Journal of Physics: Condensed Matter最新文献

This pedagogic review aims to give a gentle introduction to an exactly solvable model, the Hatsugai-Kohmoto (HK) model, which has infinite-ranged interaction but conserves the center of mass. Although this model is invented in 1992, intensive studies on its properties ranging from unconventional superconductivity, topological ordered states to non-Fermi liquid behaviors are made since 2020. We focus on its emergent non-Fermi liquid behavior and provide discussion on its thermodynamics, single-particle and two-particle correlation functions. Perturbation around solvable limit has also been explored with the help of perturbation theory, renormalization group and exact diagonalization calculation. We hope the present review will be helpful for graduate students or researchers interested in HK-like models or more generic strongly correlated electron systems.

The influence of the Zeeman energy and the Landau levels (LLs) arising from an applied magnetic fieldBupon the critical temperatureT_cis studied using a fully quantum mechanical method within the framework of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity that forms from an ultraclean metal. As in semiclassical treatments, we found that two electrons can form Cooper pairs with opposite spins and momenta in theBdirection while either in the same or in neighboring LLs. However, the fully quantum mechanical treatment of the LLs causesT_c(B) for electrons paired on the same LL to oscillate about the critical temperature of the BCS theory, similar to that of the de Haas-van Alphen effect. The Zeeman energy causesT_c(B) to decrease in an oscillatory fashion with increasingBfor electrons paired either on the same or on neighboring LLs. For the Zeemang > 1, pairing on neighboring LLs results in the highestT_c(B). Forg < 1, pairing on the same LLs gives the highestT_c(B). In addition,T_c(B) for electrons paired on neighboring LLs exhibits an apparent symmetry aroundg = 2, as the oscillatory critical temperature behaviors are nearly identical forg=2±δ.

Using first-principles calculations, this study systematically investigates the electronic properties and optical activity of AlB₂-type Dirac MBenes alongwith their correlations. Insights from phonon-spectral calculations andab-initiomolecular dynamics simulations substantiates the thermally and dynamically stable character of Dirac MBenes. Electronic dispersions reveals that all Dirac MBenes exhibits finitely gapped Dirac cones (DCs) at the fermi level, while FeB₂MBene behaves as a zero-band-gap semimetal akin to graphene. Such gap in DCs is desirable and crucial for optoelectronic applications. The interplay of out-of-planed_xzandd_yzorbitals of metal atom and hybrids in-plane dxyand dx2-y2orbitals from metal atom with p orbitals from boron atoms can be attributed to the emergence of DCs in MBenes. The calculations clearly reveal that the static dielectric constant and the energy gap within the DCs are critical factors influencing the electron-hole screening effect, consequently effecting the exciton binding energy. Further, it has been demonstrated that the exciton binding energies are consistent with predictions made by the two-dimensional (2D) Mott-Wannier model, confirming the Mott-Wannier characteristics of excitons in AlB₂-type Dirac MBenes, with the exception of partial Frenkel character in FeB₂MBene. Furthermore, it is demonstrated that Dirac MBenes exhibit a significant light absorption capacity in the near-infrared (NIR) and visible regions, with electron-hole interactions slightly modifying the optical spectral profile, making them promising for optoelectronic and photovoltaic applications. Subsequently, covariance analysis indicates that moderate energy gaps and high static dielectric constants are conducive to hosting Mott-Wannier excitons. Additionally, careful control of d-state valence electrons and hole doping can regulate the Mott-Wannier and Frenkel character of excitons in 2D Dirac materials. Thus, this comprehensive and systematic analysis of the electronic properties and optical-excitonic behavior of AlB₂-type Dirac MBenes, alongwith their correlations, enhances our understanding of this emerging family of 2D materials.

We show progressive enhancement in ferroelectric polarization with increasing Sm doping concentration along with retrieving tetragonality from the hexagonal paraelectric phase for Ba_1-xSm_xFe_0.2Ti_0.8O₃(BTO) solid solution (x= 0, 0.10, 0.15, 0.20). The cooperative off-centering phenomenon due to octahedral distortion promotes interpolar clustering resulting in the formation of polar nano region causing an enhancement in saturation polarization and coercive field with Sm doping. Ferroelectric domain fringes observed in scanning and transmission electron microscopic images indicate the presence of ferroelectric ordering for the highest Sm-doped compound. Formation ofSmBa∘-FeTi'defect complex results in a reduction of oxygen vacancy (V_O) defects which is observed from x-ray photoelectron spectroscopy. The reduction in V_Oimproves the domain switching by domain wall depinning and facilitates the increment in saturation polarization with Sm doping. The x-ray absorption spectra at TiL_3,2edges show the increase oft2gegratio indicating enhanced Ti 3d-O 2porbital hybridization which is supported by DFT calculation. This DFT calculation also confers that the increment in Ti 3d/Sm 4d-O 2porbital hybridization further weakens the short-range repulsion between core orbitals and facilitates the enhancement of saturation polarization.

Recent theoretical and experimental studies on the frustration-induced skyrmion crystal (SkX) in centrosymmetric magnets are reviewed, with some emphasis on their symmetry and topological aspects. Special importance of frustration and chirality is highlighted. Theories cover the studies based on both the spin models and the electronic models. In the former, the frustrated Heisenberg models on the triangular or the square lattices interacting either via the long-range RKKY interaction or via the competing short-range exchange interactions are treated, where frustration is borne by the oscillating nature of the long-range RKKY interaction or by the competition between the shorter-range exchange interactions. Special attention is paid to the role played by the magnetic anisotropy including the dipolar interaction. The electronic models discussed are mainly the Kondo lattice model on the triangular lattice, which reduces to the RKKY Heisenberg in the weak-coupling limit. Experiments on centrosymmetric SkX-hosting magnets cover the hexagonal or trigonal magnets Gd₂PdSi₃(triangular) and Gd₃Ru₄Al₁₂(breathing kagome), and the tetragonal magnets GdRu₂Si₂and EuAl₄. Various experimental data, including magnetization or susceptibility, specific heat, Hall resistivity, resonant magneticx-ray scattering, neutron scattering, Lorentz transmission electron microscopy, etc are reviewed and discussed in conjunction with the theoretical results. The nature of a variety of phases surrounding the SkX phase in the phase diagram, many of which are of multiple-qcharacter, is also examined. Finally, some discussion is given about the physical origin of the centrosymmetric SkX formation, its unique features in comparison with the non-centrosymmetric SkX induced by the antisymmetric Dzaloshinskii-Moriya interaction, together with some open and challenging problems for the future.

Ferrovalley materials are valleytronic materials with intrinsic ferromagnetism, in which the presence of spontaneous valley polarization is more conducive to practical applications. The optical properties of ferrovalley are important for selectively exciting electrons at the valley. In this paper, the electronic and optical spectrum of the H-phase FeCl₂monolayer is studied using first-principles calculations as an example. We use hybrid functional HSE06 and GW₀methods with spin-orbit coupling for our calculations, the band gap of H-FeCl₂is about 3.975 and 4.072 eV at K and -K valley, which is significantly larger than that obtained by the PBE method, with a 97 meV valley splitting. It is shown that the monolayer H-FeCl₂is a ferrovalley material with an ultra-wide band gap and large intrinsic valley polarization, which has strong electronic correlation and many-body effects. Calculation of the imaginary part of the dielectric function using GW-BSE method shows that the energy corresponding to the exciton peak is 2.421 and 2.491 eV, much smaller than the GW band gap. The exciton binding energy is about 1.554 and 1.581 eV at K and -K valley, indicating a large exciton effect. And the exciton binding energy of the two valleys are unequal, with a difference of 27 meV. It is found that splitting occurs at the first exciton peak in the ferrovalley material, and the splitting value is inequivalent to the bandgap splitting at the valley, which is instructive for further research as well as application of the valleytronics.

The tunnel magnetoresistance (TMR) effect is one of the representative phenomena in spintronics. Ferromagnets, which have a net spin polarization, have been utilized for the TMR effect. Recently, by contrast, the TMR effect with antiferromagnets, which do not possess a macroscopic spin polarization, has been proposed, and also been observed in experiments. In this topical review, we discuss recent developments in the TMR effect, particularly focusing on the TMR effect with antiferromagnets. First, we review how the TMR effect can occur in antiferromagnetic tunnel junctions. The Julliere model, which has been conventionally utilized to grasp the TMR effect with ferromagnets, breaks down for the antiferromagnetic TMR effect. Instead, we see that the momentum dependent spin splitting explains the antiferromagnetic TMR effect. After that, we revisit the TMR effect from viewpoint of the local density of states (LDOS). We particularly focus on the LDOS inside the barrier, and show that the product of the LDOS will qualitatively capture the TMR effect not only in the ferromagnetic tunnel junctions but also in the ferrimagnetic and antiferromagnetic tunnel junctions. This method is expected to work usefully for designing magnetic tunnel junctions.

Purely electrical control of the Dzyaloshinskii-Moriya interaction (DMI) without any external magnetic field is explored in a magnetic Weyl semimetal (WSM). The underlying mechanism for the DMI in the WSM is the recently identified asymmetrical indirect spin-spin interaction compatible with the inversion symmetry of the structure. While the necessary imbalance in the fermion population of opposite chirality is normally achieved with non-orthogonal external electric and magnetic fields (i.e. the axial anomaly), it is found that the intrinsic axial magnetic field characteristic to an inhomogeneous magnetic texture can play the role of the magnetic field. When applied to the magnetic domain walls as specific examples, our theoretical analysis clearly illustrates that the resulting DMI is pinned by and can in turn significantly affect the wall textures. As the appearance and strength of the DMI can be solely controlled by the applied electric field, this mechanism enables electrical modulation of magnetic domains including their excitation in the WSMs. Numerical calculations highlight significant advantages of the WSM over the conventional magnetic materials in spintronic applications such as the racetrack memory.

The ZrSiS-class of layered materials offer interesting topological and magnetic characteristics suitable for spintronics applications. In this work, we have synthesized a polycrystalline NdBiTe using solid-state reaction technique and have examined the magnetic properties in 2-300 K temperature range using temperature and field-dependent magnetization measurements. Our magnetic and specific heat data demonstrates a long-range antiferromagnetic (AFM) ordering in the material below 4.5 K. Furthermore, our isothermal magnetization data show a signature of spin-reorientation below Neel temperature. The observed nonlinearity in inverse susceptibility vs temperature data, and a hump in specific heat in 5-20 K range, indicate the existence of crystal field splitting in the material. Our transport properties measurements show the metallic behavior with positive magnetoresistance in the temperature range of 2-300 K. The observed rise in resistivity as function of temperature below Neel temperature infers the strongly correlated fermions, which is consistent with the observed large Sommerfeld coefficient. Consistent with experimental results, our first-principles calculations predict an AFM semimetallic nature of NdBiTe. Further, our spin-orbit coupled simulations of electronic structure show a signature of weak topological nature of the material.

MoS₂has recently emerged as a promising material for enabling quantum devices and spintronic applications. In this context, an improved physical understanding of theg-factor of MoS₂depending on device geometry is of great importance. Resistively detected electron spin resonance (RD-ESR) could be employed to determine theg-factor in micron-scale devices. However, its application and RD-ESR studies have been limited by Schottky or high-resistance contacts to MoS₂. Here, we exploit naturallyn-doped few-layer MoS₂devices with ohmic tin (Sn) contacts that allow the electrical study of spin phenomena. Resonant excitation of electron spins and resistive detection is a possible path to exploit the spin effects in MoS₂devices. Using RD-ESR, we determine theg-factor of few-layer MoS₂to be ∼1.92 and observe that theg-factor value is independent of the charge carrier density within the limits of our measurements.