We numerically study optical conductivity $\sigma \left(\omega \right)$ near the “antiferromagnetic” phase transition in the square-lattice Hubbard model at half filling. We use a cluster dynamical mean field theory and calculate conductivity including vertex corrections and, to this end, we have reformulated the vertex corrections in the antiferromagnetic phase. We find that the vertex corrections change various important details in temperature and $\omega$ dependencies of conductivity in the square lattice, and this contrasts sharply the case of the Mott transition in the frustrated triangular lattice. Generally, the vertex corrections enhance variations in the $\omega$ dependence, and sharpen the Drude peak and a high-$\omega$ incoherent peak in the paramagnetic phase. They also enhance the dip in $\sigma \left(\omega \right)$ at $\omega =0$ in the antiferromagnetic phase. Therefore, the dc conductivity is enhanced in the paramagnetic phase and suppressed in the antiferromagnetic phase, but this change occurs slightly below the transition temperature. We also find a temperature region above the transition temperature in which the dc conductivity shows an insulating behavior but $\sigma \left(\omega \right)$ retains the Drude peak, and this region is stabilized by the vertex corrections. We also investigate which fluctuations are important in the vertex corrections and analyze momentum dependence of the vertex function in detail.

We study the orbital diamagnetic susceptibility in excitonic condensation phase using the mean-field approximation for a two-band model defined on a square lattice. We find that, in semiconductors, the excitonic condensation acquires a finite diamagnetic susceptibility due to spontaneous hybridization between the valence and the conduction bands, whereas in semimetals, the diamagnetic susceptibility in the normal phase is suppressed by the excitonic condensation. We also study the orbital diamagnetic and Pauli paramagnetic susceptibilities of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ using a two-dimensional three-band model and find that the calculated temperature dependence of the magnetic susceptibility is in qualitative agreement with experiment.

Recent work has shown that a variety of novel phases of matter arise in periodically driven Floquet systems. Among these are many-body localized phases which spontaneously break global symmetries and exhibit novel multiplets of Floquet eigenstates separated by quantized quasienergies. Here we show that these properties are stable to all weak local deformations of the underlying Floquet drives—including those that explicitly break the defining symmetries—and that the models considered until now occupy submanifolds within these larger “absolutely stable” phases. While these absolutely stable phases have no explicit global symmetries, they spontaneously break Hamiltonian-dependent emergent symmetries, and thus continue to exhibit the novel multiplet structure. The multiplet structure in turn encodes characteristic oscillations of the emergent order parameter at multiples of the fundamental period. Altogether these phases exhibit a form of simultaneous long-range order in space and time which is new to quantum systems. We describe how this spatiotemporal order can be detected in experiments involving quenches from a broad class of initial states.

Motivated by recent Mössbauer experiments on strongly correlated mixed-valence systems, we revisit the Kubo-Anderson stochastic theory of spectral line shapes. Using a Majorana representation for the nuclear spin we demonstrate how to recast the classic line-shape theory in a field-theoretic and diagrammatic language. We show that the leading contribution to the self-energy can reproduce most of the observed line-shape features including splitting and line-shape narrowing, while the vertex and the self-consistency corrections can be systematically included in the calculation. This approach permits us to predict the line shape produced by an arbitrary bulk charge fluctuation spectrum providing a model-independent way to extract the local charge fluctuation spectrum of the surrounding medium. We also derive an inverse formula to extract the charge fluctuation from the measured line shape.

Graphene on hexagonal boron nitride (h-BN) is an atomically flat conducting system that is ideally suited for probing the effect of Zeeman splitting on electron transport. We demonstrate by magnetotransport measurements that a parallel magnetic field up to 30 Tesla does not affect the transport properties of graphene on h-BN even at charge neutrality where such an effect is expected to be maximal. The only magnetoresistance detected at low carrier concentrations is shown to be associated with a small perpendicular component of the field which cannot be fully eliminated in the experiment. Despite the high mobility of charge carriers at low temperatures, we argue that the effects of Zeeman splitting are fully masked by electrostatic potential fluctuations at charge neutrality.

We analyze the energy spectrum of graphene in the presence of spin-orbit coupling and a one-dimensional periodically modulated Zeeman field, focusing on the stability and location of Dirac points it may support. It is found that the Dirac points at the $K$ and $K^{\prime }$ points are generically moved to other locations in the Brillouin zone, but that they remain present when the Zeeman field $\vec{\mathrm{\Delta }}\left(x\right)$ integrates to zero within a unit cell. A large variety of locations for the Dirac points is shown to be possible: when $\vec{\mathrm{\Delta }}\parallel \widehat{z}$ they are shifted from their original locations along the direction perpendicular to the superlattice axis, while realizations of $\vec{\mathrm{\Delta }}\left(x\right)$ that rotate periodically move the Dirac points to locations that can reflect the orbit of the rotating electron spin as it moves through a unit cell. When a uniform Zeeman field is applied in addition to a periodic $\vec{\mathrm{\Delta }}\parallel \widehat{z}$ integrating to zero, the system can be brought into a metallic, Dirac semimetal, or insulating state, depending on the direction of the uniform field. The latter is shown to be an anomalous quantum Hall insulator.

We formulate a theory of spin-transfer torques in textured antiferromagnets, which covers the small to large limits of the exchange coupling energy relative to the kinetic energy of the intersublattice electron dynamics. Our theory suggests a natural definition of the efficiency of spin-transfer torques in antiferromagnets in terms of well-defined material parameters, revealing that the charge current couples predominantly to the antiferromagnetic order parameter and the sublattice-canting moment in, respectively, the limits of large and small exchange coupling. The effects can be quantified by analyzing the antiferromagnetic spin-wave dispersions in the presence of charge current: in the limit of large exchange coupling the spin-wave Doppler shift always occurs, whereas, in the opposite limit, the only spin-wave modes to react to the charge current are ones that carry a pronounced sublattice-canting moment. The findings offer a framework for understanding and designing spin-transfer torques in antiferromagnets belonging to different classes of sublattice structures such as, e.g., bipartite and layered antiferromagnets.

Helical Majorana edge states at the 2D boundaries of 3D topological superconductors can be gapped by a surface Zeeman field. Here we study the effect nested defects imprinted on the Zeeman field can have on the edge states. We demonstrate that depending on the configuration of the field, we can induce dimensional reduction of gapless Majorana modes from 2D to 1D or quasi-0D at magnetic domain walls. We determine the nature of the Majorana localization on these defects as a function of the magnitude and configuration of the Zeeman field. Finally, we observe a generalization of the index theorem governing the number of gapless modes at the interface between topologically nontrivial systems with partial Chern numbers.

We present evidence of near-infrared photoluminescence (PL) signature of nitrogen vacancy centers ${\left({\mathrm{N}}_{\mathrm{C}}{\mathrm{V}}_{\mathrm{Si}}\right)}^{-}$ in silicon carbide (SiC). This center exhibits an $S=1$ ground state spin similar to the ${\mathrm{NV}}^{-}$ center in diamond. We have performed photoluminescence excitation measurements at cryogenic temperature and demonstrated efficient photoexcitation of distinct photoluminescence from $({\mathrm{N}}_{\mathrm{C}}{\mathrm{V}}_{\mathrm{Si}}{)}^{-}$ in $4H$-SiC. Furthermore, by correlating the energies of measured zero phonon lines (ZPLs) with theoretical values derived from hybrid density functional theory each of the ZPLs has been associated to the respective occupation of hexagonal $\left(h\right)$ and quasicubic $\left(k\right)$ lattice sites in close analogy to neutral divacancy centers ${\left({\mathrm{V}}_{\mathrm{C}}{\mathrm{V}}_{\mathrm{Si}}\right)}^0$ in the same material. Finally, with the appropriate choice of excitation energy we demonstrated the selective excitation of ${\left({\mathrm{N}}_{\mathrm{C}}{\mathrm{V}}_{\mathrm{Si}}\right)}^{-}$ PL with no contamination by ${\left({\mathrm{V}}_{\mathrm{C}}{\mathrm{V}}_{\mathrm{Si}}\right)}^0$ PL, thereby opening the way towards the optical detection of ${\left({\mathrm{N}}_{\mathrm{C}}{\mathrm{V}}_{\mathrm{Si}}\right)}^{-}$ electron spin resonance.

We investigate the evolution of magnetic excitations in ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ in the paramagnetic metallic phase using a three-band tight-binding model. The effect of Mn or Ti dopant ions on the ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ band structure has been included by taking into account the dopant-induced suppression of the oxygen octahedral rotation in the tight-binding band structure. We find that the low-energy spin fluctuations are dominated by three wave vectors around $\vec{q}=((0,0),(\pi /2,\pi /2))$ and $(\pi ,0)$, which compete with each other. As the octahedral rotation is suppressed with increasing doping, the three wave vectors evolve differently. In particular, the undoped compound has dominant wave vectors at $\vec{q}=((0,0),(\pi /2,\pi /2))$, but doping ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ leads to a significant enhancement in the spin susceptibility at the $\vec{q}=(\pi ,0)$ wave vector, bringing the system closer to a magnetic instability. All the features calculated from our model are in agreement with neutron scattering experiments. We have also studied the effect of a $c$-axis Zeeman field on the low-energy spin fluctuations. We find that an increasing magnetic field suppresses the antiferromagnetic (AFM) fluctuations and leads to stronger competition between the AFM and ferromagnetic spin fluctuations. The magnetic field dependence observed in our calculations therefore supports the scenario that the observed nematic phase in the metamagnetic region in ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ is intimately related to the presence of a competing ferromagnetic instability.

The compounds designated as $R_3{T}_4{\mathrm{Sn}}_{13}$ ($R$ = rare earth and $T$ = transition metal) are hypothesized to be strongly correlated electron systems. Some of these compounds exhibit structural phase transitions with speculated charge-density-wave (CDW) formations. We carried out x-ray diffraction measurements in order to investigate the second-order phase transition of $R_3{\mathrm{Co}}_4{\mathrm{Sn}}_{13}$ ($R=\mathrm{La} \text{and} \mathrm{Ce}$) by using single-crystalline samples synthesized by the molten Sn-flux method. Both compounds exhibit the structural superlattice transformations characterized by the wave vector $\mathit{q}=(1/2,1/2,0)$ below $T_{\mathrm{D}}\simeq 160$ K, which originate from electronic states owing to the common Co and Sn. The space group of the low-temperature phase is evidenced as a chiral cubic $I{2}_{1}3$. The magnetic-susceptibility enhancement and the Co-ion valence shift below $T_{\mathrm{D}}$ are signatures of a phase transition not fully attributed to the conventional CDW formation.

Ultracold bosons in a triangular lattice are a promising candidate for observing quantum spin liquid behavior. Here we investigate, for such system, the role of a harmonic trap giving rise to an inhomogeneous density. We construct a modified spin-wave theory for arbitrary filling and predict the breakdown of order for certain values of the lattice anisotropy. These regimes, identified with the spin liquid phases, are found to be quite robust upon changes in the filling factor. This result is backed by an exact diagonalization study on a small lattice.

Heteroepitaxial growth on a pit-patterned substrate is investigated by phase-field simulations, tackling both film and substrate geometry and elastic properties. The dynamics results from material deposition and redistribution via surface diffusion according to the tendency toward free-energy minimization. The balance between surface energy, misfit strain, and wetting effects is taken into account. Numerical solution by finite-element method permits to properly account for the role played by the actual pit morphology in determining strain relaxation. The mechanisms leading to island growth into the pit are discussed. Different growth parameters and pattern geometries are considered and their effects on island ordering are explained and related to experimental evidences in literature.

The charge-current-induced spin polarization is a key property of topological insulators for their applications in spintronics. However, topological surface states are expected to give rise to only one type of spin polarization for a given current direction, which has been a limiting factor for spin manipulations. Here, we report that in devices based on the bulk-insulating topological insulator ${\mathrm{BiSbTeSe}}_2$, an unexpected switching of spin polarization was observed upon changing the chemical potential. The spin polarization expected from the topological surface states was detected in a heavily electron-doped device, whereas the opposite polarization was reproducibly observed in devices with low carrier densities. We propose that the latter type of spin polarization stems from topologically trivial two-dimensional states with a large Rashba spin splitting, which are caused by a strong band bending at the surface of ${\mathrm{BiSbTeSe}}_2$ beneath the ferromagnetic electrode used as a spin detector. This finding paves the way for realizing the “spin transistor” operation in future topological spintronic devices.

Modulation of electronic states in two-dimensional materials can be achieved by using in-plane variations of the band gap or the average potential in lateral quantum structures. In the atomic configurations with hexagonal symmetry, this approach makes it possible to tailor the valleytronic properties for potential device applications. In this work, we present a multiband theory to calculate the valley-dependent electron transport in graphene-based lateral quantum structures. As an example, we consider the structures with a single interface that exhibits an energy gap or potential discontinuity. The theoretical formalism proceeds within the tight-binding description, by first deriving the local bulk complex band structures in the regions of a constant gap or potential and, next, joining the local wave functions across the interface via a cell-averaged current operator to ensure the current continuity. The theory is applied to the study of electron reflection off and transmission through an interface. Both reflection and transmission are found to exhibit valley-contrast behavior that can be used to generate valley-polarized electron sources. The results vary with the type of interfaces, as well as between monolayer and bilayer graphene-based structures. In the monolayer case, the valley contrast originates from the band warping and only becomes sizable for incident carriers of high energy, whereas in AB-stacked bilayer graphene, the vertical interlayer coupling emerges as an additional important cause for valley contrast, and the favorable carrier energy is also found to be drastically lower. Our numerical results clearly demonstrate the propitious valleytronic properties of bilayer graphene structures.

Graphene is an ideal platform to study many-body effects due to its semimetallic character and the possibility to dope it over a wide range. Here we study the width of graphene's occupied $\pi$ band as a function of doping using angle-resolved photoemission. Upon increasing electron doping, we observe the expected shift of the band to higher binding energies. However, this shift is not rigid and the bottom of the band moves less than the Dirac point. We show that the observed shift cannot be accounted for by single-particle effects and local self-energies alone, but that nonlocal many-body effects, in particular exchange interactions, must be taken into account.

We investigate the dependence of friction forces on normal load in incommensurate micrometer-size contacts between atomically smooth single-crystal graphite surfaces under ambient conditions. Our experimental results show that these contacts exhibit superlubricity (superlow friction), which is robust against the application of normal load. The measured friction coefficients are essentially zero and independent of the external normal load up to the maximum pressure of our experiment, 1.67 MPa. The observation of load-independent superlubricity in microscale contacts is a promising result for numerous practical applications.

Tensile strain along the $c_R$ axis in epitaxial ${\mathrm{VO}}_2$ films raises the temperature of the metal insulator transition and is expected to stabilize the intermediate monoclinic M2 phase. We employ surface-sensitive x-ray spectroscopy to distinguish from the ${\mathrm{TiO}}_2$ substrate and identify the phases of ${\mathrm{VO}}_2$ as a function of temperature in epitaxial ${\mathrm{VO}}_2/{\mathrm{TiO}}_2$ thin films with well-defined biaxial strain. Although qualitatively similar to our Landau-Ginzburg theory predicted phase diagrams, the M2 phase is stabilized by nearly an order of magnitude more strain than expected for the measured temperature window. Our results reveal that the elongation of the $c_R$ axis is insufficient for describing the transition pathway of ${\mathrm{VO}}_2$ epitaxial films and that a strain induced increase of electron correlation effects must be considered.