We present a comprehensive study of $n$-alkane adsorbed on metal surfaces using the van der Waals density functional (vdW-DF) method. This method outperforms the density functional theory calculations within the local density or generalized gradient approximation in terms of a better description of a wide range of physisorption systems. Especially, our results demonstrate that vdW-DFs with improved accuracy nicely predict the adsorption heights of $n$-alkanes on close-packed metal surfaces. We also show that the C–H stretching frequencies and infrared absorption spectra are useful for discriminating the competing adsorption structures. Detailed analyses of the properties of $n$-alkanes on metal surfaces, however, reveal some discrepancies with experimental results, suggesting that further improvement of vdW-DF or the application of a more sophisticated technique is necessary for a systematic description of the $n$-alkane-metal interfaces with higher accuracy.

We theoretically investigate the coupling between surface optical phonons and holes in a system composed of a $p$-type doped GaS monolayer with a Mexican-hat valence band positioned on a polar substrate. We calculate the dynamical dielectric function within the random phase approximation by considering the hole-hole interaction in the presence of long-range polar Fröhlich interaction at zero temperature. We obtain the hybrid hole-phonon collective excitation dispersion numerically and in the long-wavelength limit, also analytically and explore the effects of density, type of substrate and separation distance between GaS monolayer and substrate on the coupled modes. Our results clearly indicate a strong coupling between the surface optical (SO) phonons of substrate and holes at all hole densities. Furthermore, we compute the real and imaginary parts of the quasiparticle self-energy, spectral function and scattering rate by using the leading-order dynamic screening approximation ($G^{0}W$ approximation) in both coupled and uncoupled systems. Interestingly, calculations of scattering rate reveal a distinctive feature by appearing two branches related to the inner and outer ring-shaped equal-energy surfaces. In the case of uncoupled state, an abrupt increase as a result of the plasmon emission is observed in one of the branches. However, the existence of two decay channels in coupled system leads to one (two) upward step(s) in the inner (outer) ring branch. Finally, we show that the scattering rate is apparently affected by the hole density and separation of $p$-type GaS monolayer and substrate.

Imaginary gauge transformation (IGT) provides a clear understanding of the non-Hermitian skin effect by transforming the non-Hermitian Hamiltonians with real spectra into Hermitian ones. In this paper, we extend this approach to the complex spectrum regime in a general nonreciprocal lattice model. We unveil the validity of IGT hinges on a class of pseudo-Hermitian symmetry. The generalized Brillouin zone of Hamiltonians respect such pseudo-Hermiticity is demonstrated to be a circle, which enables easy access to the continuum bands, localization length of skin modes, and relevant topological numbers. Furthermore, we investigate the applicability of IGT and the underlying pseudo-Hermiticity beyond nearest-neighbor hopping, offering a graphical interpretation. Our theoretical framework is applied to establish bulk-boundary correspondence in the nonreciprocal trimer Su-Schrieffer-Heeger model and to analyze the localization behaviors of skin modes in the two-dimensional Hatano-Nelson model.

We apply continuous similarity transformations (CSTs) to study the zero-temperature breakdown of long-range ordered quantum antiferromagnets. The CST flow equations are truncated in momentum space by the scaling dimension so that all operators with scaling dimension up to two are taken into account. We determine the quantum phase transition out of the Néel-ordered phase in the unfrustrated square lattice Heisenberg bilayer as well as the quantum melting of the Néel-ordered and columnar phase in the highly frustrated $J_1\text{−}{J}_2$ model on the square lattice. In all cases the CST is set up to isolate the ground state so that the stability of the flow equations, the ground-state energy, and the sublattice magnetization are used to explore the long-range ordered phases. We extract quantum critical points, which agree well with values in the literature. Further, we estimate the associated critical exponents $\alpha$ and $\beta$, which turns out to be a challenging task for the CST approach.

Despite its unique structural features, the magnetism of single-layered cuprate with five oxygen coordination ($T$*-type structure) has not been investigated thus far. Here, we report the results of muon spin relaxation and magnetic susceptibility measurements to elucidate the magnetism of $T$*-type ${\mathrm{La}}_{1-x/2}{\mathrm{Eu}}_{1-x/2}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LESCO) via magnetic Fe and nonmagnetic Zn substitution. We clarified the inducement of the spin-glass (SG)-like magnetically ordered state in ${\mathrm{La}}_{1-x/2}{\mathrm{Eu}}_{1-x/2}{\mathrm{Sr}}_x{\mathrm{Cu}}_y{\mathrm{Fe}}_{1-y}{\mathrm{O}}_4$ with $x=0.24+y$ and the nonmagnetic state in ${\mathrm{La}}_{1-x/2}{\mathrm{Eu}}_{1-x/2}{\mathrm{Sr}}_x{\mathrm{Cu}}_y{\mathrm{Zn}}_{1-y}{\mathrm{O}}_4$ with $x=0.24$ after the suppression of superconductivity for $y\ge 0.025$. The SG state lies below $\sim 7\mathrm{K}$ in a wide Sr concentration range between 0.19 and 0.34 in $5\%$ Fe-substituted LESCO. The short-range SG state is consistent with that originating from the Ruderman-Kittel-Kasuya-Yosida interaction in a metallic state. Thus the results provide the first evidence for a Fermi liquid (FL) state in the pristine $T$*-type LESCO. Taking into account the results of an oxygen $K$-edge x-ray absorption spectroscopy measurement. [J. Phys. Soc. Jpn. 89, 075002 (2020)] reporting the actual hole concentrations in LESCO, our results demonstrate the existence of the FL state in a lower hole-concentration region, compared to that in $T$-type ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. The emergence of the FL state in a lower hole-concentration region is possibly associated with a smaller charge transfer gap energy in the parent material with five oxygen coordination.

In recent years, counter-intuitive results have shown that the quantum Hall edge states with topological protection can be dissipative. In this paper, we point out that the non-equilibrium nature of edge states in quantum Hall interferometers leads to inevitable dissipation. We consider a graphene interferometer operating in the integer quantum Hall regime and simulate the inelastic scattering that causes both dissipation and dephasing in the interferometer using non-equilibrium Green's function and virtual leads. We describe the dissipation process with the numerical results of the spatial distribution of heat generation and the evolution of electron energy distribution. In addition, with the enhancement of dephasing, a competition between Aharonov-Bohm interference and topologically protected quantized Hall plateaus is observed in the oscillations and fluctuations of the Hall resistances. At a suitable dephasing strength, quantum Hall plateaus can be promoted by dephasing. Our results not only give clues for the design of dissipation-free devices but also provide a platform for studying the non-equilibrium relaxation and the dissipation mechanism of topological states.

We investigate a generalized multiorbital tight-binding model on a triangular lattice, a system prevalent in a wide range of two-dimensional materials and particularly relevant for simulating transition metal dichalcogenide monolayers. We show that the interplay between spin-orbit coupling and different symmetry-breaking mechanisms leads to the emergence of four distinct topological phases [Eck et al., Phys. Rev. B 107, 115130 (2023)]. Remarkably, this interplay also triggers the orbital Hall effect with distinguished characteristics. Furthermore, by employing the Landauer-Büttiker formula, we establish that in the orbital Hall insulating phase, the orbital angular momentum is carried by edge states present in nanoribbons with specific terminations. We also show that they do not have the same topological protection against the disorder of the edge states as a first-order topological insulator.

Higher-order topological insulators (HOTIs) have attracted significant interest in recent years due to their unique properties, but the material realization is mainly limited to nonmagnetic systems. In this work, through tight-binding modeling and first-principles calculations, we reveal the experimentally synthesized two-dimensional (2D) $\mathrm{VS}{\mathrm{e}}_2$ as an example of a room-temperature magnetic HOTI. The nontrivial nature is characterized by an inverted band feature with a large gap and spin-polarized corner states with quantized fractional charge. We demonstrate that the topological corner states are robust against magnetization canting, defects, and strain, suggesting the great potential for experimental detection. Remarkably, the magnetic HOTI phase can be extended to other 2D dichalcogenides and dihalogenides, including $\mathrm{V}{X}_2, \mathrm{Sc}{X^{\prime }}_2, \mathrm{Y}{X^{\prime }}_2, \mathrm{Ru}{X^{\prime }}_2$ ($X=\mathrm{S}$, Se, Te; $X^{\prime }=\mathrm{Cl}$, Br, I) as well as their Janus structures. Our work not only provides a series of promising candidates for room-temperature magnetic HOTIs, but also sheds light on future design and regulation of novel quantum states for real applications.

We present a comprehensive magnetization study of a newly synthesized $\mathrm{C}{\mathrm{r}}_{1+x}\mathrm{T}{\mathrm{e}}_2$ (x∼0.15) compound with ferromagnetic order at $T_C$∼191 K. The results of magnetization and magnetic entropy change suggest a strong interplay of in-plane ferromagnetic spin fluctuations with easy $c$-axis ferromagnetic critical fluctuations, resulting in competing magnetocrystalline anisotropy. The universal critical scaling analysis further reveals the spin-dimension ($n$) crossover from Ising ($n=1$) to $\mathit{XY}$ ($n=2$) across $T_C$. This is in contrast to concentrations above $x\ge 0.25$ with either Ising ($n=1$) or Heisenberg ($n=3$) spin characteristics across the transition. Our study indicates that the magnetic properties of $\mathrm{C}{\mathrm{r}}_{1+x}\mathrm{T}{\mathrm{e}}_2$ undergo qualitative change below certain critical concentration.

Since the discovery of two-dimensional ferroelectric and ferromagnetic materials, the van der Waals (vdW) heterostructures constructed by ferroelectric and ferromagnetic monolayers soon became the ideal platforms to achieve converse magnetoelectric functions at the nanoscale, namely to use the electric field to control magnetization. In this Letter, by employing density functional theory calculations and dynamic simulations of the atomic spin model, we study the key role of the interfacial Dzyaloshinshii-Moriya interaction (DMI) in ${\mathrm{CrI}}_3\text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ vdW heterostructures. Our work demonstrates feasible DMI torques pumped by ferroelectric switching, which can drive current-free and low-energy-consumption domain wall motion. Moreover, such an interfacial DMI can also significantly enlarge the Walker field in the magnetic-field-driven domain wall technique.

We investigate chains of interacting spinless fermions subject to a finite external field $F$ (also called Stark chains) and focus on the regime where the charge thermalization follows the subdiffusive hydrodynamics. First, we study reduced models conserving the dipole moment and derive an explicit Einstein relation which links the subdiffusive transport coefficient with the correlations of the dipolar current. This relation explains why the decay rate ${\mathrm{\Gamma }}_q$ of the density modulation with wave vector $q$ shows $q^4$ dependence. In the case of the Stark model, a similar Einstein relation is also derived and tested using various numerical methods. They confirm an exponential reduction of the transport coefficient with increasing $F$. On the other hand, our study of the Stark model indicates that upon increasing $q$ there is a crossover from subdiffusive behavior, ${\mathrm{\Gamma }}_q\propto q^4$, to the normal diffusive relaxation, ${\mathrm{\Gamma }}_q\propto q^2$, at the wave vector $q^{*}$ which vanishes for $F\to 0$.

Adiabatic preparation of a critical ground state is hampered by the closing of its energy gap as the system size increases. However, this gap is directly relevant only for a uniform ramp, where a control parameter in the Hamiltonian is tuned uniformly in space towards the quantum critical point. Here, we consider inhomogeneous ramps in two dimensions: initially, the parameter is made critical at the center of a lattice, from where the critical region expands at a fixed velocity. In the 1D and 2D quantum Ising models, which have a well-defined speed of sound at the critical point, the ramp becomes adiabatic with a subsonic velocity. This subsonic ramp can prepare the critical state faster than a uniform one. Moreover, in both a model of $p$-wave paired 2D fermions and the Kitaev model, the critical dispersion is anisotropic—linear with a nonzero velocity in one direction and quadratic in the other—but the gap is still inversely proportional to the linear size of the critical region, with a coefficient proportional to the nonzero velocity. This suffices to make the inhomogeneous ramp adiabatic below a finite crossover velocity and superior to the homogeneous one.

The quantitative description of long-range order remains a challenge in quantum many-body physics. We provide zero-temperature results from two complementary methods for the ground-state energy per site, the sublattice magnetization, the spin gap, and the transverse spin correlation length for the spin-1/2 quantum XXZ antiferromagnet on the square lattice. On the one hand, we use exact, large-scale quantum Monte Carlo (QMC) simulations. On the other hand, we use the semianalytic approach based on continuous similarity transformations in terms of elementary magnon excitations. Our findings confirm the applicability and quantitative validity of both approaches along the full parameter axis from the Ising point to the symmetry-restoring phase transition at the Heisenberg point and further provide quantitative reference results in the thermodynamic limit. In addition, we analytically derive the relation between the dispersion and the correlation length at zero temperature in arbitrary dimension, and discuss improved second-moment QMC estimators.

Recently, the first predicted metal clathrate hydride ${\mathrm{CaH}}_6$ was successfully synthesized experimentally and shown to have a superconducting critical temperature $T_{\mathrm{c}}$ exceeding 200 K. However, experimental findings suggest the existence of additional unidentified phases besides ${\mathrm{CaH}}_6$. In this study, we performed an extensive structure search of the $\mathrm{Ca}–\mathrm{H}$ system at 200 GPa, revealing a thermodynamically stable type-I clathrate hydride, ${\mathrm{Ca}}_8{\mathrm{H}}_{46}$, along with two analogous structures featuring hydrogen vacancies. ${\mathrm{Ca}}_8{\mathrm{H}}_{46}$ exhibits a high $T_{\mathrm{c}}$ range of 210–223 K at 150 GPa, and the presence of hydrogen vacancies shifts the $T_{\mathrm{c}}$. Our simulated x-ray-diffraction patterns, calculated equation of state, and $T_{\mathrm{c}}$'s for ${\mathrm{Ca}}_8{\mathrm{H}}_x$ $(x=44-46)$ agree with the experimental results, confirming their presence in multiple synthesized phases. Our findings serve to revise the Ca–H phase diagram under high pressure and offer valuable insights into the effects of hydrogen vacancies in ${\mathrm{Ca}}_8{\mathrm{H}}_{46}$, which explain the anomalous decrease in $T_{\mathrm{c}}$ observed in experiments.

I consider a nonlinear response of conventional superconductors contaminated with nonmagnetic impurities or imperfections to a circular polarized light. I focus on dc contributions to the induced current density which describe the emergence of the static magnetization in a superconductor. This effect is known as inverse Faraday effect. By employing quasiclassical theory of superconductivity, I derive an expression for the induced static magnetization as a function of frequency of external ac field and disorder scattering rate. The scattering of electrons off potential impurities is taken into account within the framework of the self-consistent Born approximation. It is found that the magnitude of the inverse Faraday effect decreases with an increase in disorder scattering rate. I have also discovered that the value of the induced magnetization has a characteristic minimum at a frequency which approximately equals twice the value of the pairing gap in a clean superconductor. This minimum appears due to the resonant excitation of the amplitude Higgs mode.

A tight-binding model for $e_g$ orbitals on a square lattice is investigated. We consider only the nearest-neighbor hopping and the model is characterized by two hopping parameters, $t_1$ and $t_2$. There are Dirac points in the electronic band structure and the type of the Dirac points (type I or type II) depends on the ratio $t_2/t_1$. For the case of the type-I Dirac points, edge states appear for a lattice with edges perpendicular to the [11] direction. The edge states at a certain momentum along the edges have octupole moments with opposite signs between the right and left edges. Thus, these edge states can be called helical octupolar edge states. This study bridges the research fields of topological phenomena and multipole physics.

The Bose-Hubbard model (BHM) has been widely explored to develop a profound understanding of the strongly correlated behavior of interacting bosons. Quantum simulators not only allow the exploration of the BHM but also extend it to models with interesting phenomena such as gapped phases with multiple orders and topological phases. In this work, an extended Bose-Hubbard model involving a dimerized one-dimensional model of long-range interacting hard-core bosons is studied. Bond-order density wave phases (BODW) are characterized in terms of their symmetry breaking and topological properties. At certain fillings, interactions combined with dimerized hoppings give rise to an emergent symmetry-breaking leading to BODW phases, which differs from the case of non-interacting models that require an explicit breaking of the symmetry. Specifically, the BODW phase at filling $\rho =1/3$ possesses no analog in the noninteracting model in terms of its symmetry-breaking properties and the unit cell structure. Upon changing the dimerization pattern, the system realizes topologically trivial BODW phases. At filling $\rho =1/4$, on-site density modulations are shown to stabilize the topological BODW phase. Our work provides the bridge between interacting and noninteracting BODW phases and highlights the significance of long-range interactions in a dimerized lattice by showing unique BODW phases that do not exist in the noninteracting model.

Single-walled carbon nanotubes (SWCNTs), with their high oscillator strength and large exciton binding energies, are becoming highly promising materials for the study of exciton-polaritons in the near infrared (NIR) at room temperature. However, current SWCNT-based strong coupling systems generally involve either lossy plasmonic nanostructures or bulk cavity-type resonators with poor compactability and tunability. Here, an ultracompact and low-loss all-dielectric metasurface combined with a phase change material (PCM) is proposed to enable active tuning of SWCNT-based exciton-polaritons in the NIR region. We demonstrate control of exciting quasibound states in the continuum (QBICs) by changing temperatures in the metasurface, which enables an active and flexible tuning exciton-QBIC interactions. We reveal that the dominant excitations of the magnetic dipole and high-order electric quadrupole at QBIC resonance play an important role in field enhancement, thus greatly facilitating the exciton-QBIC interaction. A quantum model fully describes the microscopic coupling dynamics, demonstrating a temperature-dependent sub- and superradiance of the polariton states. We also demonstrate a linear dependence of exciton-QBIC coupling strength on the SWCNT concentrations, which may provide an efficient approach for observing ultrastrong coupling phenomena. The PCM-assisted active tuning of exciton polaritons is expected to enable ultrafast and highly compact polariton devices at telecommunication wavelengths.

As a noble metal, gold is known to possess ultrahigh conductivity and is therefore widely used as a conductor in a variety of integrated circuits. Recently, a two-dimensional (2D) material made of a single atomic layer of gold has been successfully produced, called goldene, after the name of graphene. This immediately raises an interesting question of whether goldene is an excellent conductor, comparable to its three-dimensional counterpart. In the present paper, by performing the first principles calculations on the conductivity of goldene arising from electron-phonon (eph) scattering, we find that such a 2D gold has a very high intrinsic conductivity at room temperature, which is in the same order of magnitudes as that of a lightly doped graphene and much larger than other 2D materials accessible so far. This result suggests that goldene, instead of its 3D bulk, is an excellent conductor in future electronic devices based on 2D materials, Then, we make a detailed analysis of the individual roles of the electronic structure and e-ph scattering strength in contributing to the intrinsic electric and thermal conductivity of goldene. Finally, we establish a simple deformational potential model to describe the e-ph interaction, which works very well to reproduce the numerical result of first-principles calculation of the intrinsic conductivity of goldene.