The interplay of symmetry and topological order leads to a variety of distinct phases of matter, the symmetry enriched topological (SET) phases. Here we discuss physical observables that distinguish different SETs in the context of ${\mathbb{Z}}_2$ quantum spin liquids with SU(2) spin rotation invariance. We focus on the cylinder geometry, and show that ground-state quantum numbers for different topological sectors are robust invariants which can be used to identify the SET phase. More generally, these invariants are related to 1D symmetry protected topological phases when viewing the cylinder geometry as a 1D spin chain. In particular, we show that the kagome spin liquid SET can be determined by measurements on one ground state, by wrapping the kagome in a few different ways on the cylinder. In addition to guiding numerical studies, this approach provides a transparent way to connect bosonic and fermionic mean-field theories of spin liquids. When fusing quasiparticles, it correctly predicts nontrivial phase factors for combining their space group quantum numbers.

Small polaron formation in transition metal oxides, like the prototypical material rutile ${\mathrm{TiO}}_2$, remains a puzzle and a challenge to simple theoretical treatment. In our combined experimental and theoretical study, we examine this problem using Raman spectroscopy of photoexcited samples and real-time time-dependent density functional theory (RT-TDDFT), which employs Ehrenfest dynamics to couple the electronic and ionic subsystems. We observe experimentally the unexpected stiffening of the $A_{1g}$ phonon mode under UV illumination and provide a theoretical explanation for this effect. Our analysis also reveals a possible reason for the observed anomalous temperature dependence of the Hall mobility. Small polaron formation in rutile ${\mathrm{TiO}}_2$ is a strongly nonadiabatic process and is adequately described by Ehrenfest dynamics at time scales of polaron formation.

Hyperbolic metamaterials (HMMs) are the cornerstone of the hyperlens, which brings the superresolution effect from the near-field to the far-field zone. For effective application of the hyperlens it should operate in the so-called canalization regime, where the phase advancement of the propagating fields is maximally suppressed and thus field broadening is minimized. For conventional hyperlenses it is relatively straightforward to achieve canalization by tuning the anisotropic permittivity tensor. However, for a dark-field hyperlens designed to image weak scatterers by filtering out background radiation (dark-field regime) this approach is not viable because design requirements for such filtering and elimination of phase advancement i.e., canalization, are mutually exclusive. Here we propose the use of magnetic ($\mu$-positive and -negative) HMMs to achieve phase cancellation at the output equivalent to the performance of a HMM in the canalized regime. The proposed structure offers additional flexibility over simple HMMs in tuning light propagation. We show that in this “pseudocanalizing” configuration the quality of an image is comparable to a conventional hyperlens, while the desired filtering of the incident illumination associated with the dark-field hyperlens is preserved.

Fabrication of highly mobile spin-polarized two-dimensional electron gas (2DEG) is crucially important for both fundamental and applied research. Usually, spin polarization appears below 10 K for the 2DEG of $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ interface, stemming from the magnetic ordering of $\mathrm{T}{\mathrm{i}}^{3+}$ ions with the mediation of itinerant electrons. Herein, we report a magnetic 2DEG at a $\mathrm{L}{\mathrm{a}}_{7/8}\mathrm{S}{\mathrm{r}}_{1/8}\mathrm{Mn}{\mathrm{O}}_3$-buffered $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ interface, which simultaneously shows electrically tunable anomalous Hall effect and high conductivity. The spin-polarized temperature for the 2DEG is promoted to 30 K while the mobility remains high. The magnetism likely results from a gradient manganese interdiffusion into $\mathrm{SrTi}{\mathrm{O}}_3$. The present work demonstrates the great potential of manganite-buffered $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ interfaces for spintronic applications.

Atomistically thin adsorbate layers on surfaces with a lattice mismatch display complex spatial patterns and ordering due to strain-driven self-organization. In this work, a general formalism to model such ultrathin adsorption layers that properly takes into account the competition between strain and adhesion energy of the layers is presented. The model is based on the amplitude expansion of the two-dimensional phase field crystal (PFC) model, which retains atomistic length scales but allows relaxation of the layers at diffusive time scales. The specific systems considered here include cases where both the film and the adsorption potential can have either honeycomb (H) or triangular (T) symmetry. These systems include the so-called $(1\times 1)$, $(\sqrt{3}\times \sqrt{3})\mathrm{R}{30}^{\circ }$, $(2\times 2)$, $(\sqrt{7}\times \sqrt{7})\mathrm{R}19.1^{\circ }$, and other higher order states that can contain a multitude of degenerate commensurate ground states. The relevant phase diagrams for many combinations of the H and T systems are mapped out as a function of adhesion strength and misfit strain. The coarsening patterns in some of these systems is also examined. The predictions are in good agreement with existing experimental data for selected strained ultrathin adsorption layers.

We theoretically study the conditional counting statistics of electron transport through a system consisting of a single quantum dot (SQD) or coherently coupled double quantum dots (DQD's) monitored by a nearby quantum point contact (QPC) using the generating functional approach with the maximum eigenvalue of the evolution equation matrix method, the quantum trajectory theory method (Monte Carlo method), and an efficient method we develop. The conditional current cumulants that are significantly different from their unconditional counterparts can provide additional information and insight into the electron transport properties of mesoscopic nanostructure systems. The efficient method we develop for calculating the conditional counting statistics is numerically stable, and is capable of calculating the conditional counting statistics for a more complex system than the maximum eigenvalue method and for a wider range of parameters than the quantum trajectory method. We apply our method to investigate how the QPC shot noise affects the conditional counting statistics of the SQD system, going beyond the treatment and parameter regime studied in the literature. We also investigate the case when the interdot coherent coupling is comparable to the dephasing rate caused by the back-action of the QPC in the DQD system, in which there is considerable discrepancy in the calculated conditional current cumulants between the population rate (master-) equation approach of sequential tunneling and the full quantum master-equation approach of coherent tunneling.

We observe that second-harmonic (SH) light generated by a femtosecond probe pulse from Si nanograting surfaces can be enhanced by a correlating pump pulse. Time-resolved anisotropy analysis shows that the enhancement arises from transient stimulation by the pump because it polarizes the periodically distributed charge from the probe photoinjection. In addition, dipole and quadrupole SH components are stimulated differently depending on the pump polarization. This effect reveals ultrafast SH dynamics in centrosymmetric materials and provides guidance for measuring nanoscale features.

We develop a machine learning method to construct accurate ground-state wave functions of strongly interacting and entangled quantum spin as well as fermionic models on lattices. A restricted Boltzmann machine algorithm in the form of an artificial neural network is combined with a conventional variational Monte Carlo method with pair product (geminal) wave functions and quantum number projections. The combination allows an application of the machine learning scheme to interacting fermionic systems. The combined method substantially improves the accuracy beyond that ever achieved by each method separately, in the Heisenberg as well as Hubbard models on square lattices, thus proving its power as a highly accurate quantum many-body solver.

The physical properties of $\mathrm{M}{\mathrm{n}}_3\mathrm{CuN}$ were studied using thin films. We found that an annealing process was very effective to improve the film quality, the key of which was the use of Ti that prevented the formation of oxide impurities. Using these high-quality thin films, we found strong strain dependence for the ferromagnetic transition temperature ($T_C$) and a sign change of the Hall coefficient at $T_C$. The analysis of Hall coefficient data revealed a sizable decrease of hole concentration and a large increase of electron mobility below $T_C$, which is discussed in relation to the electronic structure of this material.

In the organic charge transfer salt $\kappa$-(BETS)${}_2\mathrm{Mn}{\left[\mathrm{N}{\left(\mathrm{CN}\right)}_2\right]}_3$ the metallic conductivity is provided by itinerant $\pi$ electrons in the layers of BETS molecules, whereas magnetization is largely dominated by the localized $d$ electrons of the ${\mathrm{Mn}}^{2+}$ ions in the insulating anionic layers. We study magnetic properties of the compound in its low-temperature, Mott-insulating state by means of magnetic torque technique. The complex behavior of the torque can be consistently explained by the coexistence of two weakly interacting magnetic subsystems associated with paramagnetic $d$-electron spins and antiferromagnetically ordered $\pi$-electron spins, respectively. Based on the experimental data, we determine the principal axes of magnetization of the ${\mathrm{Mn}}^{2+}$ sublattice and propose a qualitative model for the $\pi$-electron spin arrangement, implying an important role of the Dzyaloshinskii-Moriya interaction.

We study the quantum many-body ground states of electrons on the half-filled honeycomb lattice with short- and long-ranged density-density interactions as a model for graphene. To this end, we employ the recently developed truncated-unity functional renormalization group (TU-fRG) approach which allows for a high resolution of the interaction vertex' wave vector dependence. We connect to previous lattice quantum Monte Carlo (QMC) results which predict a stabilization of the semimetallic phase for realistic ab initio interaction parameters and confirm that the application of a finite biaxial strain can induce a quantum phase transition towards an ordered ground state. In contrast to lattice QMC simulations, the TU-fRG is not limited in the choice of tight-binding and interaction parameters to avoid the occurrence of a sign problem. Therefore, we also investigate a range of parameters relevant to the realistic graphene material which are not accessible by numerically exact methods. Although a plethora of charge-density waves arises under medium-range interactions, we find the antiferromagnetic spin-density wave to be the prevailing instability for long-range interactions. We further explore the impact of an extended tight-binding Hamiltonian with second-nearest-neighbor hopping and a finite chemical potential for a more accurate description of the band structure of graphene's $p_z$ electrons.

We study the symmetry of the potential superconducting order parameter in $5d$ Mott insulators with an eye toward hole-doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. Using a mean-field method, a mixed singlet-triplet superconductivity, $d+p$, is observed due to the antisymmetric exchange originating from a quasi-spin-orbit coupling. Our calculation on ribbon geometry shows the possible existence of the topologically protected edge states, because of the nodal structure of the superconducting gap. These edge modes are spin polarized and emerge as zero-energy flat bands, supporting a symmetry-protected Majorana state, verified by evaluation of the winding number and ${\mathbb{Z}}_2$ topological invariant. At the end, a possible experimental approach for observation of these edge states and determination of the superconducting gap symmetry is discussed based on the quasiparticle interference technique.

We present spatial frequency maps of power flow in metamaterials and photonic crystals in order to provide insights into their electromagnetic responses and further our understanding of backward power in periodic structures. Since 2001, many different structures across the electromagnetic spectrum have been presented in the literature as exhibiting an isotropic negative effective index. Although these structures all exhibit circular or spherical equifrequency contours that resemble those of left-handed media, here we show through $k$-space diagrams that the distribution of power in the spatial frequency domain can vary considerably across these structures. In particular, we show that backward power arises from high-order right-handed harmonics in photonic crystals, magnetodielectric crystals, and across the layers of coupled-plasmonic-waveguide metamaterials, while arising from left-handed harmonic pairs in split-ring resonator and wire composites, plasmonic crystals, and along the layers of coupled-plasmonic-waveguide metamaterials. We also show that the fishnet structure exhibits the same left-handed harmonic pairs as the latter group. These observations allow us to categorize different metamaterials according to their spatial spectral source of backward power and identify the mechanism behind negative refraction at a given interface. Finally, we discuss how $k$-space maps of power flow can be used to explain the high or low transmittance of power into different metamaterial or photonic crystal structures.

A new heat harvester, the phonovoltaic (pV) cell, was recently proposed. The device converts optical phonons into power before they become heat. Due to the low entropy of a typical hot optical phonon population, the phonovoltaic can operate at high fractions of the Carnot limit and harvest heat more efficiently than conventional heat harvesting technologies such as the thermoelectric generator. Previously, the optical phonon source was presumed to produce optical phonons with a single polarization and momentum. Here, we examine a realistic optical phonon source in a potential pV application and the effects this has on pV operation. Supplementing this work is our investigation of bilayer graphene as a new pV material. Our ab initio calculations show that bilayer graphene has a figure of merit exceeding 0.9, well above previously investigated materials. This allows a room-temperature pV to recycle 65% of a highly nonequilibrium, minimum entropy population of phonons. However, full-band Monte Carlo simulations of the electron and phonon dynamics in a bilayer graphene field-effect transistor (FET) show that the optical phonons emitted by field-accelerated electrons can only be recycled in situ with an efficiency of 50%, and this efficiency falls as the field strength grows. Still, an appropriately designed FET-pV can recycle the phonons produced therein in situ with a much higher efficiency than a thermoelectric generator can harvest heat produced by a FET ex situ.

We consider the problem of local tunneling into cuprate superconductors, combining model-based calculations for the superconducting order parameter with wave function information obtained from first-principles electronic structure. For some time it has been proposed that scanning tunneling microscopy (STM) spectra do not reflect the properties of the superconducting layer in the ${\mathrm{CuO}}_2$ plane directly beneath the STM tip, but rather a weighted sum of spatially proximate states determined by the details of the tunneling process. These ``filter'' ideas have been countered with the argument that similar conductance patterns have been seen around impurities and charge ordered states in systems with atomically quite different barrier layers. Here we use a recently developed Wannier function-based method to calculate topographies, spectra, conductance maps, and normalized conductance maps close to impurities. We find that it is the local planar Cu $d_{x^2-y^2}$ Wannier function, qualitatively similar for many systems, that controls the form of the tunneling spectrum and the spatial patterns near perturbations. We explain how, despite the fact that STM observables depend on the materials-specific details of the tunneling process and setup parameters, there is an overall universality in the qualitative features of conductance spectra. In particular, we discuss why STM results on ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ (BSCCO) and ${\mathrm{Ca}}_{2-x}{\mathrm{Na}}_x{\mathrm{CuO}}_2{\mathrm{Cl}}_2$ (NaCCOC) are essentially identical.

We report the spin-to-charge current conversion in an intrinsic topological insulator (TI) ${\left(\mathrm{B}{\mathrm{i}}_{0.22}\mathrm{S}{\mathrm{b}}_{0.78}\right)}_2\mathrm{T}{\mathrm{e}}_3$ film at room temperature. The spin currents are generated in a thin layer of permalloy (Py) by two different processes, the spin pumping effect (SPE) and the spin Seebeck effect (SSE). In the first, we use microwave-driven ferromagnetic resonance of the Py film to generate a SPE spin current that is injected into the TI ${\left(\mathrm{B}{\mathrm{i}}_{0.22}\mathrm{S}{\mathrm{b}}_{0.78}\right)}_2\mathrm{T}{\mathrm{e}}_3$ layer in direct contact with Py. In the second, we use the SSE in the longitudinal configuration in Py without contamination by the anomalous Nernst effect, which was made possible with a thin NiO layer between the Py and ${\left(\mathrm{B}{\mathrm{i}}_{0.22}\mathrm{S}{\mathrm{b}}_{0.78}\right)}_2\mathrm{T}{\mathrm{e}}_3$ layers. The spin-to-charge current conversion is dominated by the TI surface states and is attributed to the inverse Edelstein effect (IEE), which is made possible by the spin-momentum locking in the electron Fermi contours due to the Rashba field. The measurements by the two techniques yield very similar values for the IEE parameter, which are larger than the reported values in the previous studies on topological insulators.

We present accurate electrical resistivity measurements along the two principle crystallographic axes of the pressure-induced heavy-fermion superconductor ${\mathrm{CeRhIn}}_5$ up to 5.63 GPa. For both directions, a valence crossover line is identified in the $p\text{−}T$ plane and the extrapolation of this line to zero temperature coincides with the collapse of the magnetic ordering temperature. Furthermore, it is found that the $p\text{−}T$ phase diagram of ${\mathrm{CeRhIn}}_5$ in the valence crossover region is very similar to that of ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. These results point to the essential role of Ce-$4f$ electron delocalization in both destroying magnetic order and realizing superconductivity in ${\mathrm{CeRhIn}}_5$ under pressure.