Doping a conventional ferroelectric insulator can yield intriguing changes in its electronic transport properties, in particular when a polar-to-nonpolar transition occurs. In this work, we study the effect of electron doping on the resistivity and Seebeck coefficient of the prototypical ferroelectric ${\mathrm{BaTiO}}_3$, which undergoes a tetragonal-to-cubic second-order phase transition at a critical doping concentration. The transport properties are computed using two first-principles methods: (1) The constant relaxation time approximation and (2) the fully ab initio variational approach considering explicitly electron-phonon coupling. We show that the doping effect on the transport properties is manifold, via upshift of the Fermi level, variation of the band structure (tetragonal towards cubic), and phonon softening around the transition point, which causes nonmonotonic changes of the resistivity and Seebeck coefficient, especially the anomalies close to the transition point. Results of the two methods are compared, and the effect of temperature is also discussed.

Unveiling the ingredients that can operate shear bands is a cardinal task to tailor the mechanical properties of metallic glasses. Yet the effect of heterogeneous inclusions on shear banding is still poorly understood. Here, we perform molecular dynamics simulations of the shear deformation of CuZr metallic glasses with a portion of atoms pinned. We find a brittle-to-ductile yielding transition upon pinning, with the critical point where the alloy is the most unstable to the brittle failure shifting to large strains. In the brittle regime the shear band width increases first, then shrinks with the pinning concentration. This is interpreted as the perturbation of the aggregated events in the shear bands: the pinning causes a transition from the dilution of shear bands to the constraint, characterized by a nonmonotonic evolution of the nonaffine correlation length. The nonmonotonicity suggests an optimal microalloying addition to enhance the plasticity. This finding is significant for understanding the mechanism of tuning the mechanical performance of metallic glasses with foreign inclusions.

Halide perovskites have attracted intense research interest owing to their multifaceted and versatile applications in optoelectronics. This intrigue is further fueled by their propensity to undergo intricate structural modifications under extreme conditions, thereby instigating property changes. Within this context, in this paper, we delve deep into the intricate interplay of structural and vibrational attributes within the inorganic-metal halide perovskite-like $\mathrm{CsCu}{\mathrm{Cl}}_3$. Our approach employs Raman spectroscopy and synchrotron powder x-ray diffraction (SPXRD) techniques harnessed under the dual conditions of low temperatures and high pressures (HPs). We have observed a distinct spin-phonon coupling mechanism by employing Raman spectroscopy at low temperatures; this coupling has been manifested as a renormalization phonon phenomenon that occurs notably at $T^{*}=15$ K. The correlation between spin and phonon dynamics becomes pronounced through a notable hardening of phonon temperature dependence, a behavior intricately linked to the material antiferromagnetic transition at $T_N=10.7$ K. SPXRD under HP showed a first-order structural phase transition (SPT) at the critical pressure $P_c=3.69$ GPa, leading to the transformation from the hexagonal $P{6}_{5}22$ to a base-centered monoclinic cell. Notably, the coexistence of both phases is discernible within the pressure range from 2.79 to 3.57 GPa, indicating that the SPT involves the reorganization of the internal ${\left[{\mathrm{Cu}}_2{\mathrm{Cl}}_9\right]}^{5-}$ dimer unit, with the Cl-Cu-Cl bending contributing more than stretching modes. Furthermore, we demonstrate that the SPT is reversible, but residual strain pressure influences the modification of the critical pressure $P_c$ value upon pressure decrease.

Triggered by external fields, typical ferroelectrics (FEs) undergo a sharp phase transition from the nonpolarized paraelectric (PE) phase, which induces the emergence of spontaneous electric polarization. It is widely accepted that by introducing a large number of point defects, conventional FEs can be transformed into relaxor FEs, which exhibit unique physical properties, such as polar nanoregions (PNRs) and a corresponding gradual phase transition. In recent years, the discovery of two-dimensional (2D) FEs has attracted significant interest driven by the increasing demand for miniaturized nanoelectronics. Despite the identification of various 2D FE materials, there has been no report on 2D relaxor FEs so far. In this study, we explore the strain-induced FE phase transitions in a Se-doped PbTe monolayer through molecular dynamics simulations utilizing a developed deep-learning potential. Our findings reveal the gradual emergence of PNRs during the strain-induced FE phase transition of $\mathrm{P}{\mathrm{b}}_{50}\mathrm{T}{\mathrm{e}}_{50-x}\mathrm{S}{\mathrm{e}}_x$ ($x\ge 4$). Additionally, we showcase the dynamic nature of the PNRs through a comparison between the instantaneous and time-averaged dynamic pair distribution functions. Furthermore, we demonstrate that the evolution of the PNRs’ size and percolation strength with increasing strain can be well fitted by power laws derived from the percolation theory, which is a key characteristic of the relaxor-to-FE phase transition. Finally, a strain-temperature PE-relaxor-FE phase diagram for $\mathrm{P}{\mathrm{b}}_{50}\mathrm{T}{\mathrm{e}}_{46}\mathrm{S}{\mathrm{e}}_4$ is presented, which provides valuable insights for potential applications involving 2D Se-doped PbTe.

The integration of topological concepts into electronic energy band theory has been a transformative development in condensed matter physics. Since then, this paradigm has broadened its reach, extending to a variety of physical systems, including open ones. In this study, we employ analogs of the generalized $n$-dimensional Su-Schrieffer-Heeger model, a cornerstone in understanding topological insulators and higher-order topological states, to unveil a dimensional hierarchy of topological states within thermal diffusive networks. Unlike their electronic counterparts, the topological states in these networks are characterized by confined temperature profiles of dimension $(n-d)$ with constant diffusive rates, where $n$ represents the system's dimension and $d$ is the order of the topological state. Our findings demonstrate the existence of topological corner states in thermal diffusive systems up to $n=3$, along with surface and hinge states. We also identify and discuss an intermediate-order topological phase in the case $n=3$, characterized by the presence of hinge states but the absence of corner states. Furthermore, our work delves into the influence of chiral symmetry in these thermal networks, particularly focusing on topological thermal states with a near-zero diffusion rate. This research lays the foundation for advanced thermal management strategies that utilize topological states in multiple dimensions.

It is demonstrated that novel spin frustration can be induced in ferromagnets with nonuniform opposite Landé $g$ factors. The frustrated state is characterized by a mutual interplay of typical ferromagnetic (FM) and antiferromagnetic (AFM) features, such as the zero-field susceptibility being FM-like at low temperatures but AFM-like at high temperatures. It is also found to contain an exotic zero-temperature “half fire, half ice” critical point at which the spins on one sublattice are fully disordered and on the other one are fully ordered. We suggest that such frustration may occur in a number of copper-iridium oxides such as ${\mathrm{Sr}}_3{\mathrm{CuIrO}}_6$. We also anticipate a realization of the frustration and “partial fire, partial ice” states in certain antiferromagnets, lattice gas, and neuron systems.

Achieving multifunctional integration in two dimensions is highly desired for developing next-generation nanoelectronic devices but remains a challenge. Here, combining the advantages of cold metal and a bipolar magnetic semiconductor (BMS), three distinct functions, i.e., a spin filter, spin valve, and negative differential resistance, are simultaneously realized in a cold metal $2H\text{−}{\mathrm{TaS}}_2/\mathrm{BMS}2H\text{−}{\mathrm{VS}}_2/\mathrm{cold}$ metal $2H\text{−}{\mathrm{TaS}}_2$ lateral homologous heterojunction, where the BMS $2H\text{−}{\mathrm{VS}}_2$ is controlled by two electric gates placed on the left and right parts. The spin filter and spin valve effects originate from the bipolar spin-polarized band structure of $2H\text{−}{\mathrm{VS}}_2$, where its carriers' spin orientation can be easily flipped by shifting the Fermi level into either the valence or conduction band. Thus, in the junction, the current's spin polarization (spin up or spin down) and intensity (high or low conductance) can be switched by adjusting the polarities of the two applied gates: Once two gate voltages are both positive or negative, the junction works as a spin filter with either a fully spin-down or fully spin-up polarized current. While the two gate voltages possess opposite signs, the junction's current is prohibited due to mismatched spin orientations of carriers in the two parts of $2H\text{−}{\mathrm{VS}}_2$, leading to the spin valve effect. At the same time, the intrinsic energy gaps around the Fermi level in cold metal $2H\text{−}{\mathrm{TaS}}_2$ and the band-to-band tunneling of carriers from $2H\text{−}{\mathrm{TaS}}_2$ to $2H\text{−}{\mathrm{VS}}_2$ endow the junction with a significant negative differential resistance behavior with a large peak-to-valley current ratio.

Magnetic order and microwave absorption in amorphous ferromagnets and materials sintered from nanoscale ferromagnetic grains are investigated analytically and numerically within the random-anisotropy model. We show that a scaling argument specific to static randomness allows one to make conclusions about the behavior of a large system with a weak disorder by studying a smaller system with a strong disorder. The breakdown of the scaling on increasing the strength of the magnetic anisotropy and/or the size of the grain separates two distinct regimes in magnetic ordering and frequency dependence of the absorbed microwave power. Analytical results are confirmed by numerical experiments on spin lattices containing up to ${10}^7$ spins. Our findings should help design materials with desired magnetic and microwave properties. The method can be extended to other systems with quenched randomness.

The spin-wave dynamics in the thin CoFeB film in Damon-Eshbach geometry are studied in three cases of boundary conditions—free boundary conditions, symmetrical surface anisotropy, and one-sided surface anisotropy. The analytical model created by Wolfram and De Wames was extended to include perpendicular surface anisotropy in boundary conditions. Its comparison with numerical simulations demonstrates perfect agreement between the approaches. The analysis of the dispersion relation indicates that the presence of surface anisotropy increases the avoided crossing size between the Damon-Eshbach mode and perpendicular standing modes. Additionally, asymmetrical one-sided surface anisotropy induces nonreciprocity in the dispersion relation. In-depth analysis of the avoided crossing size is conducted for systems with different boundary conditions, different thicknesses, surface anisotropy constant values, and external magnetic fields. It shows the significant role of the strength of surface localization of the Damon-Eshbach mode and the symmetry of perpendicular standing modes in the avoided crossing broadening. Interestingly, for a specific set of parameters, the interaction between the particular modes can be suppressed, resulting in a mode crossing. Such a crossing, which occurs only for one direction of the wave vector in a one-sided surface anisotropy system, can be utilized in nonreciprocal devices.

Compounds exhibiting half-metallic character and metal-insulator transitions draw considerable attention in condensed-matter and materials physics due to their potential use in spintronics. Here we show that specific electron doping plays a crucial role in tailoring thermodynamic, structural, electronic, and magnetic properties of materials derived from low-symmetry magnetic intermetallics, exemplified on triclinic ${\mathrm{Mn}}_4{\mathrm{Al}}_{11}$. Upon chemical doping of Al by Ge, we predict improved phase stability and adherence to a generalized “18-n rule” governing closed-shell configurations in intermetallic with narrow bandgaps. Validating experiments include measurements of phase stability and electronic transport properties of electron-doped ${\mathrm{Mn}}_4{\mathrm{Al}}_{10}\mathrm{Ge}$ that crystallizes in the ${\mathrm{Mn}}_4{\mathrm{Al}}_{11}$-type structure, confirming the bandgap opening as predicted by chemical analysis and density-functional theory. We also demonstrate theoretically that hydrostatic pressure enhances the predicted half-metallic gap in the up-spin channel, which leads to a ferrimagnetic-to-ferromagnetic transition driven by Mn-Mn charge ordering in the ${\mathrm{Mn}}_4{\mathrm{Al}}_{11}$ parent. We have also discussed the generality of our approach in predicting the design of other classes of intermetallics including half and full Heusler compounds.

We consider the effects of quasiperiodic spatial modulation on the quantum Hall plateau transition by analyzing the Chalker-Coddington network model with quasiperiodically modulated link phases. In the conventional case (uncorrelated random phases), there is a critical point separating topologically distinct integer quantum Hall insulators. Surprisingly, the quasiperiodic version of the model supports an extended critical phase for some angles of modulation. We characterize this critical phase and the transitions between critical and insulating phases. For quasiperiodic potentials with two incommensurate wavelengths, the transitions we find are in a different universality class from the random transition. With the addition of more wavelengths they undergo a crossover to the uncorrelated random case. We expect our results to be relevant to the quantum Hall phases of twisted bilayer graphene or other moiré systems with large unit cells.

The signature of quantum phase transition is generally wiped out at finite temperature. A few quantities that have been observed to carry this signature through a nonanalytic behavior are also limited to low temperatures only. With an aim to identify a suitable dynamical quantity at a high temperature, we have recently constructed a function from quantum fidelity, which has the potential to bear a nonanalytic signature at the quantum critical point beyond low-temperature regime. In this paper, we elaborate our earlier paper and demonstrate the behavior of the corresponding rate function and the robustness of the nonanalyticity for a number of many-body Hamiltonians in different dimensions. We have also shown that our rate function reduces to that used in the demonstration of the dynamical quantum phase transition (DQPT) at zero temperature. It has been further observed that, unlike DQPT, the long-time limit of the rate function can faithfully detect the equilibrium quantum phase transition as well.

The Ising model with nearest-neighbor and next-nearest-neighbor interactions of the coupling constants $J_1$ and $J_2$, respectively, is investigated on a square lattice. For $J_1=2$ and $J_2=1$, the model becomes frustrated because ground states are infinitely degenerate. We obtain the density of states by using the Wang-Landau Monte Carlo method and calculate the specific heat. We find two separate peaks in the specific heat: a sharp peak related to the critical behavior and a round peak related to the specific heat of a disordered system such as spin glass. As the system size increases, the sharp-peak temperature decreases towards zero, and the maximum height of the sharp peak increases logarithmically, supporting that the spatial correlation length diverges exponentially at zero temperature. The partition-function zeros calculated by the density of states also suggest the zero-temperature phase transition.

Spiral spin liquids are correlated states of matter in which a frustrated magnetic system evades order by fluctuating between a set of (nearly) degenerate spin spirals. Here, we investigate the response of spiral spin liquids to quenched disorder in a $J_1-J_2$ honeycomb-lattice Heisenberg model. At the single-impurity level, we identify different order-by-quenched-disorder mechanisms and analyze the ensuing spin textures. In particular, we show that the latter generally display Friedel-like oscillations, which encode direct information about the spiral contour, i.e., the classical ground-state manifold. At finite defect concentrations, we perform extensive numerical simulations and characterize the resulting phases at zero temperature. As a result, we find that the competition between incompatible order-by-quenched-disorder mechanisms can lead to spiral spin glass states already at low to moderate disorder. Finally, we discuss extensions of our conclusions to nonzero temperatures and higher-dimensional systems, as well as their applications to experiments.

Ferromagnetic resonance is used to reveal features of the buried electronic band structure at interfaces between ferromagnetic metals and topological insulators. By monitoring the evolution of magnetic damping, the application of this method to a hybrid structure consisting of a ferromagnetic layer and a 3D topological insulator reveals a clear fingerprint of the Dirac point and exhibits additional features of the interfacial band structure not otherwise observable. The underlying spin-pumping mechanism is discussed in the framework of dissipation of angular momentum by topological surface states (TSSs). Tuning of the Fermi level within the TSS was verified both by varying the stoichiometry of the topological insulator layer and by electrostatic backgating and the damping values obtained in both cases show a remarkable agreement. The high-energy resolution of this method additionally allows us to resolve the energetic shift of the local Dirac points generated by local variations of the electrostatic potential. Calculations based on the chiral tunneling process naturally occurring in TSSs agree well with the experimental results.

The parton theory constructs candidate fractional quantum Hall states by decomposing the physical particles into unphysical partons, placing the partons in integer quantum Hall states, and then gluing the partons back into the physical particles. Field theoretical formulations execute the gluing process through the device of emergent gauge fields. Here we study numerically the process of going from the integer quantum Hall effect of two species of fermionic partons to the fractional quantum Hall effect of bosons by introducing an attractive interaction between the fermions of different species and continuously increasing its strength to glue them into bosons. To properly capture the physics in the bulk, we implement this process in a lattice version of the spherical geometry, which allows us to keep the full Hilbert space. Even though the two end-point states are topologically distinct, we find that, for the small system sizes accessible to our study, the energy gap remains open, indicating a crossover between these two states.

Recent studies on the electronic structures of the $5f$-electron based antiferromagnetic compounds, i.e., uranium dipnictides, have aroused widespread interest in the complex interplay among different channels of interactions, such as Kondo entanglement and antiferromagnetic ordering. Here, we use scanning tunneling microscopy/spectroscopy to explore the complex low-energy excitations in the $5f$-electron based antiferromagnet $\mathrm{U}{\mathrm{As}}_2$. The crystal-field excitations are revealed as peaks above the Fermi level in the $\mathrm{d}I/\mathrm{d}V$ spectra. Temperature-dependent spectroscopic measurements find that Kondo resonance and antiferromagnetic order are manifested as two peaks below the Fermi level, demonstrating that Kondo coherence can be undisturbedly established in the antiferromagnetic phase. The crystal-field excitations, Kondo resonance, and antiferromagnetic state are all locally altered by the presence of As-atom vacancies at the atomic scale and these phenomena prove that the nonmagnetic atom vacancy has a strong influence on low-energy excitations in heavy fermion compounds.

We have successfully synthesized a triangular lattice magnet, ${\mathrm{NaTmTe}}_2$, and conducted thermodynamic and inelastic neutron scattering (INS) investigations using single-crystal samples. Our magnetic susceptibility $\chi \left(T\right)$ measurements indicate antiferromagnetic interactions between ${\mathrm{Tm}}^{3+}$ moments. In our INS experiments, we observe a series of dispersive crystal electric field (CEF) excitations. These dispersions could be attributed to quantum tunneling effects arising from anisotropic spin exchange interactions, spontaneously establishing connections between distinct CEF states. Additionally, the field-dependent behavior of the CEF states exhibits pronounced nonlinear characteristics, suggestive of quantum fluctuations within the rare-earth moments. Utilizing the mean field technology of the random phase approximation, we compute the magnetic excitation spectra and thermodynamic properties, unveiling the emergence of a unique ground state primarily driven by quantum effects.

Hourglass-type Weyl fermions with the maximally topological charge of $\left|C\right|=3$, namely hourglass-type charge-three Weyl fermions (CTWFs), have seldom been reported due to the nonnegligible spin-orbit coupling effect and Pauli exclusion principles. Here, based on symmetry arguments and the low-energy effective $\mathbf{k}·\mathbf{p}$ model, we confirm that two chiral space groups (SGs), i.e., SG $\mathrm{P}{6}_3$ (No. 173) and SG $\mathrm{P}{6}_{3}22$ (No. 182), can host ideal hourglass-type CTWFs in spinless systems. Moreover, we take ${\mathrm{Li}}_2{\mathrm{CO}}_4$ as a concrete example to show the ideal hourglass-type CTWFs, which possess ultralong sextuple- and triple-helicoid Fermi arcs, making them easier to be detected by experiments. Our work not only offers an avenue to search for ideal hourglass-type CTWFs but also provides a promising platform for experiments to verify this kind of unconventional quasiparticles with the ultralong sextuple- and triple-helicoid surface arcs.

${\mathrm{Lu}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ is one of the cage-type superconductors with superconducting transition temperature $T_c=4.12$ K. Here we investigate the effect of pressure on the superconductivity in ${\mathrm{Lu}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$ by combining high-pressure electrical transport, synchrotron x-ray diffraction (XRD), and chemical doping. Application of high pressure can enhance both the metallicity and the superconducting transition temperature in ${\mathrm{Lu}}_5{\mathrm{Rh}}_6{\mathrm{Sn}}_{18}$. $T_c$ is found to show a continuous increase reaching up to 5.50 K at 11.4 GPa. Our high-pressure synchrotron XRD measurements demonstrate the stability of the pristine crystal structure up to 12.0 GPa. In contrast, $T_c$ is suppressed after the substitution of La ions in Lu sites, inducing negative chemical pressure. Our study provides valuable insights into the improvement of superconductivity in caged compounds.