A time periodic driving on a topologically trivial system induces edge modes and topological properties. In this work we consider triplet and singlet superconductors subject to periodic variations of the chemical potential, spin-orbit coupling, and magnetization in both topologically trivial and nontrivial phases, and study their influence on the charge and spin currents that propagate along the edges of the two-dimensional system, for moderate to large driving frequencies. Currents associated with the edge modes are induced in the trivial phases and enhanced in the topological phases. In some cases there is a sign reversal of the currents as a consequence of the periodic driving. The edge states associated with the finite quasienergy states at the edge of the Floquet zone are in general robust, while the stability of the zero quasienergy states depends on the parameters. Also, the spin polarization of the Floquet spectrum quasienergies is strong as for the unperturbed topological phases. It is found that in some cases the unperturbed edge states are immersed in a continuum of states due to the perturbation, particularly if the driving frequency is not large enough. However, their contribution to the edge currents and spin polarization is still significant.

We use tunable laser-based angle-resolved photoemission spectroscopy to study the electronic structure of the multiband superconductor ${\mathrm{MgB}}_2$. These results form the baseline for detailed studies of superconductivity in multiband systems. We find that the magnitude of the superconducting gap on both $\sigma$ bands follows a BCS-like variation with temperature with ${\mathrm{\Delta }}_0\sim 7\mathrm{meV}$. The value of the gap is isotropic within experimental uncertainty and in agreement with a pure $s$-wave pairing symmetry. We also observe in-gap states confined to $k_F$ of the $\sigma$ band that occur at some locations of the sample surface. The energy of this excitation, $\sim 3$ meV, is somewhat larger than the previously reported gap on $\pi$ Fermi sheet and therefore we cannot exclude the possibility of interband scattering as its origin.

Within density-functional theory, we apply an electronic-structure-based thermodynamic theory to calculate short-ranged order (SRO) in homogeneously disordered substitutional $N$-component alloys, and its electronic origin. Using the geometric properties of an $\left(N-1\right)$ simplex that describes the Gibbs (compositional) space, we derive the analytic transform of the SRO eigenvectors that provides a unique description of high-temperature SRO in $N$-component alloys and the incipient low-temperature long-range order. We apply the electronic-based thermodynamic theory and the new general analysis to ternaries ($A1$ Cu-Ni-Zn and $A2$ Nb-Al-Ti) for validation, and then to quinary Al-Co-Cr-Fe-Ni high-entropy alloys for predictive assessment.

We present the results of detailed dielectric investigations of the relaxation dynamics in ${\mathrm{DyMnO}}_3$ multiferroic manganite. A strong low-frequency relaxation process near the paraelectric-ferroelectric phase transition is observed. We provide experimental evidence to show that this relaxation mode corresponds to a chirality switching of the spin cycloid in ${\mathrm{DyMnO}}_3$. We demonstrate that the relaxation dynamics in ${\mathrm{DyMnO}}_3$ is typical for an order-disorder phase transition and may be understood within a simple model with a double-well potential. These results suggest the interpretation of the paraelectric sinusoidal phase in manganites as a dynamical equilibrium of magnetic cycloids with opposite chiralities.

The cubic noncentrosymmetric structure of the B20 compounds produces the helical (homochiral) structure with the wave vector $k_s=D/J$ balanced by the competition of two interactions: the large ferromagnetic exchange interaction $J$ and small antisymmetric Dzyaloshinskii-Moriya interaction $D$. The difference in the energies between the ferromagnetic collinear and helical states can be experimentally measured by the critical magnetic field $H_{c2}$ needed to transform the helix into the field-induced ferromagnet. We show that the cubic anisotropy, first, can limit the stability of the helix phase in the range of small $k_s$ and, second, makes its own contribution to the value of the critical field $H_{c2}$. We illustrate our findings taking an example of a transformation of the helix structure to the ferromagnet at $x\to x_c$ in the solid solutions ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Ge}$. We demonstrate that the mechanism of the transformation is realized via the competition between the cubic anisotropy and the Dzyaloshinskii-Moriya interaction.

We report time-of-flight neutron scattering measurements of the magnetic spectrum of ${\mathrm{Tb}}^{3+}$ in ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$. The data, which extend up to 120 meV and have calibrated intensity, enable us to consolidate and extend previous studies of the single-ion crystal field spectrum. We successfully refine a model for the crystal field potential in ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ without relying on data from other rare-earth titanate pyrochlores, and we confirm that the ground state is a non-Kramers doublet with predominantly $|\pm 4\rangle$ components. We compare the model critically with earlier models.

Topological chiral phases are ubiquitous in the physics of the fractional quantum Hall effect. Nonchiral topological spin liquids are also well known. Here, using the framework of projected entangled pair states (PEPS), we construct a family of chiral spin liquids on the square lattice, which are generalized spin-1/2 resonating valence bond (RVB) states obtained from deformed local tensors with $d+id$ symmetry. On a cylinder, we construct four topological sectors with even or odd number of spinons on the boundary and even or odd number of (${\mathbb{Z}}_2$) fluxes penetrating the cylinder, which, we argue, remain orthogonal in the limit of infinite perimeter. The analysis of the transfer matrix provides evidence of short-range (long-range) triplet (singlet) correlations as for the critical (nonchiral) RVB state. The entanglement spectrum exhibits chiral edge modes, which we confront to predictions of the conformal field theory, and the corresponding entanglement Hamiltonian is shown to be long-ranged.

Distinguishing the zero-bias peak (ZBP) caused by Majorana fermions from that caused by other effects remains a challenge in detecting the topological superconducting phase in a quantum wire. In this paper, we show that one can distinguish Majorana fermions from the trivial zero-energy Andreev bound states caused by impurities in the material by making a Josephson junction of the quantum wire attached to a side lead and then measuring the tunneling conductance through it as the phase difference across the junction $\varphi$ varies. We calculate the tunneling conductance in the presence of an additional zero-energy impurity level at the Josephson junction. Even if the ZBPs exist in both the topological and trivial phases, we can identify the topological superconducting phase by a conductance peak at $\varphi =\pi$ and a nearby butterfly pattern.

We present a high-energy-resolution inelastic x-ray scattering investigation of the soft phonon mode in the charge-density-wave (CDW) system $\mathrm{TbT}{\mathrm{e}}_3$. We analyze our data based on lattice dynamical calculations using density-functional-perturbation theory and find clear evidence that strongly momentum-dependent electron-phonon coupling defines the periodicity of the CDW superstructure: Our experiment reveals strong phonon softening and increased phonon linewidths over a large part in reciprocal space adjacent to the CDW ordering vector ${\mathit{q}}_{\mathrm{CDW}}=\left(0,0,0.3\right)$. Further, ${\mathit{q}}_{\mathrm{CDW}}$ is clearly offset from the wave vector of (weak) Fermi surface nesting ${\mathit{q}}_{\mathrm{FS}}=(0,0,0.25)$, and our detailed analysis indicates that electron-phonon coupling is responsible for this shift. Hence, we can add $\mathrm{TbT}{\mathrm{e}}_3$, which was previously considered as a canonical CDW compound following the Peierls scenario, to the list of distinct charge-density-wave materials characterized by momentum-dependent electron-phonon coupling.

We have synthesized and studied single-crystal $\mathrm{B}{\mathrm{a}}_5\mathrm{AlI}{\mathrm{r}}_2{\mathrm{O}}_{11}$ that features dimer chains of two inequivalent octahedra occupied by tetravalent $\mathrm{I}{\mathrm{r}}^{4+}\left(5d^5\right)$ and pentavalent $\mathrm{I}{\mathrm{r}}^{5+}\left(5d^4\right)$ ions, respectively. $\mathrm{B}{\mathrm{a}}_5\mathrm{AlI}{\mathrm{r}}_2{\mathrm{O}}_{11}$ is a Mott insulator that undergoes a subtle structural phase transition near $T_S=210\mathrm{K}$ and a magnetic transition at $T_M=4.5\mathrm{K}$; the latter transition is surprisingly resistant to applied magnetic fields ${\mu }_{o}H\le 12\mathrm{T}$ but more sensitive to modest applied pressure $\left(d{T}_M/dp\approx +0.61\mathrm{K}/\mathrm{GPa}\right)$. All results indicate that the phase transition at $T_S$ signals an enhanced charge order that induces electrical dipoles and strong dielectric response near $T_S$. It is clear that the strong covalency and spin-orbit interaction (SOI) suppress double exchange in Ir dimers and stabilize a novel magnetic state that is neither $S=3/2$ nor $J=1/2$, but rather lies in an “intermediate” regime between these two states. The novel behavior of $\mathrm{B}{\mathrm{a}}_5\mathrm{AlI}{\mathrm{r}}_2{\mathrm{O}}_{11}$ therefore provides unique insights into the physics of SOI along with strong covalency in competition with double-exchange interactions of comparable strength.

Rare-earth nitrides are a promising class of materials for application in spintronics, with GdN a particularly well-studied example. Here we perform band-structure calculations employing a hybrid density functional, which enables the band gap to be more accurately predicted through the inclusion of short-range exact exchange. The sensitivity of the band gap to the exchange term is demonstrated. The surface electronic structure is simulated through the use of slab models of the GdN(111) surface, which provide a consistent description of metallic surface states in the majority-spin channel.

We have carried out a systematic study on the effect of Cu doping on nuclear, magnetic, and dielectric properties in ${\mathrm{Mn}}_{1-x}{\mathrm{Cu}}_x{\mathrm{WO}}_4$ for $0\le x\le 0.19$ by a synergic use of different techniques, viz, heat capacity, magnetization, dielectric, and neutron powder diffraction measurements. Via heat capacity and magnetization measurements we show that with increasing Cu concentration magnetic frustration decreases, which leads to the stabilization of commensurate magnetic ordering. This was further verified by temperature-dependent unit cell volume changes derived from neutron diffraction measurements which was modeled by the Grüneisen approximation. Dielectric measurements show a low temperature phase transition below about 9–10 K. Furthermore, magnetic refinements reveal no changes below this transition indicating a possible spin-flop transition which is unique to the Cu doped system. From these combined studies we have constructed a magnetoelectric phase diagram of this compound.

We use the continuous-time interaction expansion (CT-INT) quantum Monte Carlo method to calculate the phonon spectral function of the one-dimensional Holstein-Hubbard model at half-filling. Our results are consistent with a soft-mode Peierls transition in the adiabatic regime, and the existence of a central peak related to long-range order in the Peierls phase. We explain a previously observed feature at small momenta in terms of a hybridization of charge and phonon excitations. Tuning the system from a Peierls to a metallic phase with a nonzero Hubbard interaction suppresses the central peak, but a significant renormalization of the phonon dispersion remains. In contrast, the dispersion is only weakly modified in the Mott phase. We discuss finite-size effects, the relation to the dynamic charge structure factor, as well as additional sum rules and their implications. Finally, we reveal the existence of a discrete symmetry in a continuum field theory of the Holstein model, which is spontaneously broken in the Peierls phase.

Last year, Salfi et al. made the first direct measurements of a donor wave function and found extremely good theoretical agreement with atomistic tight-binding theory results [Salfi et al., Nat. Mater. 13, 605 (2014)]. Here, we show that multivalley effective mass theory, applied properly, does achieve close agreement with tight-binding results and hence gives reliable predictions. To demonstrate this, we variationally solve the coupled six-valley Shindo-Nara equations, including silicon's full Bloch functions. Surprisingly, we find that including the full Bloch functions necessitates a tetrahedral, rather than spherical, donor central cell correction to accurately reproduce the experimental energy spectrum of a phosphorus impurity in silicon. We cross-validate this method against atomistic tight-binding calculations, showing that the two theories agree well for the calculation of donor-donor tunnel coupling. Further, we benchmark our results by performing a statistical uncertainty analysis, confirming that derived quantities such as the wave function profile and tunnel couplings are robust with respect to variational energy fluctuations. Finally, we apply this method to exhaustively enumerate the tunnel coupling for all donor-donor configurations within a large search volume, demonstrating conclusively that the tunnel coupling has no spatially stable regions. Although this instability is problematic for reliably coupling donor pairs for two-qubit operations, we identify specific target locations where donor qubits can be placed with scanning tunneling microscopy technology to achieve reliably large tunnel couplings.

An ordered alkaline-earth submonolayer on a clean $\mathrm{Si}\left(001\right)$ surface provides a template for growth of the atomically sharp, crystalline Si-oxide interface that is ubiquitous in the semiconductor device industry. It has been suggested that submonolayers of Sr or Ba on $\mathrm{Ge}\left(001\right)$ could play a similar role as on structurally identical $\mathrm{Si}\left(001\right)$, overcoming known limitations of the $\mathrm{Ge}\left(001\right)$ substrate such as amorphization of its oxidation layers. In this paper the initial stage of the Ba oxidation process, i.e., adsorption and organization of Ba atoms on the $\mathrm{Ge}\left(001\right)$ surface as a function of temperature $(270-770\mathrm{K})$ for coverage 1.0 monolayer (ML) and $0.15-0.4\mathrm{ML}$, is studied using scanning tunneling microscopy (STM) and density functional theory (DFT). Three types of features have been identified on the Ba-covered $\mathrm{Ge}\left(001\right)$ surface. They originate from isolated Ba adatoms, isolated Ba ad-dimers, and the Ba ad-dimers assembled into short-range, randomly distributed chains that run across the Ge dimer rows. We find from both STM measurements and DFT calculations that the latter is the dominant structure on $\mathrm{Ge}\left(001\right)$ with increasing coverage.

We investigate the effect of phenomenological relaxation parameters on the third-order optical nonlinearity of doped graphene by perturbatively solving the semiconductor Bloch equation around the Dirac points. An analytic expression for the nonlinear conductivity at zero temperature is obtained under the linear dispersion approximation. With this analytic formula as a starting point, we construct the conductivity at finite temperature and study the optical response to a laser pulse of finite duration. We illustrate the dependence of several nonlinear optical effects, such as third harmonic generation, Kerr effects and two photon absorption, parametric frequency conversion, and two-color coherent current injection, on the relaxation parameters, temperature, and pulse duration. In the special case where one of the electric fields is taken as a dc field, we investigate the dc-current- and dc-field-induced second-order nonlinearities, including dc-current-induced second harmonic generation and difference frequency generation.

Superfluorescence (SF) is a many-body process in which a macroscopic polarization spontaneously builds up from an initially incoherent ensemble of excited dipoles and then cooperatively decays, producing a delayed pulse of coherent radiation. SF arising from electron-hole recombination has recently been observed in ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{As}/\mathrm{GaAs}$ quantum wells [G. T. Noe et al., Nature Phys. 8, 219 (2012) and J.-H. Kim et al., Sci. Rep. 3, 3283 (2013)], but its observability conditions have not been fully established. Here, by performing magnetic field $\left(B\right)$, temperature $\left(T\right)$, and pump power $\left(P\right)$ dependent studies of SF intensity, linewidth, and delay time through time-integrated and time-resolved magnetophotoluminescence spectroscopy, we have mapped out the $B-T-P$ region in which SF is observable. In general, SF can be observed only at sufficiently low temperatures, sufficiently high magnetic fields, and sufficiently high laser powers with characteristic threshold behavior. We provide theoretical insights into these behaviors based primarily on considerations on how the growth rate of macroscopic coherence depends on these parameters. These results provide fundamental new insight into electron-hole SF, highlighting the importance of Coulomb interactions among photogenerated carriers as well as various scattering processes that are absent in SF phenomena in atomic and molecular systems.

We exploit the near field enhancement of nanoantennas to investigate the Raman spectra of otherwise not optically detectable carbon nanotubes (CNTs). We demonstrate that a top-down fabrication approach is particularly promising when applied to CNTs, owing to the sharp dependence of the scattered intensity on the angle between incident light polarization and CNT axis. In contrast to tip enhancement techniques, our method enables us to control the light polarization in the sample plane, locally amplifying and rotating the incident field and hence optimizing the Raman signal. Such promising features are confirmed by numerical simulations presented here. The relative ease of fabrication and alignment makes this technique suitable for the realization of integrated devices that combine scanning probe, optical, and transport characterization.

An efficient scheme is introduced for a fast and smooth convergence to the thermodynamic limit of electronic properties obtained with finite-size calculations on correlated Hamiltonians. This is obtained by modifying the energy levels of the free electron part of the Hamiltonian in a way consistent with the corresponding one-particle density of states in the thermodynamic limit. After this modification, free electron ground state energies, exact in the thermodynamic limit, are obtained with finite-size calculations and for all the particular fillings that satisfy the so called “closed-shell condition.” For those fillings the auxiliary field quantum Monte Carlo technique is particularly efficient and, by combining it with the present method, we provide strong numerical evidence that phase separation occurs in the low doping region and moderate $U\lesssim 4t$ regime of this model.