We put forth a mechanism for enhancing the interlayer transport in cuprate superconductors, by optically driving plasmonic excitations along the $c$ axis with a frequency that is blue-detuned from the Higgs frequency. The plasmonic excitations induce a collective oscillation of the Higgs field which induces a parametric enhancement of the superconducting response, as we demonstrate with a minimal analytical model. Furthermore, we perform simulations of a particle-hole symmetric $U\left(1\right)$ lattice gauge theory and find good agreement with our analytical prediction. We map out the renormalization of the interlayer coupling as a function of the parameters of the optical field and demonstrate that the Higgs mode mediated enhancement can be larger than $50\%$.

Whereas identification of Cooper-pair symmetry is the first and crucial step in the investigation of unconventional superconductors, only a few have been established so far because of their own difficulties. To solve this problem, we develop a theory for identification of pairing symmetry using knowledge of topological superconductivity. Establishing the multipole theory of emergent Majorana fermions in time-reversal-invariant topological superconductors, we discover a one-to-one correspondence between the electromagnetic response of Majorana fermions and Cooper-pair symmetry. The emergent Majorana fermions host magnetic structures that share the same irreducible representation with Cooper pairs under crystalline symmetry. We furthermore reveal that Majorana fermions in high-spin or nonsymmorphic superconductors may exhibit magnetic octupole responses, which give a direct evidence of these exotic superconducting states. Electric responses of multiple Majorana Kramers pairs are also clarified. Our theory provides the fundamentals for identification of unconventional Cooper pairings through surface-spin-sensitive measurements as well as that for manipulation of Majorana fermions by external electromagnetic fields.

Majorana zero modes in a superconductor-semiconductor nanowire have been extensively studied during the past decade. Disorder remains a serious problem, preventing the definitive observation of topological Majorana bound states. Thus, it is worthwhile to revisit the simple model, the Kitaev chain, and study the effects of weak and strong disorder on the Kitaev chain. By comparing the role of disorder in a Kitaev chain with that in a nanowire, we find that disorder affects both systems but in a nonuniversal manner. In general, disorder has a much stronger effect on the nanowire than the Kitaev chain, particularly for weak to intermediate disorder. For strong disorder, both the Kitaev chain and nanowire manifest random featureless behavior due to universal Anderson localization. Only the vanishing and strong disorder regimes are thus universal, manifesting respectively topological superconductivity and Anderson localization, but the experimentally relevant intermediate disorder regime is nonuniversal with the details dependent on the disorder realization in the system.

We report a systematic study of the superconducting (SC) and normal-state anisotropy of ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{0.6}{\mathrm{Se}}_{0.4}$ single crystals with controlled amounts of excess Fe ($y$ = 0, 0.07, and 0.14). The SC state anisotropy ${\gamma }_H$ was obtained by measuring the upper critical fields under high magnetic fields over 50 T for both $H\parallel ab$ and $H\parallel c$. On the other hand, the normal state anisotropy ${\gamma }_{\rho }$ was obtained by measuring the resistivity with current flowing in the $ab$ plane (${\rho }_{ab}$) and along the $c$ axis (${\rho }_c$). To precisely measure ${\rho }_{ab}$ and ${\rho }_c$ in the same part of a specimen avoiding the variation dependent on pieces or parts, we adopt a new method using a microfabricated bridge with an additional neck part along the $c$ axis. The ${\gamma }_H$ decreases from a value dependent on the amount of excess Fe at $T_{\mathrm{c}}$ to a common value $\sim 1$ at 2 K. The different ${\gamma }_H$ at $T_{\mathrm{c}}$ ($\sim 1.5$ for $y$ = 0, and 2.5 for $y$ = 0.14) suggests that the anisotropy of effective mass $m_c^{*}/m_{ab}^{*}$ increases from $\sim 2.25$ ($y$ = 0) to 6.25 ($y$ = 0.14) with the excess Fe. The almost isotropic ${\gamma }_H$ at low temperatures is due to the strong spin paramagnetic effect at $H\parallel ab$. By contrast, the ${\gamma }_{\rho }$ shows a much larger value of $\sim 17$ ($y$ = 0) to $\sim 50$ ($y$ = 0.14) at the temperature just above $T_{\mathrm{c}}$. Combined the results of ${\gamma }_H$ and ${\gamma }_{\rho }$ near $T_{\mathrm{c}}$, we found out that the discrepant anisotropies between the SC and normal states originates from a large anisotropy of scattering time ${\tau }_{ab}/{\tau }_c\sim$ 7.8. The ${\tau }_{ab}/{\tau }_c$ is found to be independent of the excess Fe.

We investigate the entanglement entropy (EE) and charge fluctuations in models where the low-energy physics is governed by massless Dirac fermions. We focus on the response to flux insertion which, for the EE, is widely assumed to be universal, i.e., independent of the microscopic details. We provide an analytical derivation of the EE and charge fluctuations for the seminal example of graphene, using the dimensional reduction of its tight-binding model to the one-dimensional Su-Schrieffer-Heeger model. Our asymptotic expression for the EE matches the conformal field theory prediction. We show that the charge variance has the same asymptotic behavior, up to a constant prefactor. To check the validity of universality arguments, we numerically consider several models, with different geometries and numbers of Dirac cones, and either for strictly two-dimensional models or for a gapless surface mode of three-dimensional topological insulators. We also show that the flux response does not depend on the entangling surface geometry as long as it encloses the flux. Finally we consider the universal corner contributions to the EE. We show that in the presence of corners, the Kitaev-Preskill subtraction scheme provides nonuniversal, geometry-dependent results.

We report magnetic and electrical properties for single crystals of $\mathrm{Nd}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ and $\mathrm{Sm}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$. They crystallize into a structure that has distorted, Mn-based kagome lattices, compared to the pristine kagome lattices in heavy rare-earth-bearing $\mathrm{R}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$ compounds. They are high-temperature ferromagnets of which the R moment is parallel with the Mn moment. We observed a large intrinsic anomalous Hall effect (AHE) that is comparable to the ferrimagnetic, heavy-R siblings in a wide range of temperature. We conclude that their intrinsic AHE is stemming from the Mn-based kagome lattice, just as in the heavy $\mathrm{R}{\mathrm{Mn}}_6{\mathrm{Sn}}_6$.

We study the coherent properties of fully screened and overscreened Kondo effects based on the Nozieres local Fermi-liquid theory and Affleck-Ludwig boundary conformal-field-theory approach, respectively. Coherent transports through an $\mathrm{SU}\left(N\right)$ generalization of fully screened and the multi-$\mathcal{K}$-channel overscreened Kondo impurities at and beyond the particle-hole (PH) symmetric point are thoroughly investigated. We report distinctive Fermi-liquid coefficients characterizing the finite temperature correction to the transmission phase shift and normalized visibility in fully screened Kondo regime, which can be measured with the existing experimental setups. Our work equally uncovers the significance of temperature correction to the transmission phase shift and visibility in non-Fermi-liquid regime associated with the PH asymmetric overscreened Kondo effects with an arbitrary number of conduction channels $\mathcal{K}$. We propose viable roots of verifying our predictions in connection to the recent experiments, in particular with the experiments studying highly symmetric forms of fully screened Kondo effects and two-channel overscreened Kondo effects realized in quantum dot nanostructures.

Studies of periodically driven one-dimensional many-body systems have advanced our understanding of complex systems and stimulated promising developments in quantum simulation. It is hence of interest to go one step further, by investigating the topological and dynamical aspects of periodically driven spin ladders as clean quasi-one-dimensional systems with spin-spin interaction in the rung direction. Specifically, we find that such systems display subharmonic magnetization dynamics reminiscent to that of discrete time crystals (DTCs) at finite system sizes. Through the use of generalized Jordan-Wigner transformation, this feature can be attributed to the presence of corner Majorana $\pi$ modes (MPMs), which are of topological origin, in the systems' equivalent Majorana lattice. Special emphasis is placed on how the coupling in the rung direction of the ladder prevents degeneracy from occurring between states differing by a single spin excitation, thus preserving the MPM-induced $\pi /T$ quasienergy spacing of the Floquet eigenstates in the presence of parameter imperfection. This feature, which is absent in their strict one-dimensional counterparts, may yield fascinating consequences in future studies of higher dimensional Floquet many-body systems.

It is commonly believed that electron correlations would open up a quasiparticle gap in semiconductors. Contrary to this intuitive expectation, here we reveal that phosphorene quantum dots (PQDs) may exhibit just the opposite effect. By using a configuration interaction approach beyond the conventional double-excitation scheme, quasiparticle energies are calculated for hexagonal and rectangular PQDs in various dielectric environments. For the hexagonal PQD with a nominal gap of 2.26 eV, it is found that the quasiparticle shift decreases by more than 500 meV and eventually becomes negative when the effective dielectric constant is reduced from 20.0 to 5.0. For other trapezoidal, triangular, and rectangular PQDs, the quasiparticle shift exhibits a similar amount of decrement after the same change in the dielectric environment. Furthermore, the calculation by adopting the Rytova-Keldysh potential, which may be more suitable to describe two-dimensional screening, also shows a very similar result, although with smaller decrement of the quasiparticle shift. The origin of this anomalous quasiparticle shift is believed to be related to the long-range electron-electron interactions in the distinctive lattice structure of PQDs, as a similar phenomenon has never been found in graphene quantum dots.

We examine from first principles the flexoelectric properties of phosphorene nanoribbons under mechanical bending along armchair and zigzag directions. In both cases we find that the radial polarization depends linearly on the strain gradient. The flexoelectricity along the armchair direction is over 40% larger than along the zigzag direction. The obtained flexoelectric coefficients of phosphorene are four orders of magnitude larger than those of graphene and comparable to transition metal dichalcogenides. Analysis of charge density shows that the flexoelectricity mainly arises from the $p_z$ orbitals of phosphorus atoms. The electron mobilities in bent phosphorene can be enhanced by over 60% along the armchair direction, which is significantly higher than previous reports of mobility tuned by uniaxial strain. Our results indicate phosphorene is a candidate for a two-dimensional material applicable in flexible-electronic devices.

Since the discovery of the Harper-Hofstadter model, it has been known that condensed matter systems with periodic modulations can be promoted to nontrivial topological states with emergent gauge fields in higher dimensions. In this paper, we develop a general procedure to compute the gauge fields in higher dimensions associated to low-dimensional systems with periodic (charge- and spin-) density wave modulations. We construct two-dimensional (2D) models with modulations that can be promoted to higher-order topological phases with U(1) and SU(2) gauge fields in 3D. Corner modes in our 2D models can be pumped by adiabatic sliding of the phase of the modulation, yielding hinge modes in the promoted models. We also examine a 3D Weyl semimetal (WSM) gapped by charge-density wave (CDW) order, possessing quantum anomalous Hall surface states. We show that this 3D system is equivalent to a 4D nodal line system gapped by a U(1) gauge field with a nonzero second Chern number. We explain the recently identified interpolation between inversion-symmetry protected phases of the 3D WSM gapped by CDWs using the corresponding 4D theory. Our results can extend the search for (higher-order) topological states in higher dimensions to density wave systems.

The interlayer antiferromagnetic coupling hinders the observation of quantum anomalous Hall effect in magnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$. We demonstrate that interlayer ferromagnetism can be established by utilizing the $p$-doping method in $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ multilayers. In two septuple layers system, the interlayer ferromagnetic coupling appears by doping nonmagnetic elements (e.g., N, P, As, Na, Mg, K, and Ca), due to the redistribution of orbital occupations of Mn. We further find that Mg and Ca elements are the most suitable candidates because of their low formation energy. Although, the $p$-doped two septuple layers exhibit topologically trivial band structure, the increase of layer thickness to three (four) septuple layers with Ca (Mg) dopants leads to the formation of the quantum anomalous Hall effect. Our proposed $p$-doping strategy without introducing additional magnetic disorder not only makes $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ become an ideal platform to realize the high-temperature quantum anomalous Hall effect without external magnetic field, but also can compensate the electrons from the intrinsic $n$-type defects in $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$.

Plasmonic tapered grooves have been proven to be good candidates for the excitation of gap surface plasmons (GSPs), surface plasmons trapped vertically inside a metallic tapered groove or slit. GSPs have attracted tremendous interest due to their unique properties of concentrating light in nanosized gaps with significant field enhancement, thus offering potential applications such as ultracompact nanocircuits, broadband light absorbers, and plasmonic sensors. In this paper, we focus on GSPs supported by periodic arrays of narrow convex grooves and study in detail their properties by using visible-near-infrared (VIS-NIR) spectroscopy. We identify strong second- and third-order GSP modes excited in ultrasharp convex grooves. The dependence of GSP resonances on the groove profile is analyzed with the help of detailed full-wave simulations, revealing the fact that an ultrasharp, but finite, gap exists at the groove bottom and plays a crucial role in determining both the GSP resonance positions and the nanofocusing capability with much improved field enhancement inside the grooves. Spectral shifts of the observed GSP resonances relative to the simulation results are found in a shorter wavelength range and are qualitatively explained as nonlocal effects originating from the nonclassical microscopic behavior of local currents and charges at imperfect interfaces. Utilizing such strong and distinguishable GSP resonance line shapes in an otherwise flat reflectivity spectral baseline, we experimentally demonstrate the capability of convex groove arrays to perform dual-band refractive index sensing in the VIS-NIR range.

We present a theoretical study of two spinless fermion wires coupled to a three-dimensional semiconducting substrate. We develop a mapping of wires and substrate onto a system of two coupled two-dimensional ladder lattices using a block Lanczos algorithm. We then approximate the resulting system by narrow ladder models, which can be investigated using the density-matrix renormalization group method. In the absence of any direct wire-wire hopping we find that the substrate can mediate an effective wire-wire coupling so that the wires could form an effective two-leg ladder with a Mott charge-density-wave insulating ground state for arbitrarily small nearest-neighbor repulsion. In other cases the wires remain effectively uncoupled even for strong wire-substrate hybridizations leading to the possible stabilization of the Luttinger liquid phase at finite nearest-neighbor repulsion as found previously for single wires on substrates. These investigations show that it may be difficult to determine under which conditions the physics of correlated one-dimensional electrons can be realized in arrays of atomic wires on semiconducting substrates because they seem to depend on the model (and consequently material) particulars.

We have made systematic nuclear magnetic resonance measurements for ${}^3\mathrm{He}$ fluid formed in ${}^4\mathrm{He}$-coated nanochannels including the quantum-mechanically one-dimensional (1D) region, which is a possible candidate for a different type of Tomonaga-Luttinger (TL) liquid. Corresponding to the previous heat capacity measurements, a dimensional crossover into the 1D state at low temperatures was observed by the susceptibility. In the 1D region, increases of the spin-spin relaxation time inversely proportional to temperature were found, which is apparently similar to $T$-linear electron spin resonance linewidths for 1D spin chains. While the relevance for TL liquids is still controversial since the behavior is observed in both degenerate and nondegenerate fluids, increases of the spin-spin relaxation time were confirmed to be observed only when 1D conditions for ${}^3\mathrm{He}$ fluid are satisfied, indicating that it is a characteristic property of the 1D ${}^3\mathrm{He}$ fluid.

The spin-polarized two-dimensional electron gas (2DEG) at oxide interfaces is an emerging physical phenomenon, which is technologically important for potential device applications. However, most previous relevant studies only focused on the creation and characterization of the spin-polarized 2DEG. To push forward the device applications, the control of a spin-polarized 2DEG by electric field is an important step. Here, a model system based on antiferromagnetic and ferroelectric perovskites, i.e., the ${\mathrm{YTiO}}_3/{\mathrm{PbTiO}}_3$ superlattice, is designed to manipulate the spin-polarized 2DEG. By switching the direction of polarization, the spin-polarized 2DEG can be effectively tuned for both symmetric interfaces and asymmetric polar interfaces.

For machine learning of interatomic potentials a scalable sparse Gaussian process regression formalism is introduced with a data-efficient on-the-fly adaptive sampling algorithm. With this approach, the computational cost is effectively reduced to those of the Bayesian linear regression methods while maintaining the appealing characteristics of the exact Gaussian process regression. As a showcase, experimental melting and glass-crystallization temperatures are reproduced for ${\mathrm{Li}}_7{\mathrm{P}}_3{\mathrm{S}}_{11}$, Li diffusivity is simulated, and an unchartered phase is revealed with much lower Li diffusivity which should be circumvented.

Bragg coherent diffraction imaging has the potential to provide significant insight into the structure-properties relationship for crystalline materials by imaging, with nanoscale resolution, three-dimensional strain fields within individual grains and nanoparticles. The capability of present-day synchrotrons to locate and measure a multiplicity of Bragg reflections from a single grain makes it possible to recover the full strain tensor with nanometer resolution. Recent methods for coupling reconstructions from several peaks to determine the strain tensor have been developed and applied to synthetic data, but have not been applied to experimental data. Here, using a coupled genetic reconstruction algorithm, we reconstruct an experimental data set and demonstrate improvements in the ability to resolve vector-valued displacement fields internal to the particle as compared to what is achieved with a noncoupled approach. The coupled approach developed in this work was also validated on simulated data sets. In both simulated and experimental data, reconstructions from our coupled Bragg peak algorithm show improvements over the noncoupled independent reconstruction method of $5\%$ in terms of accuracy and $53\%$ in terms of consistency.

A pseudospin model for description of the influence of the electric field, confined to the plane of sublattice polarization, on the two-dimensional squaric acid antiferroelectrics is developed. The temperature–electric field $T\text{−}{E}_1$ phase diagram is constructed. Most of the field-induced transitions are found to be associated with polarization rotation. The system behavior is best characterized by the introduced noncollinearity angle $\theta$, which is the angle between the sublattice polarizations. The collinear ferroelectric phase and two noncollinear phases with almost antiparallel and perpendicular polarizations of the sublattices are identified. The diagram also contains the crossover region, where the noncollinearity angle varies continuously between, nominally, ${180}^{\circ }$ and ${90}^{\circ }$. The first- and second-order transition lines, supercritical lines, critical and two bicritical end points, and the tricritical point on the $T\text{−}{E}_1$ phase diagram are detected. Three plateaus are observed in the field dependence of the net polarization at low temperatures.

Noncentrosymmetric PrAlGe is considered to be a magnetic Weyl semimetal that breaks both time-reversal and inversion symmetry, in which the Weyl nodes can be moved by magnetization. However, the magnetic interaction of this system remains poorly understood. In this work we perform a systematical investigation on magnetic properties and critical behaviors in single-crystal PrAlGe. The temperature, field, and angle dependence of magnetization reveal a strong magnetic anisotropy along the $c$ axis and absolute isotropic characteristic in the $ab$ plane. By fitting to the magnetic entropy change $\left[\mathrm{\Delta }{S}_M(T,H)\right]$ around the critical temperature $T_C=16$ K, critical exponents $\beta =0.136\left(6\right), \gamma =1.801\left(7\right)$, and $\delta =14.2\left(6\right)$ are obtained. Based on the obtained critical exponents, $\mathrm{\Delta }{S}_M(T,H)$ and $M(T,H)$ curves are scaled into universality curves under the framework of universality principle. The critical behaviors and exponents suggest that the magnetic interaction in PrAlGe is of a two-dimensional Ising type, revealing a uniaxial magnetic interaction along the $c$ axis. However, the ordering moments are tilted from the $c$ axis, which causes antiferromagnetism in the $ab$ plane. The clarification of the magnetic interaction and magnetic structure is of significance for unveiling its interplay with topological properties in this system.