We investigate the quasiparticle and transport properties of a model describing interacting Dirac and Weyl semimetals in the presence of local Hubbard repulsion $U$, where we explicitly include a deviation from the linearity of the energy-momentum dispersion through an intermediate-energy scale $\mathrm{\Lambda }$. Our focus lies on the correlated phase of the semimetal. At the nodal point, the renormalization of spectral weight at a fixed temperature $T$ exhibits a weak dependence on $\mathrm{\Lambda }$ but is sensitive to the proximity to the Mott transition. Conversely, the scattering rate of quasiparticles and the resistivity display high-temperature exponents that crucially rely on $\mathrm{\Lambda }$, leading to a crossover towards a conventional Fermi-liquid behavior at finite $T$. Finally, by employing the Nernst-Einstein relation for conductivity, we identify a corresponding density crossover as a function of the chemical potential.

We characterize the current-carrying nonequilibrium steady state (NESS) in a single-band Hubbard model confronted with a static electric field in the presence of quenched disorder. Beyond the linear response regime, the electric field amplitude must compensate for at least half of the band gap to have a non-negligible stationary current. As the disorder is not expected to dissipate the extra energy injected by the field, optical phonons assisted by a fermionic heat bath serve as dissipation channels for the current-induced Joule heat generated by the accelerated electrons. The NESS of the system is addressed employing the dynamical mean-field theory using the so-called auxiliary master equation approach as impurity solver. Disorder effects are treated locally via the coherent potential approximation (CPA) and the self-consistent Born (SCB) approach. In the regime in which the two schemes yield similar results, we employ the SCB as it is computationally cheaper than the CPA. We show that, in a purely electronic setup, the disorder-induced dephasing cannot contribute states within the gap but only smear out the edges of the Hubbard bands. When phonons are taken into account, the different nature of disorder-induced dephasing and phonon-related dissipation becomes clear. We show that although both disorder and electron-phonon interaction enhance the current at off-resonant fields, disorder effects play a marginal role since they cannot provide in-gap states which are instead brought about by phonons and represent the privileged relaxation pathway for excited electrons.

Topological nodal-line semimetals are characterized by symmetry-protected one-dimensional band-touching lines or loops, which give rise to their peculiar Fermi surfaces at low energies. Furthermore, if time reversal or inversion symmetry breaking tilts the bands, anisotropic Fermi surfaces simultaneously hosting electron and hole carriers can also appear. We analytically investigate the linear density-density response function of a two-dimensional tilted nodal-line semimetal in the intrinsic and doped regimes. Despite the anisotropic electronic bands, the polarizability remains isotropic in our model system. We find that the plasmon dispersion in the long-wavelength limit exhibits a standard behavior proportional to the wave vector's square root, characteristic of two-dimensional electron liquids. Tilting enhances the plasmon frequency, and the Drude weight does not depend on the carrier density at low doping levels. Unlike the intrinsic and highly doped systems, static polarizability has two distinct singularities at finite wave vectors in the low doped regimes. These two singularities result in beat patterns in the Friedel oscillations.

The electronic properties of two-dimensional (2D) nanostructures are highly responsive to changes in their geometry, making strain-engineering a powerful method for tuning the electronic characteristics of flexible 2D nanostructures. Quasi-one-dimensional (1D) graphene nanoribbons (GNRs) are crucial quantum building blocks in the development of next-generation flexible devices and have recently been recognized for possessing distinct symmetry-protected topological phases characterized by a $Z_2$ invariant. In this study, utilizing the tight-binding (TB) model, we present compelling evidence that the topological phase transition in 1D GNRs can be effectively controlled through strain-engineering. Furthermore, we investigate the behavior of heterojunctions composed of different types of AGNR segments and reveal that strain can create or eliminate the junction state while significantly enhancing the end states. Our study presents a new method for tuning topological phase transitions in flexible quasi-1D materials, offering an efficient way to control over junction state and end states.

The interplay between magnetic and topological order can give rise to phenomena such as the quantum anomalous Hall effect. The extension of topological surface states into magnetic insulators (MIs) has been proposed as an alternative to using intrinsically magnetic topological insulators (TIs). Here, we theoretically study how this extension of surface states into a magnetic insulator is influenced both by the interface barrier potential separating a topological insulator and a magnetic insulator and by finite-size effects in such structures. We find that the gap in the surface states depends nonmonotonically on the barrier strength. A small, but finite, barrier potential turns out to be advantageous as it permits the surface states to penetrate even further into the MI. Moreover, we find that due to finite-size effects in thin samples, increasing the spin splitting in the MI can actually decrease the gap of the surface states.

Anderson localization transitions are usually referred to as disorder-driven quantum phase transitions from delocalized states to localized states. Here we report an unconventional “Anderson localization transition” in two-dimensional quantized quadrupole insulators. Such transitions are from symmetry-protected topological corner states to disorder-induced normal Anderson localized states, which can occur in the bulk as well as on the boundary. We show that these localization-localization transitions (transitions between two different types of localized states) can happen in both Hermitian and non-Hermitian quantized quadrupole insulators and investigate their criticality by finite-size scaling analysis of the corner density. The scaling analysis suggests that the correlation length of the phase transition, on the Anderson insulator side and near critical disorder $W_c$, diverges as $\xi \left(W\right)\propto exp[\alpha /\sqrt{|W-W_c|}]$, a typical feature of Berezinskii-Kosterlitz-Thouless-like transitions.

In twisted ${\mathrm{MoTe}}_2$, the latest transport measurement has reported observation of the quantum anomalous Hall effect at hole filling $\nu =-1$, which undergoes a topological phase transition to a trivial ferromagnet as layer hybridization gets suppressed by interlayer bias $D$. Here we show that this underlies the existence of an orbital Chern insulating state with gate ($D$) switchable sign in an antiferromagnetic spin background at hole filling $\nu =-2$. From momentum-space Hartree-Fock calculations, we find this state has a topological phase diagram complementary to that of the $\nu =-1$ one: by sweeping $D$ from negative to positive, the Chern number of this $\nu =-2$ state can be switched between $+1$, 0, and $-1$, accompanied by a sign change of a sizable orbital magnetization. In the range of $D$ where this antiferromagnet is the ground state, the orbital magnetization allows magnetic field initialization of the spin antiferromagnetic order and the Chern number.

We determine the phase diagrams of anisotropic Kitaev-Heisenberg models on the honeycomb lattice using parton mean-field theories based on different Majorana fermion representations of the $S=1/2$ spin operators. First, we use a two-dimensional Jordan-Wigner transformation (JWT) involving a semi-infinite snake string operator. To ensure that the fermionized Hamiltonian remains local, we consider the limit of extreme Ising exchange anisotropy in the Heisenberg sector. Second, we use the conventional Kitaev representation in terms of four Majorana fermions subject to local constraints, which we enforce through Lagrange multipliers. For both representations, we self-consistently decouple the interaction terms in the bond and magnetization channels and determine the phase diagrams as a function of the anisotropy of the Kitaev couplings and the relative strength of the Ising exchange. While both mean-field theories produce identical phase boundaries for the topological phase transition between the gapless and gapped Kitaev quantum spin liquids, the JWT fails to correctly describe the magnetic instability and finite-temperature behavior. Our results show that the magnetic phase transition is first order at low temperatures but becomes continuous above a certain temperature. At this energy scale, we also observe a finite-temperature crossover on the quantum-spin-liquid side, from a fractionalized paramagnet at low temperatures, in which gapped flux excitations are frozen out, to a conventional paramagnet at high temperatures.

Optimally-doped (OPD) ${\mathrm{Bi}}_2{\mathrm{Sr}}_{1.7}{R}_{0.3}{\mathrm{CuO}}_{6+\delta }$ ($R$-Bi2201, $R=\mathrm{Eu}$ and La) are investigated from the viewpoint of photoinduced nonequilibrium quasiparticle dynamics of high-$T_{\mathrm{c}}$ cuprates with low $T_{\mathrm{c}}$. The temperature dependencies of the superconducting (SC) and pseudogap (PG) responses demonstrate that the out-of-plane disorder enhanced by replacing La with Eu reduces the SC transition temperature and increases the PG energy. SC fluctuations are observed in each sample, especially in $R=\mathrm{La}$, where large SC fluctuations, which deeply penetrate into the PG state, are observed. From the fluence-dependent dynamics, the planar optical destruction energy densities of the SC state are found consistent with the $T_{\mathrm{c}}^2$ dependence noted previously for other high-$T_{\mathrm{c}}$ superconductors. Upon an optical quench we find that the SC state recovery starts only after the completion of the PG recovery in both samples.

Materials with low thermal conductivity are essential for thermoelectric energy conversion and thermal management applications. Lone-pair electrons (LPEs), a pair of nonbonded electrons, is generally believed to increase the phonon anharmonicity, which leads to low lattice thermal conductivity. However, the specific manifestation of LPEs on the lattice dynamics and phonon transport remains elusive. Here, using Raman scattering and first-principles calculations, we unveil mode-selective anharmonicity caused by LPEs in layered oxyselenides, quantified by the linewidth of Raman-active phonon modes. Strong anharmonicity is observed not only for the phonon modes associated with the ions that contain LPEs, but also for those modes involving ions that are bonded with the LPE-containing ions. The strength of anharmonicity also depends on the vibrational directions of the phonon modes. These results establish a general guideline for identifying LPE-induced anharmonicity and provide a new perspective on optimizing thermoelectric performance.

We investigate the dynamical relaxation behavior of the two-point correlation in extended XY models with a gapless phase after quenches from various initial states. Specifically, we study the XY chain with gapless phase induced by the following additional interactions: Dzyaloshinskii-Moriya interaction and the XZY-YZX type of three-site interaction. When quenching from the gapped phase, we observe that the additional interactions have no effect on the relaxation behavior. The relaxation behavior is $\delta C_{mn}\left(t\right)\sim t^{-3/2}$ and $\sim t^{-1/2}$ for the quench to the commensurate phase and the incommensurate phase, respectively. However, when quenching from the gapless phase, we demonstrate that the scaling behavior of $\delta C_{mn}\left(t\right)$ is changed to $\sim t^{-1}$ for the quench to the commensurate phase, and the decay of $\delta C_{mn}\left(t\right)$ follows $\sim t^{-1}$ or $\sim t^{-1/2}$ for the quench to the incommensurate phase depending on the parameters of prequench Hamiltonian. We also establish the dynamical phase diagrams based on the dynamical relaxation behavior of $\delta C_{mn}\left(t\right)$ in the extended XY models.

In this study, the correlated Stokes–anti-Stokes Raman scattering, denominated as the SaS process, is investigated in a decane sample, generalizing for a liquid the results observed in crystalline diamond. The production of SaS photon pairs is observed in both real and virtual processes by measuring nonclassical values of the second-order cross-correlation function $g_{\text{SaS}}^2\left(0\right)$. The correlated Raman spectrum is observed only for Raman shift values below approximately $2900{\mathrm{cm}}^{-1}$, which is equivalent to the highest vibrational mode of the decane molecule. Furthermore, despite the fact that decane is a liquid, the polarization and spatial profile of the SaS process in decane presents the same properties as those observed in diamond.

The group-IV ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x$ ferromagnetic semiconductor (FMS) has attracted much interest in spintronics due to its compatibility with semiconductor technology. However, ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x$ FMS prepared at high growth temperature ($T_{\mathrm{S}}$) meets dilemmas in balancing solubility and intermetallic precipitates, limiting its Curie temperature ($T_{\mathrm{C}}$) and electrical tunability. In this study, ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x$ films were prepared by magnetron sputtering at low $T_{\mathrm{S}}$ combined with rapid thermal annealing (RTA). We conducted a systematic study of the microstructure, valence state, and magnetic properties of ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x$ films. The phase diagram of ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x$ films versus Mn composition $x$ and RTA temperature $T_{\mathrm{R}}$ was plotted, and face-centered cubic structure ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x$ FMS was achieved with $x$ up to 0.1. As $x$ increased, the hole-mediated ferromagnetism of ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x$ FMS was enhanced, with the $T_{\mathrm{C}}$ reaching 282 K at $x=0.07$. We found that the magnetoresistance ratio of ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x$ FMS was significantly smaller than that of ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x$ with intermetallic precipitates by two orders, providing an easy method to confirm ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x$ FMS without intermetallic precipitates. Finally, the mechanism of suppressing the formation of intermetallic compounds by RTA is discussed. Our work extends the preparation window for high-quality ${\mathrm{Ge}}_{1-x}{\mathrm{Mn}}_x$ FMS, and also provides a promising choice for further investigation of ferromagnetic semiconductors.

Recent experiments indicate that crystalline graphene multilayers exhibit much of the richness of their twisted counterparts, including cascades of symmetry-broken states and unconventional superconductivity. Interfacing Bernal bilayer graphene with a ${\mathrm{WSe}}_2$ monolayer was shown to dramatically enhance superconductivity—suggesting that proximity-induced spin-orbit coupling plays a key role in promoting Cooper pairing. Motivated by this observation, we study the phase diagram of spin-orbit-coupled rhombohedral trilayer graphene via self-consistent Hartree-Fock simulations, elucidating the interplay between displacement field effects, long-range Coulomb repulsion, short-range (Hund's) interactions, and substrate-induced Ising spin-orbit coupling. In addition to generalized Stoner ferromagnets, we find various flavors of intervalley coherent ground states distinguished by their transformation properties under electronic time reversal, ${\text{C}}_3$ rotations, and an effective antiunitary symmetry. We pay particular attention to broken-symmetry phases that yield Fermi surfaces compatible with zero-momentum Cooper pairing, identifying promising candidate orders that may support spin-orbit-enhanced superconductivity.

We use the Lindblad equation approach to investigate topological phases hosting more than one localized state at each side of a disordered Su-Schrieffer-Heeger chain with properly tuned long-range hoppings. Inducing a nonequilibrium steady state across the chain, we probe the robustness of each phase and the fate of the edge modes looking at the distribution of electrons along the chain and the corresponding standard deviation in the presence of different kinds of disorder either preserving the symmetries of the Hamiltonian or not.

The recent discovery of superconductivity in magic-angle twisted trilayer graphene (MATTG) has sparked significant interest. Here we focus on MATTG, where the low energy flat bands and linearly dispersive Dirac bands coexist and can be decoupled by external fields. Using a continuum model, we examine the low-energy electronic structures and superconductivity for various external fields and small lateral shifts. We find that small lateral shifts play a crucial role by interacting with magnetic fields to increase the density of states in the flat bands, resulting in an anomalous superconducting phenomenon that strengthens with increasing magnetic field strength. Electric displacement fields and sublattice polarization, even though both break the ${\mathcal{C}}_2{\mathcal{M}}_h$ symmetry, contribute in opposite ways to the robustness of superconductivity. The interplay of these mechanisms can give rise to reentrant superconductivity under specific parameter settings. Our findings provide insights into the external field-regulated robustness and reentrant superconductivity in the MATTG system.

Exploring spin-valley coupling in two-dimensional (2D) materials with strong spin-orbit coupling (SOC) is of great significance for fundamental physics and practical applications. Using first-principles calculations, we investigate the valley-related properties of Janus $XM{\mathrm{SiN}}_2$ $(X=\mathrm{S}, \mathrm{Se}; M=\mathrm{Mo}, \mathrm{Cr})$ monolayer. The Janus $XM{\mathrm{SiN}}_2$ monolayer forms a pair of nonequivalent valleys, and the conduction and valence bands are degenerated at the valleys. The inversion symmetry breaking and the SOC effect induce remarkable valley spin splitting and Rashba spin splitting. Our calculations indicate that not only valley-contrasting transport properties but also optical selection rules with spin-valley coupling result in the coexistence of spin and valley Hall effects in the Janus $XM{\mathrm{SiN}}_2$ monolayer. Moreover, we demonstrate that the valley-spin physics of the Janus $XM{\mathrm{SiN}}_2$ monolayer can be modulated by in-plane biaxial strains, allowing its extraordinary potential for spintronics and valleytronic applications. We also show that V-doped ${\mathrm{SMoSiN}}_2$ monolayer can exhibit giant valley polarization of 89.51 meV $(-24.48 \mathrm{meV})$ for the valence (conduction) band. These findings could be helpful for the valleytronic applications of the Janus $XM{\mathrm{SiN}}_2$ monolayer.

One of the central issues concerning unconventional superconductivity is the nature of the charge-density-wave (CDW) order and its implication for the pairing mechanism. Recently, a CDW order has been discovered in undoped infinite-layer ${\mathrm{NdNiO}}_2$ nickelates, which cannot be explained by popular Fermi-surface-based mechanisms (such as Fermi surface nesting or strong electron-phonon coupling) due to the observed insulating behavior. In this paper, we investigate the CDW order in undoped ${\mathrm{NdNiO}}_2$ using first-principles calculations. We find that the CDW order formation is independent of the Fermi surface but is strongly correlated with the intrinsic antiferromagnetic (AFM) order. We propose a CDW mechanism based on enhancement of AFM coupling induced by symmetry reduction. We demonstrate that with the loss of fourfold rotational symmetry, the $\mathrm{Ni}\text{−}3d_{x^2-y^2}$ and $\mathrm{Nd}\text{−}5d_{z^2}$ bonding establishes a Ni-Nd-Ni AFM superexchange channel. The AFM coupling between two adjacent Ni atoms in the same Ni-O plane is enhanced in the lower-symmetry CDW structure, making the CDW order energetically favorable and stabilized when the in-plane checkerboard AFM order is presented. Our findings suggest that AFM and lattice are strongly coupled in ${\mathrm{NdNiO}}_2$ nickelates, which provides inspiration for the origin of CDW order in other strongly correlated systems and for the elucidation of the mysterious pairing mechanism.