We analyze theoretically the optical generation of a spin-polarized charge current (photogalvanic effect) and spin polarization in graphene with Rashba spin-orbit coupling. An external magnetic field is applied in the graphene plane, which plays a crucial role in the mechanism of current generation. We predict a highly efficient resonant photogalvanic effect in a narrow frequency range that is determined by the magnetic field. A relatively less efficient photogalvanic effect appears in a broader frequency range, determined by the electron concentration and spin-orbit coupling strength.

We report nearly complete preservation of “spin memory” between optical absorption and photoluminescence (PL) in nanometer slabs of GaSe pumped with up to 0.2 eV excess energy. At cryogenic temperatures, the initial degree of circular polarization $\left({\rho }_0\right)$ of PL approaches unity, with the major fraction of the spin polarization decaying with a time constant $>500$ ps in sub-100-nm GaSe nanoslabs. Even at room temperature, ${\rho }_0$ as large as 0.7 is observed, while pumping 1 eV above the band edge yields ${\rho }_0=0.15$. Angular momentum preservation for both electrons and holes is due to the separation of the nondegenerate conduction and valence bands from other bands. In contrast to valley polarization in atomically thin transition-metal dichalcogenides, here optical spin polarization is preserved in nanoslabs of 100 layers or more of GaSe.

In this paper, we perform a comparative study based on ab initio modeling for perovskite $AB{\mathrm{O}}_3$ (001) surfaces and surface defect energetics in order to understand the influence of polarity and redox active Mn in the $\mathrm{LaMn}{\mathrm{O}}_3$ system. We consider $\mathrm{LaMn}{\mathrm{O}}_3,\mathrm{LaAl}{\mathrm{O}}_3,\mathrm{SrTi}{\mathrm{O}}_3$, and briefly $\mathrm{LaFe}{\mathrm{O}}_3$ systems for comparison, which illustrate the interplay between properties of polar surfaces and the varying $d$-electron shell of transition metals. We are motivated by the need to understand the surfaces of mixed electronic and ionic conductors typically used in solid oxide fuel cell cathodes and other ion conducting technologies, which are represented here by the $\mathrm{LaMn}{\mathrm{O}}_3$ system. We focus on the influence of the metal character and surface polarity on the surface and surface defect chemistry in these selected systems. We demonstrate that the facile redox of the TM $\left(3d^4\right)$ in $\mathrm{LaMn}{\mathrm{O}}_3$ with partial $e_g$ orbital occupation (or specifically $e_g$ occupancy close to 1) allows the polar surfaces to be compensated by changes in charge density over relatively short length scales (3 to 4 unit cells or ∼1.5 nm) near the surface as compared to $\mathrm{LaAl}{\mathrm{O}}_3$. In contrast to $\mathrm{LaAl}{\mathrm{O}}_3$, this low-energy and short-range screening mechanism leads to low surface energies without any additional reconstruction, rapidly converging surface properties with film thickness (by ∼8 unit cells), bulklike defect chemistry more than ∼1.5 nm from the surface, and surface defect energetics that are primarily governed by the local charge doping or the created electric field near the polar surfaces. We show that $\mathrm{LaMn}{\mathrm{O}}_3$ exhibits very different surface properties from $\mathrm{LaAl}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$, thereby demonstrating that these properties are due to the presence of the redox active transition metal with partial $e_g$ orbital occupation and a polar surface, respectively. These understandings can help guide qualitative analysis, computational study, and design of surfaces of mixed electronic and ionic conductors.

We present time- and angle-resolved photoemission spectroscopy (trARPES) measurements on the charge density wave system ${\mathrm{CeTe}}_3$. Optical excitation transiently populates the unoccupied band structure and reveals a gap size of $2\mathrm{\Delta }=0.59\mathrm{eV}$. The occupied Te-$5p$ band dispersion is coherently modified by three modes at ${\mathrm{\Omega }}_1=2.2,{\mathrm{\Omega }}_2=2.7$, and ${\mathrm{\Omega }}_3=3\mathrm{THz}$. All three modes lead to small rigid energy shifts whereas $\mathrm{\Delta }$ is only affected by ${\mathrm{\Omega }}_1$ and ${\mathrm{\Omega }}_2$. Their spatial polarization is analyzed by fits of a transient model dispersion and density-functional theory frozen phonon calculations. We conclude that modes ${\mathrm{\Omega }}_1$ and ${\mathrm{\Omega }}_2$ result from in-plane ionic lattice motions, which modulate the charge order and that ${\mathrm{\Omega }}_3$ originates from a generic out-of-plane $A_1\mathrm{g}$ phonon. We thereby demonstrate how the rich information from trARPES allows identification of collective modes and their spatial polarization, which explains the mode-dependent coupling to charge order.

This Rapid Communication reports on the modulation of the position and linewidth of the $G$ and $2D$ Raman bands for monolayer graphene through coupling to surface plasmon polaritons (SPPs). It is shown that gold nanoresonators tuned to the graphene Stokes emission frequency, not the laser pump, broaden the $G$ and $2D$ peaks and shift them to lower wave numbers, in addition to enhancing the Raman scattering cross section. The variation of the band displacement and broadening with the SPPs’ frequency demonstrates a strongly dispersive phenomenon, ruling out chemical, mechanical, and thermal mechanisms. It is believed that strong graphene-SPP coupling is responsible for the frequency pulling and line-shape broadening.

Valence electron energy-loss (VEEL) spectroscopy in the transmission electron microscope is combined to ab initio calculations to investigate the dielectric properties of multilayer (ML) two-dimensional ${\mathrm{Ti}}_3{\mathrm{C}}_2{T}_2$ ($T\equiv \text{OH}$ or F) MXene. Besides evidencing important similarities between the ${\mathrm{ML}-\mathrm{Ti}}_3{\mathrm{C}}_2{T}_2$ and TiC valence electron gas behaviors, a clear interband transition characteristic of the most stable site of the $T$ functionalization groups is identified in the VEEL spectrum. This signature, highly dependent on the $T$-group localization on the surface, has a prominent effect on the optical properties of the ML, leading to $40\%$ variations in the optical conductivity in the middle of the visible spectrum. Such a dependence could be of crucial interest for optical transparent thin films or sensing applications.

Recent experimental realization of ${\mathrm{CaSi}}_2$ paves the way of restoring the Dirac dispersion in layered materials by means of intercalation [E. Noguchi et al., Adv. Mater. 27, 856 (2015)]. Here we investigate this new material with several possible stacking sequences within ab initio calculations. The robust Dirac dispersion, a characteristic of monolayer honeycomb lattice, moves below the Fermi energy due to electron doping of silicene layers by intercalated Ca atoms. Moreover, the Dirac cone shifts away from the high symmetric point of the hexagonal Brillouin zone with the opening of a small gap. This happens due to the sublattice symmetry breaking and consequent enhanced asymmetric interlayer hopping, enforced by the enhanced buckling of the honeycomb layers. Our further studies on Ca intercalated multilayer germanene compound provide the microscopic understanding of the Dirac electrons in intercalated buckled layered materials.

Organic charge-transfer salts exhibit thermal expansion anomalies similar to those found in other strongly correlated electron systems. The thermal expansion can be anisotropic and have a nonmonotonic temperature dependence. We show how these anomalies can arise from electronic effects and be significantly enhanced, particularly at temperatures below 100 K, by strong electronic correlations. For the relevant Hubbard model the thermal expansion is related to the dependence of the entropy on the parameters ($t, t^{\prime }$, and $U$) in the Hamiltonian or the temperature dependence of bond orders and double occupancy. The latter are calculated on finite lattices with the finite-temperature Lanczos method. Although many features seen in experimental data, in both the metallic and Mott insulating phase, are described qualitatively, the calculated magnitude of the thermal expansion is smaller than that observed experimentally.

Muon spin rotation measurements, supported by magnetization experiments, have been carried out in a stoichiometric high-$T_c$ parent compound ${\mathrm{La}}_2{\mathrm{CuO}}_4$ in a temperature range from 2 to 340 K and in transverse magnetic fields up to 5 T. Along with the antiferromagnetic local field, muon spin rotation spectra indicate the presence of an additional source of magnetic field on the muon. The characteristic splitting of about 45 G, which comes from this additional magnetic field, is consistent with spontaneous circulating currents.

In this paper we investigate the effect of disorder on highly correlated electron systems, which exhibit metal-insulator transition (MIT) and structural-phase transition (SPT). We show that the effect of ion irradiation is strikingly different between ${\mathrm{V}}_2{\mathrm{O}}_3$ and $\mathrm{V}{\mathrm{O}}_2$, two otherwise similar materials. Upon irradiation, the MIT and SPT temperatures in ${\mathrm{V}}_2{\mathrm{O}}_3$ decrease drastically at low absolute dosages, much lower than for $\mathrm{V}{\mathrm{O}}_2$. At a low threshold dose, the insulating state of ${\mathrm{V}}_2{\mathrm{O}}_3$ drastically collapses into a metallic state. Contrary to this, irradiation of $\mathrm{V}{\mathrm{O}}_2$ leads to a much milder reduction of the MIT and SPT temperatures and to a weak, gradual decrease of the insulating state resistivity—not suppressed even at one order of magnitude higher doses than the ${\mathrm{V}}_2{\mathrm{O}}_3$ threshold. These major differences imply that the phase transition in ${\mathrm{V}}_2{\mathrm{O}}_3$ arises from global (rather than local as in $\mathrm{V}{\mathrm{O}}_2$) physical mechanisms that are extremely sensitive to disorder. This shows that the MIT and SPT may have substantially different physical origins in different systems, with the consequent major implications for theoretical descriptions of the MIT in highly correlated electron systems.

A bilayer of bismuth is recognized as a prototype two-dimensional topological insulator. Here we present a simple and well reproducible top-down approach to prepare a flat and well ordered bismuth bilayer with a lateral size of several hundred nanometers on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$(0001). Using scanning tunneling microscopy, surface x-ray diffraction, and Auger electron spectroscopy we show that exposure of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$(0001) to atomic hydrogen completely removes selenium from the top quintuple layer. The band structure of the system, calculated from first principles for the experimentally derived atomic structure, is in excellent agreement with recent photoemission data. Our results open interesting perspectives for the study of topological insulators in general.

Using the nonequilibrium Green's function method within the framework of density functional theory, we investigate various transport properties, such as $I-V$ characteristics, shot noise, thermopower, dynamical conductance, of Au- and Na-encapsulated ${\text{WSe}}_2$ nanotube heterojunctions. First-principles transport calculations show that from $I-V$ curves large rectification ratio is found in the (8,0) heterojunction and for shot noise it exhibits sub-Poissonian behaviors under positive biases (on Au-encapsulated tubes) while Poissonian behaviors are found under negative biases. For thermopower, it is found that as one sweeps the Fermi energy, the thermopower can change its sign. For dynamic conductance, the (5,5) heterojunction exhibits capacitivelike behavior. We find that the spin-orbit interaction (SOI) is very important for ${\text{WSe}}_2$ nanotubes. Due to the band splitting originated from SOI, the intrinsic band gap of Au-doped (5,5) nanotube is reduced by about 58% and that of the Na-doped system vanishes, while that of the doped (8,0) nanotubes decreases by about 40%. The reduction of band gap has an important impact on the transport properties. For instance, the transmission gap is decreased by about 48% and 16% in the transmission spectrum of the (5,5) and (8,0) heterojunctions, respectively. The current of the (5,5) heterojunction under small bias is almost doubled and the rectification ratio of the (8,0) heterojunction is enhanced by more than 120% due to SOI.

Vacancy properties in concentrated alloys continue to be of great interest because nowadays ab initio supercell simulations reach a scale where even defect properties in disordered alloys appear to be within reach. We show that vacancy properties cannot generally be extracted from supercell total energies in a consistent manner without a statistical model. Essential features of such a model are knowledge of the chemical potential and imposition of invariants. In the present work, we derive the simplest model that satisfies these requirements and we compare it with models in the literature. As illustration we compute ab initio vacancy properties of fcc Cu-Ni alloys as a function of composition and temperature. Ab initio density functional calculations were performed for SQS supercells at various compositions with and without vacancies. Various methods of extracting alloy vacancy properties were examined. A ternary cluster expansion yielded effective cluster interactions (ECIs) for the Cu-Ni-Vac system. Composition and temperature dependent alloy vacancy concentrations were obtained using statistical thermodynamic models with the ab initio ECIs. An Arrhenius analysis showed that the heat of vacancy formation was well represented by a linear function of temperature. The positive slope of the temperature dependence implies a negative configurational entropy contribution to the vacancy formation free energy in the alloy. These findings can be understood by considering local coordination effects.

We present a comprehensive study of polar and magnetic excitations in ${\mathrm{BiFeO}}_3$ ceramics and a thin film epitaxially grown on an orthorhombic (110) ${\mathrm{TbScO}}_3$ substrate. Infrared reflectivity spectroscopy was performed at temperatures from 5 to 900 K for the ceramics and below room temperature for the thin film. All 13 polar phonons allowed by the factor-group analysis were observed in the ceramic samples. The thin-film spectra revealed 12 phonon modes only and an additional weak excitation, probably of spin origin. On heating towards the ferroelectric phase transition near 1100 K, some phonons soften, leading to an increase in the static permittivity. In the ceramics, terahertz transmission spectra show five low-energy magnetic excitations including two which were not previously known to be infrared active; at 5 K, their frequencies are 53 and $56{\text{cm}}^{-1}$. Heating induces softening of all magnetic modes. At a temperature of 5 K, applying an external magnetic field of up to 7 T irreversibly alters the intensities of some of these modes. The frequencies of the observed spin excitations provide support for the recently developed complex model of magnetic interactions in ${\mathrm{BiFeO}}_3$ [R. S. Fishman, Phys. Rev. B 87, 224419 (2013)]. The simultaneous infrared and Raman activity of the spin excitations is consistent with their assignment to electromagnons.

We study the spin transport in the quantum Heisenberg spin chain subject to boundary magnetic fields and driven out of equilibrium by Lindblad dissipators. An exact solution is given in terms of matrix product states, which allows us to calculate exactly the spin current for any chain size. It is found that the system undergoes a discontinuous spin-valve-like quantum phase transition from ballistic to subdiffusive spin current, depending on the value of the boundary fields. Thus, the chain behaves as an extremely sensitive magnonic logic gate operating with the boundary fields as the base element.

We investigated the strong antiferromagnetic (AF) interlayer exchange coupling (IEC) at junctions between Fe(001) and ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)$ epitaxial films and the effects of an insertion layer of Mn or Co. In addition, we developed a nonlinear least-squares-fit technique to analyze the details of the magnetization processes and to distill the IEC constants. The analyses were performed by taking into account bilinear and biquadratic exchange couplings with a twisted magnetization state. The fitting results suggest the existence of significant distributions in bilinear and/or biquadratic IECs, which can originate from nonuniform stacking of atoms at the interface. The insertion of a Co layer with a thickness in the range of zero to two monolayers between the ${\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)$ and the Fe(001) layers significantly affects the magnetic-hysteresis (M-H) processes, whereas the insertion of a Mn layer does not. A quantitative fitting analysis of M-H curves suggests that the inserted thin Mn layer does not affect the AF-IEC significantly, whereas the Co layer suppresses the AF coupling and in fact is preferable for achieving ferromagnetic coupling, which is consistent with the tendency predicted by our theoretical model regarding impurity effects for the IEC of $\mathrm{Fe}/{\mathrm{Fe}}_3{\mathrm{O}}_4\left(001\right)$.

We consider a system of spins on the sites of a three-dimensional pyrochlore lattice of corner-sharing tetrahedra interacting with a predominant effective $xy$ exchange. In particular, we investigate the selection of a long-range ordered state with broken discrete symmetry induced by thermal fluctuations near the critical region. At the standard mean-field theory (s-MFT) level, in a region of the parameter space of this Hamiltonian that we refer to as ${\mathrm{\Gamma }}_5$ region, the ordered state possesses an accidental U(1) degeneracy. In this paper, we show that fluctuations beyond s-MFT lift this degeneracy by selecting one of two states (so-called ${\psi }_2$ and ${\psi }_3)$ from the degenerate manifold, thus exposing a certain form of order-by-disorder (ObD). We analytically explore this selection at the microscopic level and close to criticality by elaborating upon and using an extension of the so-called TAP method, originally developed by Thouless, Anderson, and Palmer to study the effect of fluctuations in spin glasses. We also use a single-tetrahedron cluster-mean-field theory (c-MFT) to explore over what minimal length scale fluctuations can lift the degeneracy. We find the phase diagrams obtained by these two methods to be somewhat different since c-MFT only includes the shortest-range fluctuations. General symmetry arguments used to construct a Ginzburg-Landau theory to lowest order in the order parameters predict that a weak magnetic moment $m_z$ along the local $\langle 111\rangle$ $\left(\widehat{z}\right)$ direction is generically induced for a system ordering into a ${\psi }_2$ state, but not so for ${\psi }_3$ ordering. Both E-TAP and c-MFT calculations confirm this weak fluctuation-induced $m_z$ moment. Using a Ginzburg-Landau theory, we discuss the phenomenology of multiple phase transitions below the paramagnetic phase transition and within the ${\mathrm{\Gamma }}_5$ long-range ordered phase.

We report structural and physical properties of a new quasi-one-dimensional Cr-based compound, ${\mathrm{KCr}}_3{\mathrm{As}}_3$, which is prepared by potassium deintercalation from the superconductive ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. ${\mathrm{KCr}}_3{\mathrm{As}}_3$ adopts the ${\mathrm{TlFe}}_3{\mathrm{Te}}_3$-type structure with space group $P{6}_3/m$ (No. 176). The high-temperature magnetic susceptibility obeys the Curie-Weiss law with an effective magnetic moment of 0.68 ${\mu }_{\mathrm{B}}$/Cr. Below 56 K the susceptibility deviates from the high-temperature Curie-Weiss behavior, coinciding with the rapid increase in resistivity, which suggests formation of spin clusters. The short-range spin correlations are also supported by the specific-heat data. The title material does not exhibit bulk superconductivity; instead, it shows a cluster spin-glass state below $\sim 5$ K.

Spin waves in perpendicularly magnetized ${\text{Pt/Co/AlO}}_x/\text{Pt}$ ultrathin films with varying Co thicknesses (0.6–1.2 nm) have been studied with Brillouin light spectroscopy in the Damon-Eshbach geometry. The measurements reveal a pronounced nonreciprocal propagation, which increases with decreasing Co thickness. This nonreciprocity, attributed to an interfacial Dzyaloshinskii-Moriya interaction (DMI), is significantly stronger than asymmetries resulting from surface anisotropies for such modes. Results are consistent with an interfacial DMI constant $D_{\mathrm{s}}=-1.7\pm 0.11\text{pJ}$/m, which favors left-handed chiral spin structures. This suggests that such films below 1 nm in thickness should support chiral states such as skyrmions at room temperature.

The doping of the honeycomb lattice compound ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ with cations having smaller ionic radius such as lithium was supposed to drive the system into a Kitaev spin liquid phase. Even at the smallest doping level the long range magnetic order, present in the parent compound, got suppressed as evidenced by susceptibility and heat capacity measurements. At low doping levels, the long range ordered magnetic phase is accompanied by the formation of a glassy state with a transition temperature of $T_F\sim 5 \mathrm{K}$ as determined by dc and ac magnetic susceptibility. The magnetic excitation spectrum, measured by inelastic neutron scattering, shows a significant change with doping. Our inelastic neutron scattering data can be well described by linear spin wave theory, revealing the formation of a spiral magnetic phase.