Intraband and interband contributions to the current-driven spin-orbit torque in magnetic materials lacking inversion symmetry are theoretically studied using the Kubo formula. In addition to the current-driven fieldlike torque ${\mathbf{T}}_{\mathrm{FL}}={\tau }_{\mathrm{FL}}\mathbf{m}\times {\mathbf{u}}_{\mathrm{so}}$ (${\mathbf{u}}_{\mathrm{so}}$ being a unit vector determined by the symmetry of the spin-orbit coupling), we explore the intrinsic contribution arising from impurity-independent interband transitions and producing an anti-damping-like torque of the form ${\mathbf{T}}_{\mathrm{DL}}={\tau }_{\mathrm{DL}}\mathbf{m}\times ({\mathbf{u}}_{\mathrm{so}}\times \mathbf{m})$. Analytical expressions are obtained in the model case of a magnetic Rashba two-dimensional electron gas, while numerical calculations have been performed on a dilute magnetic semiconductor (Ga,Mn)As modeled by the Kohn-Luttinger Hamiltonian exchange coupled to the Mn moments. Parametric dependencies of the different torque components and similarities to the analytical results of the Rashba two-dimensional electron gas in the weak disorder limit are described.

We discuss the energy and vorticity spectra of turbulent superfluid ${}^4\mathrm{He}$ in the entire temperature range from $T=0$ up to the phase transition “$\lambda$ point,” $T_{\lambda }\simeq 2.17\mathrm{K}$. Contrary to classical developed turbulence in which there are only two typical scales, i.e., the energy injection $L$ and the dissipation scales $\eta$, here, the quantization of vorticity introduces two additional scales, the vortex core radius $a_0$ and the mean vortex spacing $\ell$. We present these spectra for the super- and the normal-fluid components in the entire range of scales from $L$ to $a_0$ including the crossover scale $\ell$ where the hydrodynamic eddy cascade is replaced by the cascade of Kelvin waves on individual vortices. At this scale, a bottleneck accumulation of the energy was found earlier at $T=0$. We show that even very small mutual friction dramatically suppresses the bottleneck effect due to the dissipation of the Kelvin waves. Using our results for the spectra we estimate the Vinen “effective viscosity” ${\nu }^{\prime }$ in the entire temperature range and show agreement with numerous experimental observations for ${\nu }^{\prime }\left(T\right)$.

We present a systematic theoretical investigation of the surface properties, stability, and reactivity of rocksalt type alkaline-earth metal oxides including MgO, CaO, SrO, and BaO. The accuracy of commonly used exchange-correlation density functionals (LDA, PBE, RPBE, PBEsol, BEEF-vdW, and hybrid HSE) and random-phase approximation (RPA) is evaluated and compared to existing experimental values. Calculated surface energies of the four most stable surface facets under vacuum conditions, the (100) surface, the metal and oxygen terminated octopolar (111), and the (110) surfaces, exhibit a monotonic increase in stability from MgO to BaO. On the MgO(100) surface, adsorption of CO, NO, and ${\text{CH}}_4$ is characterized by physisorption while ${\text{H}}_2\text{O}$ chemisorbs, which is in agreement with experimental findings. We further use the on-top metal adsorption of CO and NO molecules to map out the surface energetics of each alkaline-earth metal oxide surface. The considered functionals all qualitatively predict similar adsorption energy trends. The ordering between the adsorption energies on different surface facets can be attributed to differences in the local geometrical surface structure and the electronic structure of the metal constituent of the alkaline-earth metal oxide. The striking observation that CO adsorption strength is weaker than NO adsorption on the (100) terraces as the period of the alkaline-earth metal in the oxide increases is analyzed in detail in terms of charge redistribution within the $\sigma$ and $\pi$ channels of adsorbates. Finally, we also present oxygen adsorption and oxygen vacancy formation energies in these oxide systems.

We use angle-resolved photoemission spectroscopy (ARPES) to study the two-dimensional (2D) heavy-fermion superconductor, ${\mathrm{Ce}}_2{\mathrm{RhIn}}_8$. The Fermi surface is rather complicated and consists of several hole and electron pockets with one of the sheets displaying strong nesting properties with a $\mathbf{q}$ vector of (0.32, 0.32) $\pi /a$. We do not observe $k_z$ dispersion of the Fermi sheets, which is consistent with the expected 2D character of the electronic structure. Comparison of the ARPES data to band-structure calculations suggests that a localized picture of the $f$ electrons works best. While there is some agreement in the overall band dispersion and location of the Fermi sheets, the model does not reproduce all observed bands and is not completely accurate for those it does. Our data paves the way for improving the band-structure calculations and the general understanding of the transport and thermodynamical properties of this material.

We present a fully analytic theory for nanorod resonances including the phase of reflection from the rounded ends using a transmission line approach. It combines the circuit theory response of spherical nanoparticles with standard transmission line theory using the Sommerfeld wave dispersion. The approach agrees well with comprehensive numerical calculations.

The vibrational behavior of heavy substitutional impurities $(M=\text{Ir},\text{Os})$ in ${\text{Fe}}_{1-x}{M}_x\text{Si}\left(x=0,0.02,0.04,0.1\right)$ was investigated with a combination of inelastic neutron scattering (INS), transport measurements, and first-principles simulations. Our INS measurements on single crystals mapped the four-dimensional dynamical structure factor, $S(\mathbf{Q},E)$, for several compositions and temperatures. Our results show that both Ir and Os impurities lead to the formation of a weakly dispersive resonance vibrational mode, in the energy range of the acoustic phonon dispersions of the FeSi host. We also show that Ir doping, which introduces free carriers, leads to softened interatomic force constants compared to doping with Os, which is isoelectronic to Fe. We analyze the phonon $S(\mathbf{Q},E)$ from INS through a Green's-function model incorporating the phonon self-energy based on first-principles density functional theory simulations, and we study the disorder-induced lifetimes on large supercells. Calculations of the quasiparticle spectral functions in the doped system reveal the hybridization between the resonance and the acoustic phonon modes. Our results demonstrate a strong interaction of the host acoustic dispersions with the resonance mode, likely leading to the large observed suppression in lattice thermal conductivity.

We discuss the mechanism of coherent phonon generation by stimulated ionic Raman scattering, a process different from conventional excitation with near visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling between a directly excited infrared-active phonon mode and other Raman modes. We experimentally study the response of $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{6+x}$ to the resonant excitation of apical oxygen motions at 20 THz by midinfrared pulses, which has been shown in the past to enhance the interlayer superconducting coupling. We find coherent oscillations of four totally symmetric $\left({\mathrm{A}}_{\mathrm{g}}\right)$ Raman modes and make a critical assessment of the role of these oscillatory motions in the enhancement of superconductivity.

The effects of a chemical alteration of the antiferromagnet surface in the exchange-biased ultrathin epitaxial antiferromagnetic/ferromagnetic bilayer Mn/Co/Cu(001) have been investigated. The formation of a nanometer-thick paramagnetic oxide at the surface of the Mn layer was induced through the controlled exposure to oxygen. Although the buried Mn/Co interface was not affected by the oxidation, strong changes of the exchange-bias properties of the system, which cannot be accounted for by the mere reduction of the antiferromagnetic Mn thickness, were observed. We propose that the produced surface modification affects the magnetic configuration of the near-surface region and the domain structure in the whole antiferromagnet layer which, in turn, influences the pinned uncompensated spin density responsible for the exchange bias.

We consider the low-temperature thermal transport properties of the two-dimensional (2D) proximity-induced superconducting state formed at the interface between a three-dimensional strong topological insulator (TI) and a $d$-wave superconductor (dSC). This system is a playground for studying massless Dirac fermions, because they enter both as quasiparticles of the dSC and as surface states of the TI. For TI surface states with a single Dirac point, the four nodes in the interface-state quasiparticle excitation spectrum coalesce into a single node as the chemical potential $\mu$ is tuned from above the impurity scattering rate $\left(\left|\mu \right|\gg {\Gamma }_0\right)$ to below $\left(\left|\mu \right|\ll {\Gamma }_0\right)$. We calculate, via Kubo formula, the universal-limit $\left(T\to 0\right)$ thermal conductivity ${\kappa }_0$ as a function of $\mu$ as it is tuned through this transition. In the large- and small-$\left|\mu \right|$ limits, we obtain disorder-independent, closed-form expressions for ${\kappa }_0/T$. The large-$\left|\mu \right|$ expression is exactly half the value expected for a $d$-wave superconductor, a demonstration of the sense in which the TI surface topological metal is half of an ordinary 2D electron gas. Our numerical results for intermediate $\left|\mu \right|$ illustrate the nature of the transition between these limits, which is shown to depend on disorder in a well-defined manner.

We report the results of pressure-induced phase transitions and metallization in ${\mathrm{VO}}_2$ based on synchrotron x-ray diffraction, electrical resistivity, and Raman spectroscopy. Our isothermal compression experiments at room temperature and 383 K show that the room temperature monoclinic phase $(M1,P{2}_1/c)$ and the high-temperature rutile phase $(R,P{4}_2/mnm)$ of ${\mathrm{VO}}_2$ undergo phase transitions to a distorted $M1$ monoclinic phase $(M{1}^{\prime },P{2}_1/c)$ above 13.0 GPa and to an orthorhombic phase (${\mathrm{CaCl}}_2$-like, $Pnnm$) above 13.7 GPa, respectively. Upon further compression, both high-pressure phases transform into a new phase (phase $X$) above 34.3 and 38.3 GPa at room temperature and 383 K, respectively. The room temperature $M1\text{−}M{1}^{\prime }$ phase transition structurally resembles the $R\text{−}{\mathrm{CaCl}}_2$ phase transition at 383 K, suggesting a second-order displacive type of transition. Contrary to previous studies, our electrical resistivity results, Raman measurements, as well as ab initio calculations indicate that the new phase $X$, rather than the $M{1}^{\prime }$ phase, is responsible for the metallization under pressure. The metallization mechanism is discussed based on the proposed crystal structure.

We discuss the compressibility in the almost staggered fermionic Harper model with repulsive interactions in the vicinity of half-filling. It has been shown by Kraus et al. [Phys. Rev. B 89, 161106(R) (2014)] that for spinless electrons and nearest neighbors electron-electron interactions the compressibility in the central band is enhanced by repulsive interactions. Here we would like to investigate the sensitivity of this conclusion to the spin degree of freedom and longer range interactions. We use the Hartree-Fock (HF) approximation, as well as the density matrix renormalization group (DMRG) calculation to evaluate the compressibility. In the almost staggered Harper model, the central energy band is essentially flat and separated from the other bands by a large gap and therefore, the HF approximation is rather accurate. In both cases the compressibility of the system is enhanced compared to the noninteracting case, although the enhancement is weaker due to the inclusion of Hubbard and longer ranged interactions. We also show that the entanglement entropy is suppressed when the compressibility of the system is enhanced.

Electronic, magnetic, and thermodynamic properties of ${\text{Co}}_2\text{Cr(Ga,Si)}$-based shape-memory alloys, which exhibit reentrant martensitic transformation (RMT) behavior, were studied experimentally. For electric resistivity (ER), an inverse (semiconductor-like) temperature dependence in the parent phase was found, along with anomalous behavior below its Curie temperature. A pseudobinary phase diagram was determined, which gives a “martensite loop” clearly showing the reentrant behavior. Differential scanning calorimetry and specific-heat measurements were used to derive the entropy change $\Delta S$ between martensite and parent phases. The temperature dependence of the derived $\Delta S$ was analyzed thermodynamically to confirm the appearances of both the RMT and normal martensitic transformation. Detailed studies on the specific heat in martensite and parent phases at low temperatures were also conducted.

We construct a Holstein-Primakoff Hamiltonian for magnons in an arbitrary slowly varying spin background for a microscopic spin Hamiltonian consisting of ferromagnetic spin exchange, Dzyaloshinskii-Moriya exchange, and the Zeeman term. The Gross-Pitaevskii-type equation for magnon dynamics contains several background gauge fields pertaining to local spin chirality, inhomogeneous potential, and anomalous scattering that violates the boson number conservation. Nontrivial corrections to previous formulas derived in the literature are given. Subsequent mapping to hydrodynamic fields yields the continuity equation and the Euler equation of the magnon fluid dynamics. Magnon wave scattering off a localized Skyrmion is examined numerically based on our Gross-Pitaevskii formulation. The dependence of the effective flux experienced by the impinging magnon on the Skyrmion radius is pointed out, and compared to an analysis of the same problem using the Landau-Lifshitz-Gilbert equation.

The nanoscale structural, chemical, and electronic properties of artificial engineered superlattice thin films consisting of superconducting Co-doped $\mathrm{BaF}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2\left(\mathrm{Ba}\text{−}122\right)$ and nonsuperconducting $\mathrm{SrTi}{\mathrm{O}}_3\left(\mathrm{STO}\right)$ layers are determined by using atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy. The bonding of $\mathrm{Ba}\text{−}122/\mathrm{STO}$ occurring between As (Ba) and $\mathrm{SrO}\left(\mathrm{Ti}{\mathrm{O}}_2\right)$ terminated layers has been identified. The thin STO (3 unit cell) insertion layers are a mixture of cations (Ba, Sr, Fe, and Ti) and rich in oxygen vacancies and the Ba-122 layers (10 unit cell) are free of vertical second phases. Our results explain why these superlattices show anisotropic transport response to an external magnetic field, i.e., strong $ab$-axis pinning (enhancing critical current density) and no $c$-axis pinning, which is opposite to single layer Ba-122 thin films. These findings reveal physical and chemical properties of superconducting/nonsuperconducting heterostructures and provide important insights into engineering of superconducting devices.

We examine trends in the wave vectors and form factors of charge density wave instabilities of three-band models of the underdoped cuprates. For instabilities from a high-temperature state with a large Fermi surface, we extend a study by Bulut et al. [Phys. Rev. B 88, 155132 (2013)] to include a direct antiferromagnetic exchange coupling between the Cu sites. As in previous work, we invariably find that the primary instability has a diagonal wave vector $(\pm Q_0,\pm Q_0)$ and a $d$-form factor. The experimentally observed wave vectors along the principal axes $(\pm Q_0,0)$, $(0,\pm Q_0)$ have higher energy, but they also have a predominantly $d$-form factor. Next, we gap out the Fermi surface in the antinodal regions of the Brillouin zone by including static, long-range antiferromagnetic order at the wave vector $(\pi ,\pi )$: this is a simple model of the pseudogap in which we assume the antiferromagnetic order averages to zero by “renormalized classical” thermal fluctuations in its orientation, valid when the antiferromagnetic correlation length is large. The charge density wave instabilities of this pseudogap state are found to have the optimal wave vector $(\pm Q_0,0)$, $(0,\pm Q_0)$, with the magnitude of the $d$-form factor decreasing with increasing magnetic order.

Understanding electronic properties of substoichiometric phases of titanium oxide such as Magnéli phase $\mathrm{T}{\mathrm{i}}_4{\mathrm{O}}_7$ is crucial in designing and modeling resistive switching devices. Here we present our study on Magnéli phase $\mathrm{T}{\mathrm{i}}_4{\mathrm{O}}_7$ together with rutile $\mathrm{Ti}{\mathrm{O}}_2$ and $\mathrm{T}{\mathrm{i}}_2{\mathrm{O}}_3$ using density functional theory methods with atomic-orbital-based self-interaction correction (ASIC). We predict a new antiferromagnetic (AF) ground state in the low temperature (LT) phase, and we explain energy difference with a competing AF state using a Heisenberg model. The predicted energy ordering of these states in the LT phase is calculated to be robust in a wide range of modeled isotropic strain. We have also investigated the dependence of the electronic structures of the Ti-O phases on stoichiometry. The splitting of titanium $t_{2g}$ orbitals is enhanced with increasing oxygen deficiency as Ti-O is reduced. The electronic properties of all these phases can be reasonably well described by applying ASIC with a “standard” value for transition metal oxides of the empirical parameter α of 0.5 representing the magnitude of the applied self-interaction correction.

We link linear prediction of Chebyshev and Fourier expansions to analytic continuation. We push the resolution in the Chebyshev-based computation of $T=0$ many-body spectral functions to a much higher precision by deriving a modified Chebyshev series expansion that allows to reduce the expansion order by a factor $\sim \frac{1}{6}$. We show that in a certain limit the Chebyshev technique becomes equivalent to computing spectral functions via time evolution and subsequent Fourier transform. This introduces a novel recursive time-evolution algorithm that instead of the group operator $e^{-iHt}$ only involves the action of the generator $H$. For quantum impurity problems, we introduce an adapted discretization scheme for the bath spectral function. We discuss the relevance of these results for matrix product state (MPS) based DMRG-type algorithms, and their use within the dynamical mean-field theory (DMFT). We present strong evidence that the Chebyshev recursion extracts less spectral information from $H$ than time evolution algorithms when fixing a given amount of created entanglement.

Given a local gapped Hamiltonian with a global symmetry on a one-dimensional lattice we describe a method to identify whether the Hamiltonian belongs to a quantum phase in which the symmetry is spontaneously broken in the ground states or to a specific symmetry-protected phase, without using local or string order parameters. We obtain different matrix product state (MPS) descriptions of the symmetric ground state(s) of the Hamiltonian by restricting the MPS matrices to transform under different equivalence classes of projective representations of the symmetry. The phase of the Hamiltonian is identified by examining which MPS descriptions, if any, are injective, namely, whether the largest eigenvalue of the transfer matrix obtained from the MPS is unique. We demonstrate the method for translationally invariant Hamiltonians with a global SO(3), $Z_2$, and $Z_2\times Z_2$ symmetry on an infinite chain.

A polarization degenerate microcavity containing charge-controlled quantum dots (QDs) enables equal coupling of all polarization degrees of freedom of light to the cavity QED system, which we explore through resonant laser spectroscopy. We first measure interference of the two fine-split neutral QD transitions and find very good agreement of this V-type three-level system with a coherent polarization-dependent cavity QED model. We also study a charged QD that suffers from decoherence and find also in this case that availability of the full-polarization degrees of freedom is crucial to reveal the dynamics of the QD transitions. Our results pave the way for postselection-free quantum devices based on electron-spin–photon polarization entanglement.

We study how the electron-hole pairing reveals itself in the response of a double layer graphene system to the vector and scalar potentials. Electron-hole pairing results in a rigid (London) relation between the current and the difference of vector potentials in two adjacent layers. The diamagnetic effect can be observed in multiple connected systems in the magnetic field parallel to the graphene layers. Such an effect would be considered as a hallmark of the electron-hole pairing, but the value of the effect is extremely small. Electron-hole pairing significantly changes the response to the scalar potential, as well. It results in a complete (at zero temperature) or partial (at finite temperature) suppression of screening of the electric field of a test charge situated at some distance to the double layer system. A strong increase of the electric field induced by the test charge under decrease in temperature can be considered as a spectacular hallmark of the electron-hole pairing.