SiGe nanowires (NWs) exhibit significant potential for thermoelectric applications due to their tunable thermal transport properties. Through systematic investigation of ${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$ ($x=1\%-50\%$), we reveal a dual role of inelastic scattering in thermal transport that depends critically on Ge concentration. At low Ge concentrations (e.g., 1%), inelastic scattering reduces thermal conductivity by 25% through conventional phonon mean free path reduction, consistent with classical phonon scattering theory. Surprisingly, at higher Ge concentrations (e.g., 50%), we observe an anomalous 61% enhancement in thermal conductivity for NWs of 543.1 nm in length by inelastic scattering. This counterintuitive behavior arises from the interplay between diffusive transport and Anderson localization. Detailed transmission and characteristic length analyses reveal that inelastic scattering partially disrupts the phase coherence of phonons, leading to an increase in localization length and a enhancement of thermal transport within specific frequency regions. These findings fundamentally advance our understanding of phonon transport in disordered nanostructures and provide guiding principles for the design of materials with tailored thermal properties in thermoelectric materials and nanoscale devices.

There has been an enduring interest and controversy about whether or not one can define physically meaningful energy density and stress fields, $e\left(\mathbf{r}\right)$ and ${\sigma }_{\alpha \beta }\left(\mathbf{r}\right)$, in quantum systems. A key issue is the kinetic energy since the well-known forms, $\frac{1}{2}{\left|\mathbf{\nabla }\mathrm{\Psi }\right|}^2$ and $-\frac{1}{2}\mathrm{\Psi }{\nabla }^2\mathrm{\Psi }$, lead to different densities, and analogous issues arise for interaction energy terms. This paper considers the ground state of a system of many interacting particles in an external potential, and presents a resolution to the problems in steps. (1) For the kinetic energy all effects of exchange and correlation are shown to be unique functions defined at each point $\mathbf{r}$; all issues of nonuniqueness are relegated to terms that involve only the density $n\left(\mathbf{r}\right)$ and are equivalent to an effective single-particle problem with ground state wave function $s\left(\mathbf{r}\right)=\sqrt{n\left(\mathbf{r}\right)/N}$. (2) Interactions can be considered in two ways: in terms of potentials acting on particles or in terms of the interaction fields, e.g., the Maxwell form in terms of electric fields. In each case, the problem reduces to a mean field part that is a function of the density and a part due to correlation that is uniquely defined; however, it is different for the two cases. (3) The final results follow from the nature of energy and stress. Because the energy determines the ground state itself through the variational principle, the energy density approach leads directly to kinetic energy in the form $-\frac{1}{2}s{\nabla }^{2}s$ and interactions in terms of potentials acting on the particles. This leads naturally to density functional theory and provides an interpretation in which the energy density $e\left(\mathbf{r}\right)$ is equilibrated to minimize fluctuations with the same chemical potential at all points $\mathbf{r}$. On the other hand, stress is related to forces, and the only physically acceptable expressions for the stress field involve the combination $\frac{1}{2}[s{\nabla }^{2}s-{\left|\mathbf{\nabla }s\right|}^2]$, as derived by Schrödinger, Pauli, and others, and Coulomb interactions in terms of electric fields, not potentials. Together these results lead to well-defined formulations of energy density and stress fields that are physically motivated and based on a clear set of arguments.

Exotic electronic physics including correlated insulating states and fractional Chern insulators have been observed in twisted bilayer graphene in a magnetic field when the Fermi velocity vanishes; however, a question remains as to the stability of these states, which is controlled by the gap to the first excited state. Free-space terahertz magneto-optics can directly probe the gap to charge excitations, which bounds the stability of electronic states, but this measurement has thus far been inaccessible due to the micron size of twisted bilayer graphene samples, while the wavelength of terahertz light is up to 1 mm. Here we leverage advances in fabrication to create twisted bilayer graphene samples over $5\times 5$ mm in size with a uniform twist angle and study the magnetic field dependence of the cyclotron resonance by a complex Faraday rotation experiment in $p$-doped large angle twisted bilayer graphene. These measurements directly probe charge excitations in inter-Landau level transitions and determine the Fermi velocity as a function of twist angle.

Artificial atoms, such as quantum corrals, offer an excellent platform to study fundamental interactions between localized quantum states and nanoscale probes. We performed atomic force microscopy measurements inside square quantum corrals on Cu(111) using CO- and metal-terminated tips. Using chemically unreactive CO-terminated tips, repulsive Pauli forces can be probed, while metallic tips are attracted to the localized quantum states due to chemical bonding. We found distinct exponential decay constants of $(46\pm 6)$ pm for the repulsive and $(66\pm 5)$ pm for the attractive forces. Attractive and repulsive interactions between two natural atoms show significantly shorter decay lengths. While natural atoms feature states with a broad range of decay lengths, including very short ones from deeply bound states, quantum corrals are lacking such deeply bound and highly localized states, resulting in longer decay lengths. These results offer an alternate route to understanding and designing atomic-scale interactions in low-dimensional quantum structures and devices.

We present a detailed comparative study of two important theoretical approaches: atomistic ${sp}^3{d}^5{s}^{*}$ tight-binding and continuum eight-band $k·p$ methods, for modeling the spin and optical properties of quantum dots (QDs). Our investigation spans key physical observables, including single-particle energy levels, $g$ factors, exciton radiative lifetimes, and hyperfine-induced Overhauser field fluctuations. We perform our calculations for self-assembled InGaAs/GaAs QD systems as representative case studies. While both methods yield qualitatively consistent trends, quantitative discrepancies arise due to the different treatment of atomistic details, strain effects, and confinement. We introduce targeted corrections to the eight-band $k·p$ framework, including a modified deformation potential scheme and adjusted remote-band contributions. Furthermore, we validate the eight-band implementation of hyperfine interactions by benchmarking it against the tight-binding model, showing reasonable convergence for both electrons and holes.

We have investigated the bulk and surface electronic structure of pyrite-type ${\mathrm{CuSe}}_2$ by means of soft x-ray photoemission spectroscopy. The Cu $3d$ band around 2.5 eV below the Fermi level does not show Cu $2p\text{−}3d$ resonance indicating that the Cu $3d$ level is almost fully filled and the Cu valence is close to $+1$. The dispersions of the Cu $3d/\mathrm{Se}4p$ bands observed by angle-resolved measurements roughly follow slab calculations for (100) surface. The surface of pyrite-type ${\mathrm{CuSe}}_2$ harbors two-dimensional Dirac-like band dispersions at the corner of the surface Brillouin zone suggesting possible interplay between the surface/interface Dirac states and the exotic bulk electronic/lattice properties through the Cu $3d$-Se $4p$ interaction.

Altermagnetic materials, with real-space antiferromagnetic arrangement and reciprocal-space anisotropic spin splitting, have attracted much attention. However, the spin splitting is small in most altermagnetic materials, which is a disadvantage to their application in electronic devices. In this study, based on symmetry analysis and the first-principles electronic structure calculations, we predict two Luttinger compensated bipolarized magnetic semiconductors $\mathrm{Mn}{\left(\mathrm{CN}\right)}_2$ and $\mathrm{Co}{\left(\mathrm{CN}\right)}_2$ with $s$-wave spin splitting as in the ferromagnetic materials. Our further analysis shows that the Luttinger compensated magnetism here depends not only on spin group symmetry, but also on the crystal field splitting and the number of $d$-orbital electrons. In addition, the polarized charge density indicates that both $\mathrm{Mn}{\left(\mathrm{CN}\right)}_2$ and $\mathrm{Co}{\left(\mathrm{CN}\right)}_2$ have the quasisymmetry $\mathrm{T}\tau$, resulting from the crystal field splitting and the number of $d$-orbital electrons. The Luttinger compensated magnetism not only has the zero total magnetic moment as the antiferromagnetism, but also has the $s$-wave spin splitting as the ferromagnetism, thus our work not only provides theoretical guidance for searching Luttinger compensated magnetic materials with distinctive properties, but also provides a material basis for the application in spintronic devices.

We study topological insulators under dephasing noise. With examples of both a $2d$ Chern insulator and a $3d$ topological insulator protected by time-reversal symmetry, we demonstrate that there is a phase transition at finite dephasing strength between phases with nontrivial and trivial topological indices. Here, the topological index is defined through the correlation matrix. The transition can be diagnosed through the spectrum of the whole correlation matrix or of a local subsystem. Interestingly, even if the topological insulator is very close to the topological-trivial critical point in its Hamiltonian, it still takes finite strength of dephasing to change the topological index, suggesting the robustness of topological insulators under dephasing. We further consider Chern insulators in the presence of real-space disorder, which exhibit a ground-state transition between topological and Anderson insulating phases. We find that even strongly disordered Chern insulators, close to the critical disorder strength, exhibit robustness with respect to dephasing.

Phase transitions governed by topological defects constitute a cornerstone of modern physics. Two-dimensional (2D) Anderson transitions in chiral symmetry classes are driven by the proliferation of vortex-antivortex pairs, a mechanism analogous to the Berezinskii-Kosterlitz-Thouless transition in the 2D XY model. In this work, we extend this paradigm to three-dimensional (3D) chiral symmetry classes, where vortex loops emerge as the key topological defects governing the Anderson transition. By deriving the dual representation of the 3D nonlinear sigma model for the chiral unitary class, we develop a mean-field theory of its Anderson transition and elucidate the role of one-dimensional weak band topology in the Anderson transition. Strikingly, our dual representation of the 3D nonlinear sigma model in the chiral symmetry class uncovers its connection to the magnetostatics of 3D type-II superconductors. The metal-to-quasilocalized and quasilocalized-to-insulating transitions in 3D chiral symmetry class share a unified theoretical framework with the normal-to-mixed and mixed-to-superconducting transitions in 3D type-II superconductors under an external magnetic field, respectively.

We study the dynamics of entanglement in a one-dimensional $XXZ$ spin-$1/2$ chain with and without integrability-breaking interactions and connected to a bath. We start from a state where the system and bath are completely unentangled, and the bath is polarized spin-down. We consider two different initial states for the system: (i) a polarized spin-up state, and (ii) an infinite-temperature state. In the particle representation of the spin chain, the polarized spin-up state corresponds to a filled state, while the polarized spin-down state corresponds to an empty state. Starting from these inhomogeneous quenches, in all the above-mentioned cases, we obtain the Page curve like behavior in the entanglement. We report different power-law behavior in the growth of entanglement for different initial states and different kinds of baths (interacting and noninteracting). In an attempt to explore plausible deep connections between entanglement and Boltzmann entropy, we investigate the latter in both the filled and the infinite-temperature case, for the system and the bath. For the filled case, the Boltzmann entropy of the system has the form of a Page curve but quantitatively deviates from the entanglement. On the other hand, the entropy of the bath keeps increasing. Remarkably, for the infinite-temperature case, we find that the system and bath Boltzmann entropies agree with the entanglement entropy, after and before the Page time, respectively. Our findings are expected to hold for generic interacting quantum systems and could be of relevance to black-hole physics.

We demonstrate the versatility, simplicity, and power of the minimally augmented spin-wave theory in studying phase diagrams of the quantum spin models in which unexpected magnetically ordered phases occur or the existing ones expand beyond their classical stability regions. We use this method to obtain approximate phase diagrams of the two paradigmatic spin-$\frac{1}{2}$ models on the honeycomb lattice: the $J_1\text{−}{J}_3$ ferro-antiferromagnetic and $J_1\text{−}{J}_2$ antiferromagnetic $XXZ$ models. For the $J_1\text{−}{J}_3$ case, various combinations of the $XXZ$ anisotropies are analyzed. In a dramatic deviation from their classical phase diagrams, which host significant regions of the noncollinear spiral phases, quantum fluctuations stabilize several unconventional collinear phases and significantly extend conventional ones to completely supersede spiral states. These results are in close agreement with the available density-matrix renormalization group calculations. The applicability of this approach to the other models and its potential extension to different types of orders are discussed.

Two-dimensional antiferromagnetic (AFM) valley materials attract significant attention due to their combined valley polarization and magnetic order. However, the achievement of the anomalous valley Hall effect (AVHE) still faces challenges of spin degeneracy and difficulty in controlling valley polarization. In this study, through theoretical research of the H-functionalized MXene bilayer ${\mathrm{Cr}}_2{\mathrm{CH}}_2$ system, a different mechanism for regulating AVHE by interlayer slip or an applied electric field is proposed. The study finds that the AB stacking exhibits an AFM ground state and intrinsic ferroelectricity, and that reversal of the polarization can switch the layer contribution of valley polarization, enabling nonvolatile electrical control of the AVHE. This work provides a feasible strategy and design principles for developing electrically writable and readable memory devices based on AFM valley materials.

We realize the full-replica-symmetry-breaking scheme by Parisi to obtain low-temperature entropy and specific heat for the spin-glass phase, which is predicted by the random Blume-Capel model. This theory describes special features of spin-glass behavior in a series of ${\mathrm{Fe}}_x{\mathrm{TaS}}_2\left({\mathrm{Se}}_2\right)$ compounds, in which the intercalated ${\mathrm{Fe}}^{2+}$ ions can lose their magnetic moments due to a transition to a low-spin singlet state. We demonstrate that the magnetic moment loss-recovery process by iron ions and the nonstoichiometry caused by their different crystallographic positions significantly affect the shape of the temperature dependence of the entropy and the specific heat near the tricritical point of the transition to the singlet paramagnetic state. Ways to enhance nonstoichiometry in these materials are discussed to validate the theory.

In the Yb-Co-Sn system a Remeika phase ${\mathrm{Yb}}_{3+x}{\mathrm{Co}}_4{\mathrm{Sn}}_{13-x}$ is found with solid solution extending within $0\le x\le 0.5$. All these compounds crystallize with primitive cubic (space group $Pm\overline{3}n, a\approx 9.54\AA$) strongly disordered ${\mathrm{Sc}}_3{\mathrm{Ir}}_4{\mathrm{Si}}_{13+x}$ type. Stoichiometric ${\mathrm{Yb}}_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$ is a superconductor with critical temperature $T_c=3.1\left(2\right)\mathrm{K}$, lower- $[B_{\mathrm{c}}1=1.90\left(2\right)\mathrm{mT}]$ and upper $[B_{\mathrm{c}}2=2.16\left(5\right)\mathrm{T}]$ critical fields. The specific-heat jump $\mathrm{\Delta }{c}_p/\gamma T_c=1.72\left(9\right)$ together with the fact that $B_{\mathrm{c}}2\left(T_c\right)$ dependence is described by the Werthamer-Helfand-Hohenberg (WHH) model hint toward a conventional mechanism with a weak electron-phonon coupling. Although a more precise analysis is hampered by the presence of multiple superconducting transitions observed in the specific heat of this Remeika phase, the nonstoichiometric ${\mathrm{Yb}}_{3.2}{\mathrm{Co}}_4{\mathrm{Sn}}_{12.8}$ reveals $T_c=2.4\left(2\right)\mathrm{K}$ and $B_{\mathrm{c}}2=4.79\left(9\right)\mathrm{T}$ exceeding the classical Pauli limit. Strong electron-phonon coupling in this superconductor is confirmed by the high value of the ${\lambda }_{\mathrm{AD}}=0.92\left(2\right)$ parameter determined from the Allen-Dynes formula. However, the electronic specific heat of ${\mathrm{Yb}}_{3.2}{\mathrm{Co}}_4{\mathrm{Sn}}_{12.8}$ follows the exponential law $[c_{\mathrm{el}}\left(T\right)\propto e^{-\mathrm{\Delta }\left(0\right)/k_{\mathrm{B}}T}]$ and can be described by the $\alpha$-model $[\alpha \equiv \mathrm{\Delta }\left(0\right)/k_{\mathrm{B}}{T}_c=2]$ indicating this stannide to be a conventional BCS-superconductor. Both ${\mathrm{Yb}}_{3+x}{\mathrm{Co}}_4{\mathrm{Sn}}_{13-x}$ ($x=0,0.2$) compounds reveal complex phonon spectra with possible “rattling” behavior. They are also found to be metallic systems, some aspects of which can be described by a free-electron-gas model.

Pure borocarbides suffer from limited superconducting potential due to intrinsic structural instability, requiring transition/alkali metals as dual-functional stabilizers and dopants. Here, by combining high-throughput screening with anisotropic Migdal-Eliashberg theory, we identify dynamically stable borocarbides where high-$T_c$ superconductivity predominately originates from $E$ symmetry-selective electron-phonon coupling (EPC). The six distinct superconducting gaps emerge from a staircase distribution or uncoupling of EPC strength $\rho \left({\lambda }_k^{el}\right)$ across each Fermi surface sheet, constituting a metal-free system with such high gap multiplicity. Crucially, dimensional reduction from bulk to surface strengthens $E$-symmetry EPC and enhances $T_c$ from 32 K [three-dimensional (3D) bulk] to 75 K (2D surface), a result that highlights structural confinement as a key design strategy for observing high $T_c$. External strain further optimizes the competition between EPC strength and characteristic phonon frequency to achieve $T_c>90$ K. This work reveals a systematic correlation between structural dimensionality and gap multiplicity and establishes borocarbide as a tunable platform to engineer both high-$T_c$ and multigap superconductivity.

The superionic transition in lithium oxide $\left({\mathrm{Li}}_2\mathrm{O}\right)$ presents significant challenges in establishing the structure-property relationship between its microscopic dynamical behavior to macroscopic physical properties. Additionally, the fundamental mechanism driving this transition remains contentious due to competing theoretical interpretations. Using machine learning potentials that maintain ab initio-level precision while enabling extended in both temporal and spatial scales, we systematically investigate lithium-ion diffusion dynamics spanning from crystalline to superionic phases. Our results give a clear physical pattern of ${\mathrm{Li}}_2\mathrm{O}$ Arrhenius plot throughout three distinct regions. The elastic softening serves as a signal of accelerating of Li-ions movement. Superionic transition is manifested in the melting of the Li sublattice. Dynamical analysis indicates that the superionic transition is driven by the formation of Frenkel pairs. Octahedral interstitial Li-ions, increasing sharply near the superionic state, act as primary diffusion carriers. The tetrahedral Li-ions exhibit much lower individual mobility and play a secondary role in diffusion under superionic state. This highlights the unique defect-mediated nature of diffusion in superionic state.

Using variable temperature total x-ray scattering and large-scale structure modeling, we study the temperature and composition evolution of lattice distortions and charge density waves (CDWs) in the archetypal strongly correlated system $\mathrm{RENi}{\mathrm{C}}_2$ ($\mathrm{RE}=\mathrm{Dy}$, Tb, Gd, and Sm). Joint analysis of reciprocal and real space data reveals the presence of strong lattice distortions that, depending on the RE species and temperature, appear periodic or remain local, forming a complex CDW phase diagram. Apparently, the CDWs in $\mathrm{RENi}{\mathrm{C}}_2$ materials arise from a common to the system lattice instability involving diverse distortion modes that persist when magnetic order also sets in at very low temperature. The results support the notion that intrinsic lattice distortions are important to the emergence of exotic electronic phases in strongly correlated systems and call for their further investigation using the advanced approach adopted here.

Elastic diffuse scattering from six concentrated binary fcc alloys in states of thermal equilibrium, previously analyzed within the real-space approach introduced by Borie and Sparks, was reevaluated within the reciprocal-space approach of Krivoglaz and Khachaturyan. Size effect scattering was treated within the microscopic theory of elasticity. Chemical and strain-induced interaction parameters were least-squares fitted to yield short-range order (SRO) scattering and size effect (SE) scattering. The r-values in fitting were larger for the reciprocal space than for the real-space approach, as demonstrated for the neutron scattering studies. While SRO scattering and thus the effective pair interaction (EPI) parameters well agree with the real-space approach, SE scattering differs, especially close to Bragg reflections, unless the strain-induced displacement fields are treated with a cutoff in range. The separation of the EPI parameters into the chemical and strain-induced interaction parameters shows similarity with the data from electronic structure calculations. Monte Carlo simulations with total interaction parameters closely reproduce SRO scattering.