive slope κ ≈ 0.21, indicating the dominance of electron-electron interaction (EEI) over the quantum interference effect, with a clear two dimensional nature of the contribution. Raman spectroscopy showed that the sample has crystallized in the trigonal space group. Energy-dispersive x-ray spectroscopy reveales high homogeneity in the concentration and no magnetic impurities introduced during preparation or growth.
Aulden K Jones et al 2024 J. Phys.: Condens. Matter 36 355802
We present a comprehensive exploration of loop-gap resonators for electron spin resonance (ESR) studies, enabling investigations into the hybridization of solid-state magnetic materials with microwave polariton modes. The experimental setup, implemented in a Physical Property Measurement System by Quantum Design, allows for measurements of ESR spectra at temperatures as low as 2 Kelvin. The versatility of continuous wave ESR spectroscopy is demonstrated through experiments on CuSOH2O and MgCr2O4, showcasing the g-tensor and magnetic susceptibilities of these materials. The study delves into the challenges of fitting spectra under strong hybridization conditions and underscores the significance of proper calibration and stabilization. The detailed guide provided serves as a valuable resource for laboratories interested in exploring hybrid quantum systems through microwave resonators.
Maryna Pankratova et al 2024 J. Phys.: Condens. Matter
We report on the stabilization of ferromagnetic skyrmions in zero external magnetic fields, in
exchange-biased systems composed of ferromagnetic-antiferromagnetic (FM-AFM) bilayers. By per-
forming atomistic spin dynamics simulations, we study cases of compensated, uncompensated, and
partly uncompensated FM-AFM interfaces, and investigate the impact of important parameters such
as temperature, inter-plane exchange interaction, Dzyaloshinskii-Moria interaction, and magnetic
anisotropy on the skyrmions appearance and stability. The model with an uncompensated FM-AFM
interface leads to the stabilization of individual skyrmions and skyrmion lattices in the FM layer,
caused by the effective field from the AFM instead of an external magnetic field. Similarly, in the
case of a fully compensated FM-AFM interface, we show that FM skyrmions can be stabilized. We
also demonstrate that accounting of interface roughness leads to the stabilization of skyrmions both in
compensated and uncompensated interfaces. Moreover, in bilayers with a rough interface, skyrmions
in the FM layer are observed for a wide range of exchange interaction values through the FM-AFM
interface, and the chirality of the skyrmions depends critically on the exchange interaction.
Volker Heine and Siyu Chen 2024 J. Phys.: Condens. Matter 36 353002
This theoretical discussion covers several effects of metallic bonding based on a simple formula. It comes from the first steps in the Moment Method for calculating the local electronic structure of a solid (such as at a surface or in a random alloy), and depends on the square root of the total coordination number C of near neighbours. Each atom is covalently bonded to its cluster of near neighbours as a whole. The properties of metals touched on include malleability, crystal structure and phase transitions, vacancy formation energy, surface catalysis, surface reconstruction, graphite stability, and some aspects of the benzene molecule seen as an atomic metal ring. In most of these, the 'saturation' type of curvature of the square root function plays a crucial role. A short historical survey indicates the development of the ideas from Bloch (1929 Z. Phys.52 555) to recent times.
Elizabeth H Krenkel et al 2024 J. Phys.: Condens. Matter
The coexistence and competition between the charge density wave (CDW) and superconductivity was studied by varying the Rh/Ir ratio. The superconducting transition temperature, $T_c$, varies from 7 K in pure Ir ($x=0$)
to 8.3 K in pure Rh ($x=1$). Temperature-dependent electrical resistivity reveals monotonic suppression of the CDW transition temperature, $T_{\text{CDW}}(x)$. The CDW starts in pure Ir, $x=0$, at $T_{\text{CDW}}\approx40$~K and extrapolates roughly linearly to zero at $x_c \approx 0.53-0.58$ under the superconducting dome. Magnetization and transport measurements show a significant influence of CDW on superconducting and normal states. Meissner expulsion is substantially reduced in the CDW region, indicating competition between the CDW and superconductivity. The low-temperature resistivity is higher in the CDW part of the phase diagram, consistent with the reduced density of states due to CDW gapping. Its temperature dependence just above $T_c$ shows signs of non-Fermi liquid behavior in a cone-like composition pattern. We conclude that the $\text{Ca}_3(\text{Ir}_{1-x}\text{Rh}_x)_4\text{Sn}_{13}$ alloy is a good candidate for a composition-driven quantum critical point (QCP) at ambient pressure.
Temperature-dependent electrical resistivity reveals monotonic suppression of the CDW transition temperature, $T_{\text{CDW}}(x)$. The CDW starts in pure Ir, $x=0$, at $T_{\text{CDW}}\approx40$~K and extrapolates roughly linearly to zero at $x_c \approx 0.53-0.58$ under the superconducting dome. Magnetization and transport measurements show a significant influence of CDW on superconducting and normal states. Meissner expulsion is substantially reduced in the CDW region, indicating the competition between the CDW and superconductivity. The low-temperature resistivity is higher in the CDW part of the phase diagram, consistent with the reduced density of states due to CDW gapping. Its temperature dependence just above $T_c$ shows clear signs of non-Fermi-liquid behavior in a cone-like composition pattern. We conclude that the $\text{Ca}_3(\text{Ir}_{1-x}\text{Rh}_x)_4\text{Sn}_{13}$ alloy is a good candidate for a composition-driven quantum critical point (QCP) at ambient pressure.
Milosz Zdunek et al 2024 J. Phys.: Condens. Matter
The interaction between phonons and magnons is a rapidly developing area of research, particularly in the field of acoustic spintronics. To discuss this interaction, it is necessary to observe two different waves (acoustic and spin waves) with the same frequency and wavelength. In the Ni80Fe20/Au/Co/Au system deposited on a silicon substrate, we observe the interaction between spin waves and surface acoustic waves using Brillouin light scattering spectroscopy. As a result, we can selectively control (activate or deactivate) the magnetoelastic interaction between the fundamental spin wave mode and surface acoustic waves. This is achieved by adjusting the magnetostrictive layer thickness in the multilayer. We demonstrate that by adjusting the number of layers in a multilayer structure, it is possible to precisely control the dispersion of surface acoustic waves while having minimal impact on the fundamental spin wave mode.
Anton Pfannstiel et al 2024 J. Phys.: Condens. Matter 36 355701
The absorption features of optically generated, short-lived small bound electron polarons are inspected in congruent lithium tantalate, LiTaO3 (LT), in order to address the question whether it is possible to localize electrons at interstitial Ta:V defect pairs by strong, short-range electron–phonon coupling. Solid-state photoabsorption spectroscopy under light exposure and density functional theory are used for an experimental and theoretical access to the spectral features of small bound polaron states and to calculate the binding energies of the small bound Ta (antisite) and Ta:V (interstitial site) electron polarons. As a result, two energetically well separated ( eV) absorption features with a distinct dependence on the probe light polarization and peaking at 1.6 eV and 2.1 eV are discovered. We contrast our results to the interpretation of a single small bound Ta electron state with strong anisotropy of the lattice distortion and discuss the optical generation of interstitial Ta:V small polarons in the framework of optical gating of Ta:Ta bipolarons. We can conclude that the appearance of carrier localization at Ta:V must be considered as additional intermediate state for the 3D hopping transport mechanisms at room temperature in addition to Ta, as well, and, thus, impacts a variety of optical, photoelectrical and electrical applications of LT in nonlinear photonics. Furthermore, it is envisaged that LT represents a promising model system for the further examination of the small-polaron based photogalvanic effect in polar oxides with the unique feature of two, energetically well separated small polaron states.