The sensitive dependence of monolayer materials on their environment often gives rise to unexpected properties. It was recently demonstrated that monolayer FeSe on a ${\mathrm{SrTiO}}_3$ substrate exhibits a much higher superconducting critical temperature $T_c$ than the bulk material. Here, we examine the interfacial structure of FeSe/${\mathrm{SrTiO}}_3$ and the effect of an interfacial ${\mathrm{Ti}}_{1+x}{\mathrm{O}}_2$ layer on the increased $T_c$ using a combination of scanning transmission electron microscopy and density functional theory. We find ${\mathrm{Ti}}_{1+x}{\mathrm{O}}_2$ forms its own quasi-two-dimensional layer, bonding to both the substrate and the FeSe film by van der Waals interactions. The excess Ti in this layer can reconstruct the FeSe Fermi surface in a manner consistent with experimental observations. Moreover, the interfacial layer introduces symmetry-breaking distortions in the FeSe film that may favor a $T_c$ increase. These results suggest that this common substrate may be functionalized to modify the electronic structure of a variety of thin films and monolayers.

We study the stability of Li-O compounds as a function of pressure, with rich phase diagram, diverse properties, and fundamental chemical interest in mind. Using the ab initio evolutionary algorithm USPEX, we predict the stability of compounds ${\mathrm{LiO}}_4, {\mathrm{Li}}_5{\mathrm{O}}_3$, and ${\mathrm{Li}}_6\mathrm{O}$ under pressure. Unexpectedly, ${\mathrm{LiO}}_2$ will decompose to ${\mathrm{Li}}_2{\mathrm{O}}_2+{\mathrm{LiO}}_4$ in the pressure range 6–18 GPa. ${\mathrm{LiO}}_4$, formed at the pressure of just 6 GPa, can be seen as $\varepsilon -{\mathrm{O}}_8$ accepting two electrons from two Li atoms. This phase is superconducting, with $T_c$ up to 12.2 K at 10 GPa. This is remarkable, because elemental oxygen becomes superconducting at much higher pressure (96 GPa) and has much lower $T_c$ ($<0.6$ K), and suggests that chemical alloying with other elements has the potential of not only decreasing metallization pressure, but also of increasing $T_c$. Since $\varepsilon -{\mathrm{O}}_8$ is called red oxygen, ${\mathrm{LiO}}_4$ can be identified as “lithium red-oxide”, and is distinct from superoxide. Additionally, ${\mathrm{Li}}_5{\mathrm{O}}_3$ is stable at pressures above 70 GPa and can be represented as a hybrid structure $4{\mathrm{Li}}_2\mathrm{O}·{\mathrm{Li}}_2{\mathrm{O}}_2$, and electride suboxide ${\mathrm{Li}}_6\mathrm{O}$ is stable above 62 GPa.

We fabricate a large number $(>100)$ of nonlocal spin valves with mesoscopic Cu channels. A systematic method is used to extract Cu resistivity ${\rho }_{\mathrm{Cu}}$ and spin relaxation length ${\lambda }_{\mathrm{Cu}}$ from each given structure. A relationship between ${\lambda }_{\mathrm{Cu}}$ and ${\rho }_{\mathrm{Cu}}$ is established over a broad range of ${\rho }_{\mathrm{Cu}}$ (from 0.7 to $>10\mu \mathrm{\Omega }\mathrm{cm}$) with $>200$ data points obtained at 10 and 295 K. Quantitative analysis of the relationship indicates that the spin relaxation can be described by the Elliott-Yafet model with a low spin-flip probability of $\left(1.9\pm 0.2\right)\times {10}^{-4}$ for bulk scattering and a higher probability of $\left(1.6\pm 0.2\right)\times {10}^{-3}$ for surface scattering. Encouraging large values of ${\lambda }_{\mathrm{Cu}}$ ($\sim 2.0\mu \mathrm{m}$ at 10 K and $\sim 700\mathrm{nm}$ at 295 K) are achieved experimentally in structures with low ${\rho }_{\mathrm{Cu}}$ values ($\le 1.0\mu \mathrm{\Omega }\mathrm{cm}$ at 10 K and $\le 4.0\mu \mathrm{\Omega }\mathrm{cm}$ at 295 K).

The magnetic ground state of the ideal quantum kagome antiferromagnet (QKA) has been a long-standing puzzle, mainly because perturbations to the nearest-neighbor isotropic Heisenberg Hamiltonian can lead to various fundamentally different ground states. Here we investigate a recently synthesized QKA representative, ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_3$, in which perturbations commonly present in real materials, like lattice distortion and intersite ion mixing, are absent. Nevertheless, this compound enters a long-range magnetically ordered state below $T_N=15\mathrm{K}$. Our powder neutron diffraction experiment reveals that its magnetic structure corresponds to a coplanar ${120}^{\circ }$ state with negative vector spin chirality. The ordered magnetic moments are suppressed to $0.42\left(2\right){\mu }_B$, which is consistent with the previously detected spin dynamics persisting to the lowest experimentally accessible temperatures. This indicates either a coexistence of magnetic order and disorder or the presence of strong quantum fluctuations in the ground state of ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_3$. The origin of the magnetic order is sought in terms of Dzyaloshinskii-Moriya magnetic anisotropy and further-neighbor isotropic exchange interactions.

Anisotropy is the key to understanding iron-based superconductors, which usually have quasi-two-dimensional layered structures. By using torque measurements (a sensitive tool to detect the magnetic anisotropy), we investigate the superconducting anisotropy of the ${\mathrm{KCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ (12442, $T_c=33\text{K}$) single crystal. In the normal state, the torque data display a paramagnetic behavior $H^{2}sin2\theta$ in different applied magnetic fields $H$ aligned at an angle $\theta$ to the $c$ axis of the crystal, while in the mixed state vortex torque dominates the torque signal. The anisotropy parameter $\gamma$ and London penetration depth $\lambda$ were obtained from vortex torque analyzed using Kogan's formula. It was found that $\gamma$ is 16.2 at $T=25\text{K}$ ($\approx 0.76$ $T_c$), close to $\gamma \approx$ 15 at T $\approx$ 0.76 $T_c$ for ${\mathrm{CaFe}}_{0.88}{\mathrm{Co}}_{0.12}\mathrm{AsF}$, but much larger than the result of 11 and 122 families, where $\gamma$ is around 3. In addition, the obtained perpetration depth from the torque measurements is consistent with the results of $\mu \text{SR}$ measurements of ${\mathrm{KCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$. Our results suggest that the two-dimensionality plays a vital role in the superconductivity of the 12442-type iron-based superconductor.

We show that an array of qubits embedded in a waveguide can emit entangled pairs of microwave photon beams. The quadratures obtained from the homodyne detection of these outputs beams form a pair of correlated continuous variables similar to the Einstein-Podolsky-Rosen experiment. The photon pairs are produced by the decay of plasmonlike collective excitations in the qubit array. The maximum intensity of the resulting beams is bounded by only the number of emitters. We calculate the excitation decay rate both into a continuum of the photon state and into a one-mode cavity. We also determine the frequency of Rabi-like oscillations resulting from a detuning.

In one dimension (1D), a general decaying long-range interaction can be fit to a sum of exponential interactions $e^{-\lambda r_{ij}}$ with varying exponents $\lambda$, each of which can be represented by a simple matrix product operator with bond dimension $D=3$. Using this technique, efficient and accurate simulations of 1D quantum systems with long-range interactions can be performed using matrix product states. However, the extension of this construction to higher dimensions is not obvious. We report how to generalize the exponential basis to two and three dimensions by defining the basis functions as the Green's functions of the discretized Helmholtz equation for different Helmholtz parameters $\lambda$, a construction which is valid for lattices of any spatial dimension. Compact tensor network representations can then be found for the discretized Green's functions, by expressing them as correlation functions of auxiliary fermionic fields with nearest-neighbor interactions via Grassmann Gaussian integration. Interestingly, this analytic construction in three dimensions yields a $D=4$ tensor network representation of correlation functions which (asymptotically) decay as the inverse distance ($r_{ij}^{-1}$), thus generating the (screened) Coulomb potential on a cubic lattice. These techniques will be useful in tensor network simulations of realistic materials.

We consider a lattice version of the Bisognano-Wichmann (BW) modular Hamiltonian as an ansatz for the bipartite entanglement Hamiltonian of the quantum critical chains. Using numerically unbiased methods, we check the accuracy of the BW ansatz by both comparing the BW Rényi entropy to the exact results and investigating the size scaling of the norm distance between the exact reduced density matrix and the BW one. Our study encompasses a variety of models, scanning different universality classes, including integrable models such as the transverse field Ising, three-state Potts and XXZ chains, and the nonintegrable bilinear-biquadratic model. We show that the Rényi entropies obtained via the BW ansatz properly describe the scaling properties predicted by conformal field theory. Remarkably, the BW Rényi entropies also faithfully capture the corrections to the conformal field theory scaling associated with the energy density operator. In addition, we show that the norm distance between the discretized BW density matrix and the exact one asymptotically goes to zero with the system size: this indicates that the BW ansatz also can be employed to predict properties of the eigenvectors of the reduced density matrices and is thus potentially applicable to other entanglement-related quantities such as negativity.

Real-time evolution and charge pumping in magnetic Weyl semimetals are studied by solving the time-dependent Schrödinger equations. In the adiabatic limit of the real-time evolution, we show that the total pumped charge is quantized in the magnetic Weyl semimetals as in the quantum Hall system, although the Weyl semimetal has no bulk gap. We examine how the disorder affects the charge pumping. As a result, we show that the quantized pumped charge is robust against the small disorder and find that the pumped charge increases in the intermediate disorder region. We also examine the doping effects on the charge pumping and show that the remnant of the quantized pumped charge at zero doping can be detected. Our results show that the real-time evolution is a useful technique for detecting the topological properties of the systems with no bulk gap and/or disorders.

We develop a many-body approach to the behavior of exciton bound states and the conduction electron band edge in a surrounding electron-hole plasma with a focus on the absorption spectrum of Rydberg excitons in cuprous oxide. The interplay of band edge and exciton levels is analyzed numerically, whereby the self-consistent solution is compared to the semiclassical Debye approximation. Our results provide criteria which allow us to verify or rule out the different band edge models against future experimental data.

The recently found $(\mathrm{Tl},\mathrm{Au})/\mathrm{Si}\left(100\right)c(2\times 2)$ two-dimensional compound which possesses spin-split surface states with peculiar spin texture and strong band anisotropy was examined using low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy observations and in situ four-probe transport measurements. Two types of the one-dimensional incommensurate Friedel oscillations were found to develop near the surface defects with the wavelengths corresponding to the nesting vectors of the $(\mathrm{Tl},\mathrm{Au})/\mathrm{Si}\left(100\right)c(2\times 2)$ Fermi surface. In transport measurements, samples show strong localization behavior when density of defects is sufficiently high. With low density of defects, the system demonstrates a metallic behavior with fingerprints of superconducting transition at estimated $T_c=1.33\mathrm{K}$. Thus, the $(\mathrm{Tl},\mathrm{Au})/\mathrm{Si}\left(100\right)c(2\times 2)$ system hosts a combination of fascinating properties and effects, including one-dimensional Friedel oscillations, superconductivity, and a spin-split anisotropic Fermi surface, that opens an avenue for prospective investigations of the interplay of collective electronic phenomena in atomic layer material.

Thermoelectricity is a promising avenue for harvesting energy but large-scale applications are still hampered by the lack of highly efficient low-cost materials. Recently, ${\mathrm{Fe}}_{2}YZ$ Heusler compounds were predicted theoretically to be interesting candidates with large thermoelectric power factor. Here, we show that under doping conditions compatible with thermoelectric applications, these materials are prone to an unexpected magnetic instability detrimental to their thermoelectric performance. We rationalize the physics at the origin of this instability, provide guidelines for avoiding it, and discuss its impact on the thermoelectric power factor. Doing so, we also point out the shortcomings of the rigid band approximation commonly used in high-throughput theoretical searches of new thermoelectrics.

Polycrystalline graphene has an inherent tendency to buckle, i.e., develop out-of-plane, three-dimensional structure. A force applied to stretch a piece of polycrystalline graphene influences the out-of-plane structure. Even if the graphene is well relaxed, this happens in nonlinear fashion: Occasionally, a tiny increase in stretching force induces a significant displacement, in close analogy to avalanches, which in turn can create vibrations in the surrounding medium. We establish this effect in computer simulations: By continuously changing the strain, we follow the displacements of the carbon atoms that turn out to exhibit a discontinuous evolution. Furthermore, the displacements exhibit a hysteretic behavior upon the change from low to high stress and back. These behaviors open up another direction in studying dynamical elasticity of polycrystalline quasi-two-dimensional systems, and in particular the implications on their mechanical and thermal properties.

Using a realistic multiband model for two holes doped into a ${\mathrm{CuO}}_2$ layer, we devise a method to turn off the magnon-mediated interaction between the holes. This allows us to verify that this interaction is attractive, and therefore could indeed be (part of) the superconducting glue. We derive its analytical expression and show that it consists of pair-hopping $+$ spin-exchange terms. Its coupling constant is fitted from the ground state energy obtained with variational exact diagonalization, and it faithfully reproduces the effect of the magnon-mediated attraction in the entire Brillouin zone. For realistic parameter values, this effective interaction is borderline strong enough to bind the holes into preformed pairs.

$\mathrm{L}{\mathrm{i}}_2\mathrm{Ru}{\mathrm{O}}_3$ undergoes a structural transition at a relatively high temperature of 550 K with a distinct dimerization of Ru-Ru bonds on the otherwise isotropic honeycomb lattice. It exhibits a unique herringbone dimerization pattern with an unusually large value of bond shrinkage, about $\sim 0.5\AA$. However, many questions still remain about its origin and its effect on the physical properties. In this work, using high-quality single crystals we investigated the anisotropy of resistivity (ρ) and magnetic susceptibility (χ) to find a very clear anisotropy: ${\rho }_{c^{*}}>{\rho }_b>{\rho }_a$ and ${\chi }_b>{\chi }_a>{\chi }_{c^{*}}$. We also carried out density functional calculations for possible theoretical interpretations, and concluded that this anisotropic behavior is due to the correlation effects combined with the unique orbital structure and the dimerization of Ru $4d$ bands.

Resonant photoelectron spectroscopy at the Co and Mn $2p$ core absorption edges of half-metallic ${\mathrm{Co}}_2\mathrm{MnGe}$ has been performed to determine the element-specific density of states (DOS). A significant contribution of the Mn $3d$ partial DOS near the Fermi level $\left(E_{\mathrm{F}}\right)$ was clarified by measurement at the Mn $2p$ absorption edge. Further analysis by first-principles calculation revealed that it has $t_{2\mathrm{g}}$ symmetry, which must be responsible for the electrical conductivity along the line perpendicular to the film plane. The dominant normal Auger contribution observed at the Co $2p$ absorption edge indicates delocalization of photoexcited Co $3d$ electrons. The difference in the degrees of localization of the Mn $3d$ and Co $3d$ electrons in ${\mathrm{Co}}_2\mathrm{MnGe}$ is explained by the first-principles calculation. Our findings of the element-/orbital-specific electronic states near $E_{\mathrm{F}}$ will pave the way for future interface design of magnetic tunneling junctions to overcome the temperature-induced reduction of the magnetoresistance.

We compute the real-space profile of the superconducting order parameter (OP) in a hybrid ring that consists of a one-dimensional superconductor connected to a Fibonacci chain using a self-consistent approach. In our paper, the strength of the penetration as measured by the order parameter at the center of the quasicrystal depends on the structural parameter $\varphi$ or phason angle that characterizes different realizations of the Fibonacci chains of a given length. We show that the penetration strength dependence on $\varphi$ reflects properties of the topological edge states of the Fibonacci chain. We show that the induced superconducting order parameter averaged over all chains has a power-law decay as a function of distance from the superconductor-normal interface. More interestingly, we show that there are large OP fluctuations for individual chains and that the penetration strength in a finite Fibonacci chain can be larger than in a normal periodic conductor for special values of $\varphi$.

We study theoretically the magnetoresistance oscillations near a half-filled lowest Landau level ($\nu =1/2$) that result from the presence of a periodic one-dimensional electrostatic potential. We use the Dirac composite fermion theory of Son [Phys. Rev. X 5, 031027 (2015)], where the $\nu =1/2$ state is described by a $(2+1)$-dimensional theory of quantum electrodynamics. We extend previous work that studied these oscillations in the mean-field limit by considering the effects of gauge-field fluctuations within a large flavor approximation. A self-consistent analysis of the resulting Schwinger-Dyson equations suggests that fluctuations dynamically generate a Chern-Simons term for the gauge field and a magnetic field–dependent mass for the Dirac composite fermions away from $\nu =1/2$. We show how this mass results in a shift of the locations of the oscillation minima that improves the comparison with experiment [Kamburov et al., Phys. Rev. Lett. 113, 196801 (2014)]. The temperature-dependent amplitude of these oscillations may enable an alternative way to measure this mass. This amplitude may also help distinguish the Dirac and Halperin, Lee, and Read composite fermion theories of the half-filled Landau level.