We propose a different scheme for achieving valley splitting using an electric field in the materials with inversion symmetry but without time-reversal symmetry, and apply this scheme to two-dimensional transition metal trihalides $\text{V}{X}_3$ ($X=\text{Cl}$, Br). Based on ab initio calculations and the tight-binding model, we find few layer $\text{V}{X}_3$ can be readily tuned from intrinsic quantum anomalous Hall insulators to quantum valley Hall insulators by external electric fields. Especially, the electric-field-induced valley splitting of the bilayer $\text{V}{X}_3$ is extremely large, about two orders of magnitude higher than that induced by a magnetic field in the state-of-the-art valleytronic materials (e.g., ${\mathrm{MoS}}_2$ and ${\mathrm{WSe}}_2$). We further reveal rich topological phases of few layer $\text{V}{X}_3$ and valley-polarized states at the phase boundary. These findings may motivate further topology and valleytronics related researches in low-dimensional transition metal compounds.

We present detailed measurements of the Casimir-Lifshitz force between two gold surfaces, performed in both gas (nitrogen) and liquid (ethanol) environments with the same apparatus and on the same spot of the sample. Furthermore, we study the role of double-layer forces in the liquid, and we show that these electrostatic effects are important. The latter contributions are subtracted to recover the genuine Casimir force, and the experimental results are compared with calculations using Lifshitz theory. Our measurements demonstrate that carefully accounting for the actual optical properties of the surfaces is necessary for an accurate comparison with the Lifshitz theory predictions at distances smaller than 200 nm.

We use the Hamiltonian theory developed by Shankar and Murthy to study a quantum Hall system in a tilted magnetic field. With a finite width of the system in the $z$ direction, the parallel component of the magnetic field introduces anisotropy into the effective two-dimensional interactions. The effects of such anisotropy can be effectively captured by the recently proposed generalized pseudopotentials. We find that the off-diagonal components of the pseudopotentials lead to mixing of composite fermions Landau levels, which is a perturbation to the picture of $p$ filled Landau levels in composite-fermion theory. By changing the internal geometry of the composite fermions, such a perturbation can be minimized and one can find the corresponding activation gaps for different tilting angles, and we calculate the associated optimal metric. Our results show that the activation gap is remarkably robust against the in-plane magnetic field in the lowest and first Landau levels.

Structure-property relationships have always been guiding principles for materials discovery. Here we explore the relationships to discover two-dimensional (2D) materials with the goal of identifying 2D magnetic semiconductors for spintronics applications. In particular, we report a density functional theory + $U$ study of single-layer antiferromagnetic (AFM) semiconductor ${\mathrm{CoS}}_2$ with the pentagonal structure forming the so-called Cairo tessellation. We find that this single-layer magnet exhibits an indirect band gap of 1.06 eV with electron and hole effective masses of 0.52 and 1.93 $m_0$, respectively, which may lead to relatively high electron mobility. The hybrid density functional theory calculations correct the band gap to 2.24 eV. We also compute the magnetocrystalline anisotropy energy (MAE), showing that the easy axis of the AFM ordering is along the $b$ axis with a sizable MAE of 153 $\mu \mathrm{eV}$ per Co ion. We further calculate the magnon frequencies at different spin-spiral vectors, based on which we estimate the $\mathrm{N}\acute{\text{e}}\mathrm{el}$ temperatures to be 20.4 and 13.3 K using the mean field and random phase approximations, respectively. We then apply biaxial strains to tune the band gap of single-layer pentagonal ${\mathrm{CoS}}_2$. We find that the energy difference between the ferromagnetic and AFM structures strongly depends on the biaxial strain, but the ground state remains the AFM ordering. Although the low critical temperature prohibits the magnetic applications of single-layer pentagonal ${\mathrm{CoS}}_2$ at room temperature, the excellent electrical properties may find single-layer semiconductor applications in optoelectronic nanodevices.

In this work we study planar metasurfaces comprised of two layers of hexagonal arrays of circular metal patches. This two-layer geometry supports a bound surface wave that propagates along the $x$ direction. In our study, each metasurface is infinitely periodic in one direction ($x$) but only a few periods wide in the orthogonal direction ($y$). Through experiments and modeling we find evidence of a localized edge mode whose existence is dependent on the relative alignment of the two layers. This edge mode is used to guide the propagation of electromagnetic energy around both triangular and hexagonal shapes.

The topological Kondo effect arises when conduction electrons in metallic leads are coupled to a mesoscopic superconducting island with Majorana fermions. Working with its minimal setup, we study the lead electron local tunneling density of states in its thermally smeared form motivated by scanning tunneling microscopy, focusing on the component ${\rho }_{2k_F}$ oscillating at twice the Fermi wave number. As a function of temperature $T$ and at zero bias, we find that the amplitude of ${\rho }_{2k_F}$ is nonmonotonic, whereby with decreasing $T$ an exponential thermal-length-controlled increase, potentially through an intermediate Kondo logarithm, crosses over to a $T^{1/3}$ decay. The Kondo logarithm is present only for tip-junction distances sufficiently smaller than the Kondo length, thus providing information on the Kondo screening cloud. The low-temperature decay indicates non-Fermi-liquid scattering, in particular the complete suppression of single-particle scattering at the topological Kondo fixed point. For temperatures much below the Kondo temperature, we find that the ${\rho }_{2k_F}$ amplitude can be described as a universal scaling function indicative of strong correlations. In a more general context, our considerations point towards the utility of ${\rho }_{2k_F}$ in studying quantum impurity systems, including extracting information about the single-particle scattering matrix.

Using a combination of density functional theory and molecular dynamics simulations, this paper reveals the atomistic origin of diameter-dependent extreme mechanical behavior of [111] 3C-SiC nanowires obtained from an energy-based framework. Our results suggest that heterogeneity in atomic stress and variations in diameter-dependent potential-energy density have a profound impact on extreme mechanical properties in the nanowires. The heterogeneity in stress evolves from the nonuniform bond lengths mediated by low coordinated surface atoms—and it penetrates the entire cross section in thinner nanowires and constitutes the atomistic basis for their large reduction in fracture strain, toughness, and strength. Although stress heterogeneity is substantially higher in ultrathin nanowires, its intensity drops and saturates rapidly in larger nanowires following a nonlinear dependence on diameter. The maximum stress heterogeneity in a cross section localizes crack nucleation at the core in ultrathin nanowires but near the surface in larger nanowires. Moreover results show that stiffness, toughness, strength, and fracture strain of the nanowires increase nonlinearly with increasing diameter and saturate at a lower value compared to bulk SiC. In addition to resolving wide discrepancies in the reported values of the first-order elastic modulus in SiC nanowires, the findings highlight heterogeneity as a critical factor for inducing diameter-dependent extreme mechanical behavior in brittle nanowires.

Using a machine-learning accelerated crystal structural search, and ab initio calculations together with high-pressure Raman measurements, we study the crystal and electronic structures of bismuth iodide (BiI) systematically up to 50 GPa. We find that the ambient $C2/m$ phase ($\beta -\mathrm{B}{\mathrm{i}}_4{\mathrm{I}}_4$) transforms across a tetragonal $P{4}_2/mmc$ phase at 8.5 GPa and finally to a hexagonal $P{6}_3/mmc$ phase at 28.2 GPa. Our high-pressure Raman experiments identify a phase transition at around 8.6 GPa, and the experimental Raman modes evolution agrees with our calculations reasonably. Band-structures calculations suggest the BiI system undergoes a pressure-induced topological phase transition from a topological metal ($P{4}_2/mmc$ phase) to a trivial metal (the $P{6}_3/mmc$ phase). Our electron-phonon coupling calculations show both the $P{4}_2/mmc$ and $P{6}_3/mmc$ phases are superconductors and the estimated superconducting critical temperature agrees with previous measurements. Our study shows the previously reported pressure-induced superconductivity in BiI should originate from structure phase transitions.

The basic and the charge density wave (CDW) structures of the monoclinic ${\mathrm{NbS}}_3$-II polymorph were studied by synchrotron x-ray diffraction, ab initio calculations, simulation of electron diffraction patterns, and by atomic-resolution transmission electron and low-temperature scanning tunneling microscopies. It is confirmed that the basic structure belongs to the space group $P{2}_1/m$ and is described with a unit cell, formed of four pairs of symmetry-related trigonal prismatic (TP) columns [$a_0=0.96509\left(8\right)\mathrm{nm}$, $b_0=0.33459\left(2\right)\mathrm{nm}$, $c_0=1.9850\left(1\right)\mathrm{nm}$, and ${\beta }_0=110.695{\left(4\right)}^{\circ }$]. The incommensurate components of the two CDWs, $\overrightarrow{q_1}=(0,0.298,0)$ and $\overrightarrow{q_2}=(0,0.352,0)$, are related as $q_{1_b}+2q_{2_b}\approx 1$. Both CDWs form their own modulation patterns with unit cells ($a_{\mathrm{m}}=2a_0$, $b_{\mathrm{m}}=b_0/q_{\mathrm{jb}}$, $c_{\mathrm{m}}=c_0$, ${\beta }_m={\beta }_0$) and are ordered along adjacent isosceles TP columns either pairwise or with both columns modulated by either the $\overrightarrow{q_1}$ or $\overrightarrow{q_2}$ CDW only. The CDWs are ordered according to one of the two possible modulation pattern space groups, $Cm$ or $C2/m$. If considered as long-period commensurate, the entire modulated structure with both CDWs included is described within experimental error with an enlarged unit cell ($a=2a_0$, $b=37b_0$, $c=c_0$, and $\beta ={\beta }_0$) and with all atoms displaced from their average positions in accord with the specified modulation pattern.

We investigate the minus-sign problem that afflicts quantum Monte Carlo (QMC) simulations of frustrated quantum spin systems, focusing on spin $S=1/2$, two spatial dimensions, and the extended Shastry-Sutherland model. We show that formulating the Hamiltonian in the diagonal dimer basis leads to a sign problem that becomes negligible at low temperatures for small and intermediate values of the ratio of the inter- and intradimer couplings. This is a consequence of the fact that the product state of dimer singlets is the exact ground state both of the extended Shastry-Sutherland model and of a corresponding “sign-problem-free” model, obtained by changing the signs of all positive off-diagonal matrix elements in the dimer basis. By exploiting this insight, we map the sign problem throughout the extended parameter space from the Shastry-Sutherland to the fully frustrated bilayer model and compare it with the phase diagram computed by tensor-network methods. We use QMC to compute with high accuracy the temperature dependence of the magnetic specific heat and susceptibility of the Shastry-Sutherland model for large systems up to a coupling ratio of 0.526(1) and down to zero temperature. For larger coupling ratios, our QMC results assist us in benchmarking the evolution of the thermodynamic properties by systematic comparison with exact diagonalization calculations and interpolated high-temperature series expansions.

The antiferromagnetic Ising model on the pyrochlore lattice exhibits a quantum phase transition in an applied transverse field from the low-field quantum spin-ice phase to the high-field polarized regime. Recent field-theoretical analysis and series expansion results indicate this to be a discontinuous, first-order transition. Here, we explore this transition using quantum Monte Carlo simulations in order to assess this scenario and study the thermodynamic properties in the vicinity of the quantum phase transition. For this purpose, we also consider several variants of extended cluster-update schemes for the transverse-field Ising antiferromagnet on frustrated lattices and compare their performance to the conventional bond-based algorithm for the transverse-field Ising model on the pyrochlore lattice.

We theoretically demonstrate that the presence of Dzyaloshinskii-Moriya interaction (DMI) can lead to enhancement of the spin-motive force (SMF) arising due to field-induced ferromagnetic domain-wall motion. A SMF refers to an electric voltage induced by dynamical magnetic textures, which reflects the temporal and spatial variations of the magnetization. A DMI can introduce extra spatial rotation of the magnetization in the domain-wall region, which turns out to cause the enhancement of the SMF. We derive an expression for the SMF, and examine the field and DMI dependencies of the SMF. We find that the SMF can be amplified by up to an order of magnitude in the low field regime, where the external field is lower than the so-called Walker breakdown field.

We present a comparison of the characteristics of magnetic domain walls in an atomic monolayer of Co on Pt(111) and a Ni/Fe atomic bilayer on Ir(111) based on spin-polarized scanning tunneling microscopy measurements. In both cases, the films exhibit a roughly triangular dislocation line pattern created by epitaxial strain relief, as well as out-of-plane ferromagnetic order. Domains with opposite magnetization are separated by domain walls with a unique rotational sense, demonstrating the important role of the Dzyaloshinskii-Moriya interaction induced by the Co/Pt and Fe/Ir interfaces. The domain walls in Co/Pt(111) are straight and usually found in geometrical constrictions of the film, where they can minimize their length. In contrast, the domain walls in Ni/Fe/Ir(111) follow complicated paths, which can be correlated to the structural triangular pattern. The comparison between the two systems shows that the structural patterns, despite their similarity, have a different impact on the domain walls. In the Co/Pt(111) case, the magnetic state is not influenced by the dislocation line network, in contrast to the Ni/Fe/Ir(111) system in which the formation of the walls is favored at specific positions of the structural pattern.

Micron-sized superconducting interference devices ($\mu$-SQUIDs) based on constrictions optimized for minimizing thermal runaway are shown to exhibit voltage oscillations with applied magnetic flux despite their hysteretic behavior. We explain this remarkable feature by a significant supercurrent contribution surviving deep into the resistive state due to efficient heat evacuation. A resistively shunted junction model, complemented by a thermal balance determining the amplitude of the critical current, describes well all experimental observations, including the flux modulation of the (dynamic) retrapping current and voltage, by introducing a single dimensionless parameter. Compared to the nonhysteretic regime, this regime extends the voltage readout mode in a given $\mu$-SQUID to further lower temperatures. More importantly, the quantitative modeling of this regime incorporating both heating and phase dynamics paves the way for further optimization of $\mu$-SQUIDs for nanomagnetism.

Predictive theory to geometrically engineer devices and materials in continuum systems to have desired topological-like effects is developed here by bridging the gap between quantum and continuum mechanical descriptions. A structured elastic plate, a bosoniclike system in the language of quantum mechanics, is shown to exhibit topological valley modes despite the system having no direct physical connection to quantum effects. We emphasize a predictive, first-principles, approach, the strength of which is demonstrated by the ability to design well-defined broadband edge states, resistant to backscatter, using geometric differences; the mechanism underlying energy transfer around gentle and sharp corners is described. Using perturbation methods and group theory, several distinct cases of symmetry-induced Dirac cones, which when gapped yield nontrivial band gaps, are identified and classified. The propagative behavior of the edge states around gentle or sharp bends depends strongly upon the symmetry class of the bulk media and we illustrate this via numerical simulations.

We propose a numerical technique based on a combination of short-iterative Lanczos and exact diagonalization methods, suitable for simulating the time evolution of the reduced density matrix of a single qubit interacting with an environment. By choosing a mode discretization method and a flexible bath states truncation scheme, we are able to include in the physical description multiple-excitation processes, beyond weak coupling and Markov approximations. We apply our technique to the simulation of three different model Hamiltonians, which are relevant in the field of adiabatic quantum computation. We compare our results with those obtained on the basis of the widely used Lindblad master equation, as well as with well-known exact and approximated approaches. We show that our method is able to recover the thermodynamic behavior of the qubit-bath system, beyond the Born-Markov approximation. Finally, we show that even in the case of the adiabatic quantum annealing of a single qubit the bath can be beneficial in reaching the reduced system ground state.

We theoretically investigate the microwave-active resonance modes of a skyrmion crystal on a thin-plate specimen under application of an external magnetic field that is inclined from the perpendicular direction to the skyrmion plane. In addition to the well-known breathing mode and two rotation modes, we find novel resonance modes that can be regarded as combinations of the breathing and rotation modes. Motivated by the previous theoretical work of [Wang et al., Phys. Rev. B 92, 020403(R) (2015)], which demonstrated skyrmion propagation driven by breathing-mode excitation under an inclined magnetic field, we investigate the propagation of a skyrmion crystal driven by these resonance modes using micromagnetic simulations. We find that the direction and velocity of the propagation vary depending on the excited mode. In addition, it is found that a mode with a dominant counterclockwise-rotation component drives much faster propagation of the skyrmion crystal than the previously studied breathing mode. Our findings enable us to perform efficient manipulation of skyrmions in nanometer-scale devices or in magnetic materials with strong uniaxial magnetic anisotropy such as ${\mathrm{GaV}}_4{\mathrm{S}}_4$ and ${\mathrm{GaV}}_4{\mathrm{Se}}_4$, using microwave irradiation.

We study quantum phase transitions between competing orders in one-dimensional spin systems. We focus on systems that can be mapped to a dual-field double sine-Gordon model as a bosonized effective field theory. This model contains two pinning potential terms of dual fields that stabilize competing orders and allows different types of quantum phase transition to happen between two ordered phases. At the transition point, elementary excitations change from the topological soliton of one of the dual fields to that of the other, thus it can be characterized as a topological transition. We compute the dynamical susceptibilities and the entanglement entropy, which gives us access to the central charge, of the system using a numerical technique of infinite time-evolving block decimation and characterize the universality class of the transition as well as the nature of the order in each phase. The possible realizations of such transitions in experimental systems both for condensed matter and cold atomic gases are also discussed.

Motivated by the discovery of superconductivity in the two-leg, quasi-one-dimensional ladder compound ${\mathrm{BaFe}}_2{\mathrm{S}}_3$, we present a renormalization group study of electrons moving on a two-leg, two-orbital ladder, subjected to Hubbard repulsion $U$ and Hund's coupling $J$. In our calculations, we adopt tight-binding parameters obtained from ab initio studies on this material. At incommensurate filling, the long-wavelength analysis displays four phases as a function of $0\le J/U<1$. We show that a fully gapped superconductor is stabilized at sufficiently large Hund's coupling, the relative phases at the three Fermi points are “$+,-,-$”. By contrast, when the system is tuned to half-filling, umklapp scattering gives rise to Mott insulating phases. We discuss the general implications of our study for the broad class of iron-based superconductors.

We employ the functional renormalization group to study the effects of phonon-assisted tunneling on the nonequilibrium steady-state transport through a single level molecular quantum dot coupled to electronic leads. Within the framework of the spinless Anderson-Holstein model we focus on small to intermediate electron-phonon couplings and we explore the evolution from the adiabatic to the antiadiabatic limit and also from the low-temperature nonperturbative regime to the high-temperature perturbative one. We identify the phononic signatures in the bias-voltage dependence of the electrical current and the differential conductance. Considering a temperature gradient between the electronic leads we further investigate the interplay between the transport of charge and heat. Within the linear response regime we compare the temperature dependence of various thermoelectric coefficients to our earlier results obtained within the numerical renormalization group [Phys. Rev. B 96, 195156 (2017)]. Beyond the linear response regime in the context of thermoelectric generators we discuss the influence of molecular vibrations on the output power and the efficiency. We find that in the antiadiabatic limit the thermoelectric efficiency can be significantly enhanced.