We investigate itinerant ferromagnetism using a diluted spin-fermion model, derived from a repulsive Hubbard model, where itinerant fermions are coupled antiferromagnetically to spin auxiliary fields in a three-dimensional simple cubic lattice. We assign a positive finite value ($\sim \text{bandwidth}$) to the Hubbard on-site potential ($U$) at a fraction of sites to model magnetic impurities and set $U=0$ for rest of the sites, which corresponds to the nonmagnetic sites. In this inhomogeneous Hubbard-model representation for randomly distributed magnetic impurities, which is an appropriate model to study diluted spin systems like diluted magnetic semiconductors, itinerant carriers are confined to the impurity band and play a key role in determining the kinetic energy of the system and, consequently, the carrier-spin polarization. We focus, in particular, on understanding the spin-dependent transport properties of itinerant fermions in the impurity band by taking positional disorder of the auxiliary fields into account. Our semiclassical Monte Carlo calculations show that the ferromagnetic transition temperature of the carrier spins indeed shows an optimization behavior with the carrier density, similar to experimental observation in diluted magnetic semiconductors. We calculate the transport properties in detail to establish a one-to-one correspondence between the magnetic and transport properties of the carriers. In addition, the spin-polarized resistivities and density of states show that the system turns out to be a half-metallic ferromagnet. Overall, our results obtained beyond the perturbative regime are significant for understanding the ferromagnetism in diluted magnetic semiconductors.

The opacity of graphene is known to be approximately given by the fine-structure constant $\alpha$ times $\pi$. We point out the fact that the opacity is roughly independent of the frequency and polarization of the light can be attributed to the topological charge of the Dirac points. As a result, one can literally see the topological charge with the naked eye from the opacity of graphene, and moreover it implies that the fine-structure constant is topologically protected. A similar analysis suggests that 3D topological insulator thin films of any thickness also have opacity $\pi \alpha$ in the infrared region owing to the topological surface states, indicating that one can see the surface states with the naked eye through an infrared lens. For 3D Dirac or Weyl semimetals, the optical absorption power is linear to the frequency in the infrared region, with a linearity given by the fine-structure constant and the topological charge of Weyl points.

We present the application of classical and quantum-classical hybrid anomaly detection schemes to explore exotic configurations with anomalous features. We consider the Anderson model as a prototype, where we define two types of anomalies—a high conductance in the presence of strong impurity and a low conductance in the presence of weak impurity—as a function of random impurity distribution. Such anomalous outcome constitutes an imperceptible fraction of the data set and is not a part of the training process. These exotic configurations, which can be a source of rich new physics, usually remain elusive to conventional classification or regression methods and can be tracked only with a suitable anomaly detection scheme. We also present a systematic study of the performance of the classical and the quantum-classical hybrid anomaly detection method and show that the inclusion of a quantum circuit significantly enhances the performance of anomaly detection, which we quantify with suitable performance metrics. Our approach is quite generic in nature and can be used for any system that relies on a large number of parameters to find their new configurations, which can hold exotic new features.

The recent experimental claim of room-temperature ambient-pressure superconductivity in a Cu-doped lead- apatite (LK-99) has ignited substantial research interest in both experimental and theoretical domains. Previous density functional theory (DFT) calculations with the inclusion of an on-site Hubbard interaction $U$ consistently predict the presence of flat bands crossing the Fermi level. This is in contrast to DFT plus dynamical mean-field theory (DMFT) calculations, which reveal the Mott insulating behavior for the stoichiometric ${\mathrm{Pb}}_9\mathrm{Cu}{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$ compound. Nevertheless, the existing calculations are all based on the parent $P{6}_3/m\text{−}{\mathrm{Pb}}_{10}{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$ structure, which is argued to be not the ground-state structure. Here, we revisit the electronic structure of ${\mathrm{Pb}}_9\mathrm{Cu}{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$ with the energetically more favorable $P\overline{3}$ structure, fully taking into account electronic symmetry breaking. We examine all possible configurations for Cu substituting the Pb sites. Our results show that the doped Cu atoms exhibit a preference for substituting the Pb2 sites rather than the Pb1 sites. In both cases, the calculated substitutional formation energies are large, indicating the difficulty in incorporating Cu at the Pb sites. We find that most of the structures with Cu at the Pb2 site tend to be insulating, while the structures with both Cu atoms at the Pb1 sites (except one configuration) are predicted to be metallic by $\mathrm{DFT}+U$ calculations. However, when accounting for the electronic symmetry breaking, some Cu-doped configurations previously predicted to be metallic (including the structure studied in previous $\mathrm{DFT}+U$ calculations) become insulating. Our work highlights the importance of symmetry breaking in obtaining a correct electronic state for ${\mathrm{Pb}}_9\mathrm{Cu}{\left({\mathrm{PO}}_4\right)}_6\mathrm{O}$, thereby reconciling previous $\mathrm{DFT}+U$ and $\mathrm{DFT}+\mathrm{DMFT}$ calculations.

We demonstrate the existence and propagation of three hybrid modes of surface plasmon polaritons supported by two graphene monolayers coating a solid film. These modes propagate long distances with short wavelengths, which are suitable features to enhance the heat conduction along the film interfaces. For a Si film with a thickness of 15 nm and a length of 5 mm at 300 K, we find a plasmon thermal conductivity of 13.6 $\text{W}{\text{m}}^{-1}{\mathrm{K}}^{-1}$, which represents $67\%$ $(26\%)$ of its phonon Si $(\mathrm{Si}+\mathrm{graphene})$ counterpart. This thermal energy appears due to the coupling of plasmons propagating at speeds comparable to the speed of light in vacuum. The plasmonic heat conduction driven by two-dimensional materials thus appears as a major and unexpected mechanism for controlling temperature in solid-state systems at rates faster than the ones of phonons and electrons.

Simulating the nonequilibrium dynamics of thermal states is a fundamental problem across scales from high-energy to condensed-matter physics. Quantum computers may provide a way to solve this problem efficiently. Preparing a thermal state on a quantum computer is challenging, but there exist methods to circumvent this by computing a weighted sum of time-dependent matrix elements in a convenient basis. While the number of basis states can be large, in this paper we show that it can be reduced by simulating only the largest density matrix elements by weight, capturing the density matrix to a specified precision. Leveraging Hamiltonian symmetries enables further reductions. This approach paves the way to more accurate thermal-state dynamics simulations on near-term quantum hardware.

The thermodynamics of low-dimensional systems departs significantly from phenomenologically deducted macroscopic laws. Particular examples, not yet fully understood, are provided by the breakdown of Fourier's law and the ballistic transport of heat. Low-dimensional trapped ion systems provide an experimentally accessible and well-controlled platform for the study of these problems. In our paper, we study the transport of thermal energy in low-dimensional trapped ion crystals, focusing, in particular, on the influence of the Aubry-like transition that occurs when a topological defect is present in the crystal. We show that the transition significantly hinders efficient heat transport, being responsible for the rise of a marked temperature gradient in the nonequilibrium steady state. Further analysis reveals the importance of the motional eigenfrequencies of the crystal.

The spin Seebeck effect (SSE) consists of the generation of a spin current in a magnetic insulator under a temperature gradient that is converted into a charge current in an attached thin metallic layer with strong spin-orbit interaction. The theoretical models proposed to explain the experimental observations of the SSE in the so-called longitudinal configuration consider that the spin current is entirely produced by the thermal excitation of magnons in the magnetic material. Here we show that actually, at low temperatures, the SSE is entirely dominated by a phonon drag mechanism, in which phonons in the heat current generate a spin current by means of magnon-phonon interaction. The theory explains quantitatively quite well the experimental observations of Iguchi et al. [Iguchi et al., Phys. Rev. B 95, 174401 (2017)], demonstrating that the SSE in yttrium iron garnet (YIG)/platinum structures has a pronounced peak at low temperatures following the behavior of the thermal conductivity in YIG.

Quantum squeezing is an essential asset in the field of quantum science and technology. In this paper, we investigate the impact of temperature and anisotropy on squeezing of quantum fluctuations in two-mode magnon states within uniaxial antiferromagnetic materials. Through our analysis, we discover that the inherent nonlinearity in these bipartite magnon systems gives rise to a conjugate magnon squeezing effect across all energy eigenbasis states, driven by temperature and anisotropy. We show that temperature induces amplitude squeezing, whereas anisotropy leads to phase squeezing. In addition, we observe that the two-mode squeezing characteristic of magnon eigenenergy states is associated with amplitude squeezing. This highlights the constructive impact of temperature and the destructive impact of anisotropy on two-mode magnon squeezing. Nonetheless, our analysis shows that the destructive effect of anisotropy is bounded. We demonstrate this by showing that, at a given temperature, the squeezing of the momentum (phase) quadrature [or equivalently, the stretching of the position (amplitude) quadrature] approaches a constant function of anisotropy after a finite value of anisotropy. Moreover, our paper demonstrates that higher magnon squeeze factors can be achieved at higher temperatures, smaller levels of anisotropy, and closer to the Brillouin zone center. All these characteristics are specific to low-energy magnons in the uniaxial antiferromagnetic materials that we examine here.

In this work, we study a slave-rotor mean-field theory of an extended Hubbard model, applicable to transition metal dichalcogenide moiré systems, that captures both the formation of Wigner crystals as well as exotic spin states on top of these charge backgrounds. Phase diagrams are mapped out for different choices of long-range Coulomb repulsion strength, reproducing several experimentally found Wigner crystal states. Assuming unbroken time-reversal symmetry, we find several spin-liquid states as well as dimer states at fractional fillings. While spin dimer states are always found to have the lowest mean-field energy, several spin-liquid states are energetically competitive and may be stabilized by including gauge fluctuations or further interaction terms. We further discuss possible experimental signatures of these states pertinent to two-dimensional moiré heterostructures.

A rare class of square-planar iridate, ${\mathrm{Cs}}_2{\mathrm{Na}}_2{\mathrm{IrO}}_4$, where isolated ${\mathrm{IrO}}_4$ moieties are oriented orthogonally in the consecutive lattice planes, has been investigated using first-principles calculations. Microscopic magnetic exchange interactions and Wannier function analysis reveals the quasi-two-dimensional antiferromagnetic ground state with moderate mean-field transition temperature, in spite of the absence of long-range structural connectivity via ${\mathrm{IrO}}_4$ moieties. Further magnetic interactions pointed out possible canting of the Ir spins in a complex manner, due to the orthogonally oriented ${\mathrm{IrO}}_4$ planes. The estimated magnetocrystalline anisotropy is significantly large, which is further accentuated by Os doping due to the modification of the electronic configurations and electronic structure of the valence and conduction bands. The phonon modes analysis confirms very weak spin-phonon coupling and reveals the possible mechanism for the evolution of orthogonally oriented ${\mathrm{IrO}}_4$ moieties. The energetics of the muon active sites has been identified to guide the experimental measurements.

Recent experimental progress has made it possible to simulate spin-1/2 Hubbard model on triangular lattice in moiré materials formed by transition metal dichalcogenide (TMD) heterobilayer or homobilayer. In twisted TMD homobilayer, a vertical electric field can induce a valley contrasting flux in the hopping term. In this paper we study possible superconductors from a $t\text{−}J$ model with valley contrasting flux $\mathrm{\Phi }$ using the slave boson mean-field theory. We obtain a phase diagram with doping $x$ and $\mathrm{\Phi }$. A finite $\mathrm{\Phi }$ breaks spin rotation symmetry and the pairing symmetry is a superposition of spin singlet $d-id$ and spin triplet $p+ip$. There are two topological phase transitions when tuning $\mathrm{\Phi }$ from 0 to $\pi$, with three Dirac nodes at one transition and one single Dirac node at the other transition. We also discuss the effects of Van Hove singularity and a three-site correlated hopping term on the pairing strength. Lastly, we demonstrate that a small anisotropy term breaking the $C_3$ rotation can lead to a time-reversal invariant nodal superconductor connected to the $d_{x^2-y^2}$ superconductor on square lattice.

We investigate the physical mechanisms for spin-dependent recombination of a strongly bound pair of triplet excitons generated by singlet fission and forming a spin quintet (total spin of two) biexciton. For triplet excitons the spin-dependent recombination pathways can involve intersystem crossing or triplet-triplet annihilation back to the singlet ground state. However, the modeling of spin-dependent recombination for quintets is still an open question. Here we introduce two theoretical models and compare their predictions with the broadband optically detected magnetic-resonance spectrum of a long-lived quintet biexciton with known molecular structure. This spectrum measures the change in the fluorescence signal induced by microwave excitation of each of the ten possible spin transitions within the quintet manifold as a function of a magnetic field. While most of the experimental features can be reproduced for both models, the behavior of some of the transitions is only consistent with the quintet spin-recombination model inspired by triplet intersystem crossing which can reproduce accurately the experimental two-dimensional spectrum with a small number of kinetic parameters. Thus quantitative analysis of the broadband optically detected magnetic-resonance signal enables quantitative understanding of the dominant spin-recombination processes and estimation of the out-of-equilibrium spin populations.

We show that rhombohedral four-layer graphene (4LG) nearly aligned with a hexagonal boron nitride (hBN) substrate often develops nearly flat isolated low-energy bands with nonzero valley Chern numbers. The bandwidths of the isolated flat bands are controllable through an electric field and twist angle, becoming as narrow as $\sim 10$ meV for interlayer potential differences between top and bottom layers of $\left|\mathrm{\Delta }\right|\approx 10$–15 meV and $\theta \sim 0.5^{\circ }$ at the graphene and boron nitride interface. The local density of states analysis shows that the nearly flat band states are associated to the nondimer low-energy sublattice sites at the top or bottom graphene layers and their degree of localization in the moiré superlattice is strongly gate tunable, exhibiting at times large delocalization despite the narrow bandwidth. We verified that the first valence band's valley Chern numbers are $C_{V1}^{\nu =\pm 1}=\pm n$, proportional to layer number for $n\mathrm{LG}/\mathrm{BN}$ systems up to $n=8$ rhombohedral multilayers.

Analogous to graphite, bulk $h$-BN is a layered material in which the hexagonal layers may exhibit different stacking sequences. Effects of the stacking sequences on the stability, electronic, and transport properties of $h$-BN are investigated thoroughly based on fully ab initio calculations. We identify three stable $h$-BN stackings $({\mathrm{AA}}^{\prime }$, AB, and ABC) and present the tight-binding analysis based on Wannier functions as a fresh perspective on the effect of the stacking sequences on interorbital coupling. We also perform ab initio calculations of the electron-phonon interaction and phonon-limited electron mobilities for differently stacked $h$-BN. Our analysis of electron scattering rates associated with different phonon modes reveals that (i) the room-temperature electron mobilities are limited by acoustic phonons, (ii) for stackings ${\mathrm{AA}}^{\prime }$ and AB, the interlayer shear optical phonons also have significant contributions, and (iii) Fröhlich scattering (via longitudinal optical phonons) contributes negligibly to the room-temperature electron mobilities but may play a crucial role at higher temperatures. Semiempirical models, despite their widespread use owing to simplicity, cannot capture these effects precisely when applied to the electron mobility calculation of $h$-BN. We thus observe a significant discrepancy between our calculated ab initio mobilities and the semiempirical results, which highlights the importance of the accurate fully ab initio determination of the carrier mobilities in $h$-BN. Our findings can provide a benchmark for exploring the carrier transport properties of van der Waals layered materials.

Nanowires (NWs) of III-V semiconductor materials have been of interest to researchers for the last two decades. Knowledge of the subband spectrum of charge carriers in NWs and NW-based structures is very important for current applications. The electronic subband spectrum in NWs is currently known in detail, while for holes it is found with significant simplifications. One or more of the following crucial features are usually neglected: the real NW cross section shape, the crystal orientation of the NW, an accounting for the real anisotropic Hamiltonian of the bulk host material, and contributions that are due to the lack of an inversion center in the crystal lattice. Here we present a detailed calculation of hole subbands in GaAs NWs with the [111] orientation with a zinc blende crystal lattice, taking into account all the above four features. The spectrum of hole subbands based on the $4\times 4$ Luttinger Hamiltonian is numerically calculated taking into account two main contributions arising from the lack of inversion symmetry (the $T_d$ point group) in the lattice of the host crystal. Accounting for these contributions leads to the appearance of spin splitting only for some subbands, in accordance with symmetry considerations. However, a significant rearrangement also occurs in the spectrum of nonsplit subbands. The hole densities are visualized, and it is shown that the contribution of terms with $T_d$ symmetry significantly changes the structure of the multicomponent wave function. Thus, taking into account the lack of an inversion center is essential for the spectrum of hole subbands and wave functions in GaAs NWs. This can be more pronounced for NWs of III-V materials constituted by heavy elements, such as InSb, where spin-orbit interaction is stronger. The effect of a transverse electric field leading to so-called Rashba spin splitting is considered as well.

Anomalous photocurrents following optical interband excitation and the interaction with a Terahertz field are investigated in GaAs quantum wells. Our approach is based on the multiband semiconductor Bloch equations in length gauge, which are formulated in the basis of eigenfunctions of a 14-band $\mathbf{k}·\mathbf{p}$ model. Excitonic effects are included by treating the Coulomb interaction on the level of the time-dependent Hartree-Fock approximation and, furthermore, carrier longitudinal-optical and carrier longitudinal-acoustic phonon-scattering processes are included on the second-order Born-Markov level. The random $k$-dependent phase factors that originate from the numerical diagonalization of the eigenstates are treated by implementing a smooth gauge transformation. The flexible numerically implemented gauging procedure allows us to include many-body effects when integrating the semiconductor Bloch equations. Our results demonstrate that anomalous currents appear after resonant optical excitation of excitonic resonances. These currents are even stronger than those obtained when neglecting excitonic effects and exciting above the band gap. In the excitonic anomalous current transients, we identify temporal oscillations which can be attributed to the dynamics of excitonic wave packets since their frequency corresponds to the separation of the $1s$ and $2s$ exciton resonances. The numerical results are in very good agreement with experiments.

The recently reported bilinear magnetoeletric resistance (BMR) in novel materials with rich spin textures, such as bismuth selenide (${\mathrm{Bi}}_2{\mathrm{Se}}_3$) and tungsten ditelluride (${\mathrm{WTe}}_2$), opens new possibilities for probing the spin textures via magnetotransport measurements. By its nature, the BMR effect is directly linked to the crystal symmetry of the materials and its spin texture. Therefore, understanding the crystallographic dependency of the effect is crucial. Here we report the observation of crystallographically dependent BMR in thin ${\mathrm{WTe}}_2$ layers and explore how it is linked to its spin textures. The linear response measured in first-harmonic signals and the BMR measured in second-harmonic signals are both studied under a wide range of magnitudes and directions of the magnetic field and applied current and at different temperatures. We discover a threefold symmetry contribution of the BMR when current is applied along the $a$ axis of the ${\mathrm{WTe}}_2$ thin layer at 10 K, which is absent when current is applied along the $b$ axis.

We report a method to measure the temperature and transition time of cerium resolidification after shock to the liquid phase. Coating the highly reactive metal onto an inert optical window allows a supported shock to take a pure sample through an isobaric thermodynamic trajectory as diffusion cools the sample under steady stress. We measure the temperature of the liquid-to-epsilon transformation at a range of pressures to map the melt boundary under dynamic compression. We use the transformation duration to bound transformation kinetics, constrain transformation enthalpy and entropy, and explain discrepancies in predicted Hugoniot and melt coexistence in published equation-of-state models. This technique enables exploration of dynamic phase transformation in a variety of shocked materials.