We analyze the Hall conductivity ${\sigma }_{xy}\left(\omega \right)$ of a charge ordered state with momentum $\mathbf{Q}=(0,2Q)$ and calculate the intrinsic contribution to the Kerr angle ${\Theta }_K$ using the fully reconstructed tight-binding band structure for layered cuprates beyond the low energy hot spot model and particle-hole symmetry. We show that such a unidirectional charge density wave (CDW), which breaks time-reversal symmetry, as recently put forward by Wang and Chubukov [Phys. Rev. B 90, 035149 (2014)], leads to a nonzero polar Kerr effect, as observed experimentally. In addition, we model a fluctuating CDW via a large quasiparticle damping of the order of the CDW gap and discuss possible implications for the pseudogap phase. We can qualitatively reproduce previous measurements of underdoped cuprates, but making quantitative connections to experiments is hampered by the sensitivity of the polar Kerr effect with respect to the complex refractive index $n\left(\omega \right)$.

Recent investigations have shown that ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ can be made superconducting by annealing it in Se and O vapors. The current lore is that these chalcogen vapors induce superconductivity by removing the magnetic excess Fe atoms. To investigate this phenomenon, we performed a combination of magnetic susceptibility, specific heat, and transport measurements together with scanning tunneling microscopy and spectroscopy and density functional theory calculations on ${\mathrm{Fe}}_{1+y}{\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$ treated with Te vapor. We conclude that the main role of the Te vapor is to quench the magnetic moments of the excess Fe atoms by forming ${\mathrm{FeTe}}_m$ ($m\ge 1$) complexes. We show that the remaining ${\mathrm{FeTe}}_m$ complexes are still damaging to the superconductivity and therefore that their removal potentially could further improve superconductive properties in these compounds.

We present the Raman scattering spectra of the ${\mathrm{BaFe}}_2{\mathrm{X}}_3(\mathrm{X}=\mathrm{S},\mathrm{Se})$ compounds in a temperature range between 20 and 400 K. Although the crystal structures of these two compounds are both orthorhombic and very similar, they are not isostructural. The unit cell of ${\mathrm{BaFe}}_2{\mathrm{S}}_3({\mathrm{BaFe}}_2{\mathrm{Se}}_3$) is base-centered $Cmcm$ (primitive $Pnma$), giving 18 (36) modes to be observed in the Raman scattering experiment. We have detected almost all Raman active modes, predicted by factor group analysis, which can be observed from the cleavage planes of these compounds. Assignment of the observed Raman modes of ${\mathrm{BaFe}}_2{\mathrm{S}\left(\mathrm{Se}\right)}_3$ is supported by the lattice dynamics calculations. The antiferromagnetic long-range spin ordering in ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ below $T_N=255\mathrm{K}$ leaves a fingerprint both in the $A_{1g}$ and $B_{3g}$ phonon mode linewidth and energy.

We have studied the phonon spectrum of ${\text{Mg}}_2{\text{Si}}_{1-x}{\text{Sn}}_x$ alloys using both density-functional theory calculations and neutron measurements. In the low tin concentration limit a resonance is obtained in the lower part of the spectrum. At higher concentrations comparison between theory and experiments shows that a simple averaging of the force constants of the perfect crystals ${\text{Mg}}_2\text{Si}$ and ${\text{Mg}}_2\text{Sn}$ can explain the main features of the density of states. However we suggest that the tin resonance persists up to large tin concentrations and may therefore be important for transport properties.

We calculate the nonequilibrium local density of states on a vibrational quantum dot coupled to two electrodes at $T=0$ using a numerically exact diagrammatic Monte Carlo method. Our focus is on the interplay between the electron-phonon interaction strength and the bias voltage. We find that the spectral density exhibits a significant voltage dependence if the voltage window includes one or more phonon sidebands. A comparison with well-established approximate approaches indicates that this effect could be attributed to the nonequilibrium distribution of the phonons. Moreover, we discuss the long transient dynamics caused by the electron-phonon coupling.

We study the electronic and magnetic structures of multiferroic ${\mathrm{CaMn}}_7{\mathrm{O}}_{12}$ by first-principles calculations, based on the experimentally determined modulated crystal structure. We confirm the presence of a $3d$ orbital modulation of the Mn2 ${\text{(Mn}}^{3+}$) sites, previously inferred from the Jahn-Teller crystal distortions. Our results indicate that in the multiferroic phase the magnetic structure of the Mn3 ${\text{(Mn}}^{4+}$) sites is anharmonically modulated via orbitally mediated coupling with the structural modulation, and that the Dzyaloshinskii-Moriya and exchange striction mechanisms contribute equally to the polarization.

Picosecond switching of the staggered antiferromagnetic order is shown to be realizable through spin-transfer torques from a short current pulse. The coupled dynamics of sublattice magnetization is mapped onto a classical pendulum subject to gravity and a driving pulse, where switching occurs if the pendulum acquires sufficient kinetic energy during the pulse to overcome the maximum of the effective gravity potential. The optimal switching scheme is explored through the dependence of switch angle and magnetic loss on the duration and strength of the current pulse. The physics discussed here provides a general route towards multifunctional THz applications via the spin-transfer torque in antiferromagnetic materials.

We present a general study of the magnetic excitations within a weak-coupling five-orbital model relevant to itinerant iron pnictides. As a function of enhanced electronic correlations, the spin excitations in the symmetry-broken spin-density wave phase evolve from broad low-energy modes in the limit of weak interactions to sharply dispersing spin waves prevailing to higher energies at larger interaction strengths. We show how the resulting spin response at high energies depends qualitatively on the magnitude of the interactions. We also calculate the magnetic excitations in the nematic phase by including an orbital splitting, and find a pronounced $C_2$-symmetric excitation spectrum right above the transition to long-range magnetic order. Finally, we discuss the $C_2$ versus $C_4$ symmetry of the spin excitations as a function of energy for both the nematic and the spin-density wave phase.

We studied the magnetic properties of single-crystal ${\mathrm{Yb}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ using dc and ac susceptibility measurements, elastic and inelastic neutron-scattering measurements, and linear spin-wave theory. The experimental data show a ferromagnetic ordering of ${\mathrm{V}}^{4+}$ ions at 70 K, a short-range ordering of ${\mathrm{Yb}}^{3+}$ ions below 40 K, and finally a long-range noncollinear ordering of ${\mathrm{Yb}}^{3+}$ ions below 15 K. With external magnetic field oriented along the [111] axis, the Yb sublattice experiences a spin flop transition related to the “three-in one-out” spin structure. By modeling the spin-wave excitations, we extract the Hamiltonian parameters. Our results confirm that although the extra inter-sublattice Yb-V interactions dramatically increase the Yb ordering temperature to 15 K, the intra-sublattice Yb-Yb interactions, based on the pyrochlore lattice, still stabilize the Yb ions' noncollinear spin structure and spin flop transition.

We report a systematic study of nonlinearity in the ferromagnetic resonance of a series of submicron Permalloy ellipses with varying aspect ratios. At high excitation powers, the resonances are found to shift to higher or lower applied field. We focus here on the sign of the shift and its dependence on the applied field and shape-induced anisotropy of the ellipses. Using ferromagnetic resonance force microscopy, we find that the measured nonlinear coefficient changes sign as a function of anisotropy field and applied field in qualitative agreement with a macrospin analysis. This macrospin analysis also points to origins of the nonlinearity in a combination of hard-axis in-plane anisotropy and precession ellipticity. In comparison of the macrospin predictions with both experimental and micromagnetic modeling results, we measure/model values of the nonlinear coefficient that are more positive than predicted by the macrospin model. The results are useful in understanding nonlinear physics in nanomagnets and applications of spin-torque oscillators.

A unified molecular field theory (MFT) is presented that applies to both collinear and planar noncollinear Heisenberg antiferromagnets (AFs) on the same footing. The spins in the system are assumed to be identical and crystallographically equivalent. This formulation allows calculations of the anisotropic magnetic susceptibility $\chi$ versus temperature $T$ below the AF ordering temperature $T_{\mathrm{N}}$ to be carried out for arbitrary Heisenberg exchange interactions $J_{ij}$ between arbitrary neighbors $j$ of a given spin $i$ without recourse to magnetic sublattices. The Weiss temperature ${\theta }_{\mathrm{p}}$ in the Curie-Weiss law is written in terms of the $J_{ij}$ values and $T_{\mathrm{N}}$ in terms of the $J_{ij}$ values and an assumed AF structure. Other magnetic and thermal properties are then expressed in terms of quantities easily accessible from experiment as laws of corresponding states for a given spin $S$. For collinear ordering these properties are the reduced temperature $t=T/T_{\mathrm{N}}$, the ratio $f={\theta }_{\mathrm{p}}/T_{\mathrm{N}}$, and $S$. For planar noncollinear helical or cycloidal ordering, an additional parameter is the wave vector of the helix or cycloid. The MFT is also applicable to AFs with other AF structures. The MFT predicts that $\chi (T\le T_{\mathrm{N}})$ of noncollinear ${120}^{\circ }$ spin structures on triangular lattices is isotropic and independent of $S$ and $T$ and thus clarifies the origin of this universally observed behavior. The high-field magnetization and heat capacity for fields applied perpendicular to the ordering axis (collinear AFs) and ordering plane (planar noncollinear AFs) are also calculated and expressed for both types of AF structures as laws of corresponding states for a given $S$, and the reduced perpendicular field versus reduced temperature phase diagram is constructed.

We discuss a generalized self-consistent mean-field (MF) treatment, based on the selection of an arbitrary subset of operators for representing the system density matrix, and its application to the problem of entanglement evaluation in composite quantum systems. As a specific example, we examine in detail a pair MF approach to the ground state (GS) of dimerized spin-$1/2$ systems with anisotropic ferromagnetic-type $XY$ and $XYZ$ couplings in a transverse field, including chains and arrays with first neighbor and also longer range couplings. The approach is fully analytic and able to capture the main features of the GS of these systems, in contrast with the conventional single-spin MF. Its phase diagram differs significantly from that of the latter, exhibiting $\left(S_z\right)$ parity breaking just in a finite field window if the coupling between pairs is sufficiently weak, together with a fully dimerized phase below this window and a partially aligned phase above it. It is then shown that through symmetry restoration, the approach is able to correctly predict not only the concurrence of a pair, but also its entanglement with the rest of the chain, which shows a pronounced peak in the parity breaking window. Perturbative corrections allow to reproduce more subtle observables like the entanglement between weakly coupled spins and the low lying energy spectrum. All predictions are tested against exact results for finite systems.

We present first-principles calculations on the superconductivity of solid argon under high pressure. Solid argon is found to take the double hexagonal close-packed structure in pressure range from 420 to 690 GPa, where an insulator-to-metal transition occurs at around 590 GPa. The crystal structure transforms into the hexagonal close-packed structure at 690 GPa and into the face-centered cubic structure at 2300 GPa. The superconducting critical temperature is gradually increased with the successive phase transitions and reaches the maximum value of 12 K at 2600 GPa due to the enhancement of the Fermi surface nesting.

The superconducting (SC) phase in phase-separated (PS) ${\mathrm{K}}_{0.8}\mathrm{F}{\mathrm{e}}_{1.6+x}\mathrm{S}{\mathrm{e}}_2(0\le x\le 0.15)$ materials is found to crystallize on Archimedean solidlike frameworks, and this structural feature originates from a spinodal phase separation (SPS) at around $T_s\approx 540\mathrm{K}$, depending slightly on the Fe concentration, as revealed by in situ heating TEM observations and shown in a phase diagram. Two stable phases in ${\mathrm{K}}_{0.8}\mathrm{F}{\mathrm{e}}_{1.6+x}\mathrm{S}{\mathrm{e}}_2$ are demonstrated to be the SC ${\mathrm{K}}_{0.5}\mathrm{F}{\mathrm{e}}_2\mathrm{S}{\mathrm{e}}_2$ and antiferromagnetic (AFM) ${\mathrm{K}}_{0.8}\mathrm{F}{\mathrm{e}}_{1.6}\mathrm{S}{\mathrm{e}}_2$. The spinodal waves go along the systematic $\left[113\right]$ direction and result in notable lamellar structure as illustrated by strain-field theoretical simulations. The three-dimensional SC framework is constructed with hollow truncated octahedron similar to that discussed for Archimedean solids. Based on this structural model, we can efficiently calculate the volume fraction of the SC phase in this type of PS SC material.

Self-consistent $GW$ calculations and the solution of the Bethe-Salpeter equation are to date the best available approaches to simulate electronic excitations in a vast class of materials, ranging from molecules to solids. However, up to now numerical instabilities made it impossible to use these techniques to calculate optical absorption spectra of the best-known thin-film absorbers for solar cells: ${\mathrm{Cu}(\mathrm{In},\mathrm{Ga})(\mathrm{S},\mathrm{Se})}_2$ chalcopyrites and ${\mathrm{Cu}}_2{\mathrm{ZnSn}(\mathrm{S},\mathrm{Se})}_4$ kesterites/stannites. We show here how to solve this problem using a finite-difference method in $k$ space to evaluate the otherwise diverging dipole matrix elements, obtaining an excellent agreement with experiments. Having established the validity of this approach, we use it then to calculate the optical response of the less studied, but promising, ${\mathrm{Cu}}_2{\mathrm{ZnGe}(\mathrm{S},\mathrm{Se})}_4$ compounds, opening the way to predictive calculations of still unknown materials.

We note an implication of the chiral Luttinger liquid based edge state description of the fractional quantum Hall effect. By considering several examples that involve backward moving neutral modes, arising from either composite fermions with reverse flux attached or edge reconstruction, we show that nonuniversality of the edge exponent implies nonuniversality of the Hall conductance, as measured in the two-terminal conductance.

Charge dynamics in one-dimensional (1D) Mott insulators was investigated by femtosecond pump-probe absorption spectroscopy on ${\mathrm{Ca}}_2{\mathrm{CuO}}_3$. An irradiation of a femtosecond laser pulse gives rise to a Drude-like response due to the nature of spin-charge separation characteristic of 1D Mott insulators. The photoinduced metallic state decays with ∼30 fs, and photocarriers are localized as polarons via charge-phonon coupling which produce a broad midgap absorption. Calculations with the extended Hubbard-Holstein model suggest that the peak structure of the midgap absorption is a magnon sideband of the polaron absorption. This demonstrates that charge-spin coupling becomes effective via charge-phonon coupling in polarons.

Based on electronic structure calculations and theoretical analysis, we predict the (111) thin films of the SnTe class of three-dimensional (3D) topological crystalline insulators (TCIs) realize the quantum spin Hall phase in a wide range of thicknesses. The nontrivial topology originates from the intersurface coupling of the topological surface states of TCIs in the 3D limit. The intersurface coupling changes sign and gives rise to topological phase transitions as a function of film thickness. Furthermore, this coupling can be strongly affected by an external electric field, hence the quantum spin Hall phase can be effectively tuned under the experimentally accessible electric field.

In the Luttinger model with nonlocal interaction we investigate, by exact analytical methods, the time evolution of an inhomogeneous state with a localized fermion added to the noninteracting ground state. In the absence of interaction the averaged density has two peaks moving in opposite directions with constant velocities. If the state is evolved with the interacting Hamiltonian, two main effects appear. The first is that the peaks have velocities which are not constant but vary between a minimal and maximal value. The second is that a dynamical “Landau quasiparticle weight” appears in the oscillating part of the averaged density, asymptotically vanishing with time, as a consequence of the fact that fermions are not excitations of the interacting Hamiltonian.