We study theoretically the phase-dependent thermal transport in graphene-based Josephson junctions and highlight the role of Klein-like tunneling in the thermal energy transmission. We compute the thermal conductance for both short and long junctions by solving the Dirac-Bogoliubov–de Gennes equation. Numerical results show that the thermal conductance contains distinctive signatures of the existence of Andreev bound states, enabling us to identify different superconducting pairing symmetries in graphene, including $s$-, $d_{x^2-y^2}$-, $d_{xy}$-, $p_x$-, and $p_y$-wave symmetries. Importantly, our results may pave the way for applications of graphene in the field of Josephson heat interferometers.

Spin currents are key to spin torque devices, but determining the proper spin current is nontrivial. Here we derive a general quantum-mechanical formula for the intrinsic proper spin current showing that it is topological and can be finite in the gap. For topological insulators with an out of plane magnetization and the chemical potential in the surface state gap, the net spin torque is determined by the competition between the topological spin-Hall torque due to the bulk and the topological Edelstein effect due to the surface states. We also discuss spin-3/2 hole quantum wells.

The thermoelectric performance at a given output power of a voltage-probe heat engine, exposed to an external magnetic field, is investigated in linear irreversible thermodynamics. For the model, asymmetric parameter, general figures of merit, and efficiency at a given output power are analytically derived. Results show a tradeoff between efficiency and output power, and we recognize optimum-efficiency values at a given output power are enhanced compared to a Büttiker-probe heat engine due to the presence of a characteristic parameter, namely, $d_m$. Moreover, similar to a Büttiker-probe heat engine, the universal bounds on the efficiency are obtained, and the efficiency at a given output power can exceed the Curzon-Ahlborn limit. These findings have practical implications for the optimization of realistic heat engines and refrigerators. By controlling the values of the asymmetric parameter, the figures of merit, and $d_m$, it may be possible to design more efficient and powerful thermoelectric devices.

We theoretically study the band structure and optical conductivity in the Weyl and Dirac semimetal nanowires. It is found that the Fermi arc states play a crucial role in the optical conductivity. The optical conductivity shows an anisotropic dependence on the optical polarization axis. The qualitative anisotropy is attributed to the underlying band structure and distinct optical selection rules. For an optical polarization axis parallel to the nanowire, the optical resonant transitions are between Fermi arc and bulk states or bulk and bulk states. On the other hand, when the polarization axis is perpendicular to the nanowire, there are additional transitions between Fermi arc and Fermi arc states aside from the transitions aforementioned. They contribute to a giant resonant peak. The amplitude of this ultrastrong resonant peak is 10 times stronger than other responses. Moreover, these characteristic features of optical conductivities can be observed even with a finite chemical potential which is out of the bulk gap and are robust to disorders. In a word, the large resonant peak due to Fermi arc states and strong anisotropy of optical conductivities provide an optical means to probe the dynamics of the Fermi arc states in Weyl and Dirac semimetal nanowires. We hope that our study could give a valuable theoretical reference for nanoelectronics and ultrafast optoelectronic applications based on nanowires of Dirac and Weyl materials.

We have investigated the effects of strain on two-dimensional square lattices and examined the methods for inducing pseudomagnetic fields. In both the columnar and staggered $\pi$-flux square lattices, we have found that strain only modulates Fermi velocities rather than inducing pseudomagnetic fields. However, spatially nonuniform on-site potentials (anisotropic hoppings) can create pseudomagnetic fields in columnar (staggered) $\pi$-flux square lattices. On the other hand, we demonstrate that strain does induce pseudomagnetic fields in staggered zero-flux square lattices. By breaking a quarter of the bonds, we clarify that a staggered zero-flux square lattice is topologically equivalent to a honeycomb lattice and displays pseudovector potentials and pseudo-Landau-levels at the Dirac points.

The understanding of pressure-dependent fundamental properties of materials is very essential, not only for basic scientific knowledge but also for advanced technological applications. Experimental observations have fully revealed the hexagonal-to-rocksalt phase transition in the GaSe system under high pressure. In this article we systematically investigate the pressure-induced structural phase transition and related phonon, electronic, and optical properties of hexagonal $\varepsilon \text{−}\mathrm{GaSe}$ through a complete first-principles theoretical framework developed by the density-functional-theory calculations. The study focuses on geometric optimization, electronic band structures, electron localization functions, phonon spectra, dielectric properties, and optical spectra of the GaSe system under hydrostatic pressure. This work also includes an analysis of the phase transformation mechanism using the solid-state nudged elastic band method. Our research sheds light on the physics of structural phase transitions in layered materials and offers potential for the development of pressure-manipulated electronics and optoelectronics.

We discuss and demonstrate an unsupervised machine-learning procedure to detect topological order in quantum many-body systems. Using a restricted Boltzmann machine to define a variational Ansatz for the low-energy spectrum, we sample wave functions with probability decaying exponentially with their variational energy; this defines our training dataset that we use as input to a diffusion map scheme. The diffusion map provides a low-dimensional embedding of the wave functions, revealing the presence or absence of superselection sectors and, thus, topological order. We show that for the diffusion map, the required similarity measure of quantum states can be defined in terms of the network parameters, allowing for an efficient evaluation within polynomial time. However, possible “gauge redundancies” have to be carefully taken into account. As an explicit example, we apply the method to the toric code.

We examine possible ordered states of AA-stacked bilayer graphene arising due to electron-electron coupling. We show that under certain assumptions the Hamiltonian of the system possesses an SU(4) symmetry. The multicomponent order parameter is described by a $4\times 4$ matrix $\widehat{Q}$, for which a mean-field self-consistency equation is derived. This equation allows Hermitian and non-Hermitian solutions. Hermitian solutions can be grouped into three topologically distinct classes. First class corresponds to the charge density wave. Second class includes spin density wave, valley density wave, and spin-valley density wave. An ordered state in the third class is a combination of all the aforementioned density-wave types. For anti-Hermitian $\widehat{Q}$ the ordered states are characterized by spontaneous interlayer loop currents flowing in the bilayer. Depending on the topological class of the solution these currents can carry charge, spin, valley, and spin-valley quanta. We also discuss the special case when matrix $\widehat{Q}$ is not Hermitian and not anti-Hermitian. Utility and weak points of the proposed SU(4)-based classification scheme of the ordered states are analyzed.

We study the ground state properties of the hole-doped three-band Hubbard (Emery) model, describing the copper-oxygen planes of the cuprates, using large-scale two-dimensional tensor network calculations. Our simulations reveal a period 4 stripe state with spin and weak charge order over an extended doping range beyond $\delta \sim 0.12$ and stripes with larger periods at smaller doping. The period 4 stripe exhibits coexisting $d$-wave superconductivity in the doping range $0.15\lesssim \delta <0.25$, while at smaller doping around $\delta \sim 1/8$ we find a strong competition between superconducting and nonsuperconducting stripes, including also pair density-wave states with an alternating sign structure on neighboring stripes, suggesting that the fate of superconductivity around 1/8 doping may be sensitive on the model parameters.

Kagome metals are topological materials with a rich phase diagram featuring various charge density wave orders and even unconventional superconductivity. However, little is still known about possible spin polarized responses in these nonmagnetic compounds. Here, we perform ab initio calculations of the intrinsic spin Hall effect (SHE) in the kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{Cs}$, Rb, K), ${\mathrm{CsTi}}_3{\mathrm{Bi}}_5$, and ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$. We report large spin Hall conductivities, comparable with the Weyl semimetal TaAs. Additionally, in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ the SHE is strongly renormalized by the charge density wave order. We can understand these results based on the topological properties of band structures, demonstrating that the SHE is dominated by the position and shape of the Dirac nodal lines in the kagome sublattice. Our results suggest kagome materials as a promising, tunable platform for future spintronics applications.

GaAs/AlGaAs quantum dots created by the nanohole-filling droplet epitaxy are promising for applications in information photonics due to the low mechanical stress in these lattice-matched heterostructures. In this work, we have carried out a detailed study of the optical properties of such quantum dots using microphotoluminescence and reflection spectroscopy. For quantum dots grown by molecular beam epitaxy with a density of about 1 $\mu {\mathrm{m}}^{-2}$, the photoluminescence signal was scanned with a spatial resolution of about 1 $\mu \mathrm{m}$. A correlation map was constructed for the spatial derivatives of the photoluminescence spectra in order to identify the emission bands that correlate with each other. It was found that QD A $(E=1.56 \mathrm{eV})$, QD B1 $(E=1.61\mathrm{eV})$, and QD B2 $(E=1.65\mathrm{eV})$ bands originate from the same regions of the sample. The smaller width and the lower energy position of the QD A band allow it to be related to the central part of the quantum dot, while the QD B1 and the QD B2 bands' origins are smaller quantum-sized objects located around the same dot. All these bands are observed in anticorrelation with the signal from the heavy-hole exciton in the quantum well.

Surpassing the individual characteristics of the non-Hermitian skin effect (NHSE) and the scale-free (SF) effect observed recently, we systematically exploit the exponential decay behavior of bulk eigenstates via the transfer matrix approach in non-Hermitian systems. We concentrate on one-dimensional finite-size non-Hermitian systems with $2\times 2$ transfer matrices in either the absence or presence of the boundary impurity. We analytically unveil that the unidirectional SF effect emerges with the singular transfer matrices, while the hybrid scale-free skin (SFS) effect appears with the nonsingular transfer matrices even when an open boundary condition (OBC) is imposed. The unidirectional SF effect exceeds the scope of the SF effect in previous works, while the hybrid SFS effect is an interesting interplay between the skin effect and the SF effect in finite-size systems. Our results reveal that the skin effect under the OBC prevails when it coexists with the SF effect as the system approaches the thermodynamic limit in the presence of the hybrid SFS effect. Our approach paves the way for rigorous and unified explorations of the skin and SF effects in both Hermitian and non-Hermitian systems with generic boundary conditions.

We describe a numerical time-domain approach for high-accuracy calculations of Casimir-Polder forces near microstructured materials. The use of a time-domain formulation enables the investigation of a broad range of materials described by advanced material models, including nonlocal response functions. We validate the method by a number of example calculations for which we thoroughly investigate the convergence properties of the method, and comparing to analytical reference calculations, we find average relative errors as low as a few parts in a million. As an application example, we investigate the anisotropy-induced repulsive behavior of the Casimir-Polder force near a sharp gold wedge described by a hydrodynamic Drude model.

A bound state in the continuum (BIC) is a concept of a wave phenomenon that remains confined even though it coexists with a continuous radiative spectrum. In photonics, such states are promising for the enhancement of the light-matter interaction. In this work, we study the optical properties of the $a\text{−}{\mathrm{Si}}_{0.65}{\mathrm{C}}_{0.35}$:H-based planar waveguide structure covered by a two-dimensional array of gold nanodisks. To obtain BICs, we use a square lattice of nanodisks, which has one of the highest degrees of symmetry. The lattice provides light coupling to both TE and TM guided modes, while the single nanodisk supports localized plasmon resonance. We show that the structure possesses different types of BICs which are the result of the interaction of guided modes and single-particle resonances.

Majorana bound states (MBSs) at the end of nanowires have been proposed as one of the most important candidates for topological qubits. However, similar tunneling conductance features for both the MBSs and Andreev bound states have turned out to be a major obstacle in the verification of the presence of MBSs in semiconductor-superconductor heterostructures. In this article, we use a protocol to probe properties specific to the MBSs and use it to distinguish the topological zero-bias peak (ZBP) from a trivial one. For a scenario involving a quantized ZBP in the nanowire, we propose a scheme wherein the length of the topological region in the wire is altered. The tunneling conductance signatures can then be utilized to gauge the impact on the energy of the low-energy states. We show that the topological and trivial ZBPs behave differently under our protocol; in particular, the topological ZBP remains robust at zero bias throughout the protocol, while the trivial ZBP splits into two peaks at finite bias. This protocol probes the protection of near-zero-energy states due to their separable nature, allowing us to distinguish between topological and trivial ZBP.

We show that interacting bosons on a ring which are driven periodically by a rotating potential can support discrete time crystals whose absolute stability can be proven. The absolute stability is demonstrated by an exact mapping of discrete-time-crystal states to low-lying eigenstates of a time-independent model that reveals spontaneous breaking of space translation symmetry. The mapping ensures that there are no residual time-dependent terms that could lead to heating of the system and destruction of discrete time crystals. We also analyze periodically kicked bosons where the mapping is approximate only and cannot guarantee the absolute stability of discrete time crystals. Besides illustrating potential sources of instability, the kicked bosons model demonstrates a rich field for investigating the interplay between different time and space symmetry breaking, as well as the stability of time crystal behavior in contact with a thermal reservoir.

We develop a microscopic transport theory in a randomly driven fermionic model with and without a linear potential. The operator dynamics arise from the competition between noisy and static couplings, leading to diffusion regardless of ballistic transport or Stark localization in the clean limit. The universal diffusive behavior is attributed to a noise-induced bound state arising in the operator equations of motion at small momentum. By mapping the noise-averaged operator equation of motion to a one-dimensional non-Hermitian hopping model, we analytically solve for the diffusion constant, which scales nonmonotonically with noise strength, revealing regions of enhanced and suppressed diffusion from the interplay between on-site and bond dephasing noise, and a linear potential. For large on-site dephasing, the diffusion constant vanishes, indicating an emergent localization. On the other hand, the operator equation becomes the diffusion equation for strong bond dephasing and is unaffected by additional arbitrarily strong static terms that commute with the local charge, including density-density interactions. The bound state enters a continuum of scattering states at finite noise and vanishes. However, the bound state reemerges at an exceptional-like point in the spectrum after the bound-to-scattering state transition. We then characterize the fate of Stark localization in the presence of noise.

The electronic structure and magnetism of an individual Dy atom adsorbed on the MgO(001) substrate is investigated using a combination of the density functional theory with the Hubbard-I approximation to the Anderson impurity model. The divalent ${\mathrm{Dy}}^{2+}$ adatom in $f^{10}$ configuration is found. The calculated x-ray absorption and magnetic circular dichroism spectra are compared to the experimental data. Quantum tunneling between degenerate $|J=8.0,J_z=\pm 4.0\rangle$ states leads to the formation of a $|J=8.0,J_z=0.0\rangle$ ground state with an in-plane orientation of the magnetic moment. It explains the absence of remanent magnetization in a Dy adatom on the top of the MgO(001) substrate. Our studies can provide a viable route for further investigation and prediction of the rare-earth single-atom magnets.

One pillar of quantum magnonics is the exploration of the utilization of the mediation role of magnons in different platforms to develop quantum technologies. The efficient coupling between magnons and various quantum entities is a prerequisite. Here, we propose a scheme to enhance the spin-magnon coupling by the magnonic Kerr nonlinearity in a YIG sphere. We find that the Kerr-enhanced spin-magnon coupling invalidates the widely used single-Kittel-mode approximation to magnons. It is revealed that the spin decoherence induced by the multimode magnons in the strong-coupling regime becomes not severe, but suppressed, manifesting as either population trapping or persistent Rabi-like oscillation. This anomalous effect is because the spin changes to be so hybridized with the magnons that one or two bound states are formed between them. Enriching the spin-magnon coupling physics, the result supplies a guideline to control the spin-magnon interface.