Floquet topological insulators are noninteracting quantum systems that, when driven by a time-periodic field, are described by effective Hamiltonians whose bands carry nontrivial topological invariants. A longstanding question concerns the possibility of selectively populating one of these effective bands, thereby maximizing the system's resemblance to a static topological insulator. We study such Floquet systems coupled to a zero-temperature thermal reservoir that provides dissipation. We find that the resulting electronic steady states are generically characterized by a finite density of excitations above the effective ground state, even when the driving has a small amplitude and/or large frequency. We discuss the role of reservoir engineering in mitigating this problem.

Here, we report on the postfabrication tuning of thermal emission spectra using photonic crystal resonators interacting with $n$-type quantum wells. Our method is based on the voltage control of intersubband absorption in the quantum wells, which allows the continuous tuning of both the thermal emission intensity and the linewidth. By adjusting an applied bias to match the intersubband absorption rate with the radiative rate of the resonator, we realize the maximization of the emissivity (0.7) of a narrowband thermal emission peak ($Q=157$). Even narrower thermal emission ($Q>200$) is also demonstrated with the same device by increasing the applied bias.

Noninteracting insulating electronic states of matter can be classified according to their symmetries in terms of topological invariants which can be related to effective surface theories. These effective surface theories are in turn topologically protected against the effects of disorder. Topological crystalline insulators are, on the other hand, trivial in the sense of the above classification but still possess surface modes. In this paper we consider an extension of the Bernevig-Hughes-Zhang model that describes a point group insulator. We explicitly show that the surface properties of this state can be as robust as in topologically nontrivial insulators but only if the $S_z$ component of the spin is conserved. However, in the presence of Rashba spin-orbit coupling this protection vanishes, and the surface states localize, even if the crystalline symmetries are intact on average.

The interface of graphene with ferromagnets is highly relevant for spintronics, because graphene on Co(0001) shows a largely intact Dirac cone and strong hybridization with Co $3d$ states breaking the sublattice symmetry that had been considered mutually exclusive. Here we show by spin- and angle-resolved photoemission that the Dirac cone and Dirac point are also highly spin polarized $\left(\sim -25\%\right)$, which reinforces the puzzling issue of a strong graphene-substrate interaction. The problem is solved by our ab initio calculations which show that (i) the upper and lower halves of the Dirac cone belong to different sublattices and (ii) one half is spin polarized by spin-dependent hybridization because it is situated at the edge of a minority-spin band gap of the Co substrate.

The spin dynamics of strongly localized donor-bound electrons in fluorine-doped ZnSe epilayers is studied using pump-probe Kerr rotation techniques. A method exploiting the spin inertia is developed and used to measure the longitudinal spin relaxation time $T_1$ in a wide range of magnetic fields, temperatures, and pump densities. The $T_1$ time of the donor-bound electron spin of about $1.6 \mu \mathrm{s}$ remains nearly constant for external magnetic fields varied from zero up to 2.5 T (Faraday geometry) and in a temperature range 1.8–45 K. These findings impose severe restrictions on possible spin relaxation mechanisms. In our opinion they allow us to rule out scattering between free and donor-bound electrons, jumping of electrons between different donor centers, scattering between phonons and donor-bound electrons, and with less certainty charge fluctuations in the environment of the donors caused by the 1.5 ps pulsed laser excitation.

In two-dimensional topological insulators, such as inverted HgTe/CdTe quantum wells, helical quantum spin Hall (QSH) states persist even at finite magnetic fields below a critical magnetic field $B_{\mathrm{c}}$, above which only quantum Hall (QH) states can be found. Using linear-response theory, we theoretically investigate the magneto-optical properties of inverted HgTe/CdTe quantum wells, both for infinite two-dimensional and finite-strip geometries and for possible signatures of the transition between the QSH and QH regimes. In the absorption spectrum, several peaks arise due to nonequidistant Landau levels in both regimes. However, in the QSH regime, we find an additional absorption peak at low energies in the finite-strip geometry. This peak arises due to the presence of edge states in this geometry and persists for any Fermi level in the QSH regime, while in the QH regime the peak vanishes if the Fermi level is situated in the bulk gap. Thus, by sweeping the gate voltage, it is possible to experimentally distinguish between the QSH and QH regimes due to this signature. Moreover, we investigate the effect of spin-orbit coupling and finite temperature on this measurement scheme.

We utilize numerical linked-cluster expansions (NLCEs) and the determinantal quantum Monte Carlo algorithm to study pairing correlations in the square-lattice Hubbard model. To benchmark the NLCE, we first locate the finite-temperature phase transition of the attractive model to a superconducting state away from half filling. We then explore the superconducting properties of the repulsive model for the $d$-wave and extended $s$-wave pairing symmetries. The pairing structure factor shows a strong tendency to $d$-wave pairing and peaks at an interaction strength comparable to the bandwidth. The extended $s$-wave structure factor and correlation length are larger at higher temperatures but clearly saturate as temperature is lowered, whereas the $d$-wave counterparts, which start off lower at high temperatures, continue to rise near half filling. This rise is even more dramatic in the $d$-wave susceptibility. The convergence of NLCEs breaks down as the susceptibilities and correlation lengths become large, so we are unable to determine the onset of long-range order. However, our results extend the conclusion, previously restricted to only magnetic and charge correlations, that NLCEs offer a unique window into pairing in the Hubbard model at strong coupling.

We consider theoretically as a function of temperature the plasmon mode arising in three-dimensional Dirac liquids, i.e., systems with linear chiral relativistic single-particle dispersion, within the random phase approximation. We find that whereas no plasmon mode exists in the intrinsic (undoped) system at zero temperature, there is a well-defined finite-temperature plasmon with superlinear temperature dependence, rendering the plasmon dispersion widely tunable with temperature. The plasmon dispersion contains a logarithmic correction due to the ultraviolet-logarithmic renormalization of the electron charge, manifesting a fundamental many-body interaction effect as in quantum electrodynamics. The plasmon dispersion of the extrinsic (doped) system displays a minimum at finite temperature before it crosses over to the superlinear intrinsic behavior at higher temperature, implying that the high-temperature plasmon is a universal feature of Dirac liquids irrespective of doping. This striking characteristic temperature dependence of intrinsic Dirac plasmons along with the logarithmic renormalization is a unique manifestation of the three-dimensional relativistic Dirac nature of quasiparticle excitations and serves as an experimentally observable signature of three-dimensional Dirac materials.

A photoinduced phase transition was investigated in an organic charge-transfer (CT) complex ${\mathrm{M}}_2\mathrm{P}-\mathrm{TCNQ}{\mathrm{F}}_4, [{\mathrm{M}}_2\mathrm{P}$: 5,10-dihydro-5,10-dimethylphenazine, donor (D) molecule; $\mathrm{TCNQ}{\mathrm{F}}_4$: 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, acceptor (A) molecule] by means of femtosecond pump-probe reflection spectroscopy. This is an ionic compound and has a peculiar two-dimensional (2D) molecular arrangement; the same A (or D) molecules arrange along the [100] direction, and A and D molecules alternately arrange along the [111] direction. It results in a strongly anisotropic two-dimensional electronic structure. This compound shows a structural and magnetic phase transition at 122 K below which the two neighboring molecules are dimerized along both the [100] and [111] directions. We demonstrate that two kinds of photoinduced phase transitions occur by irradiation of a femtosecond laser pulse; in the high-temperature lattice-uniform phase, a quasi-one-dimensional (1D) metallic state along the AA(DD) stack is generated, and in the low-temperature lattice-dimerized phase, a quasi-2D metallic state is initially produced and molecular dimerizations are subsequently released. Mixed-stack CT compounds consisting of DA stacks are generally insulators or semiconductors in the ground state. Here, such a dynamical metallization in the DA stack is demonstrated. The release of the dimerizations drives several kinds of coherent oscillations which play an important role in the stabilization of the lattice-dimerized phase. The mechanisms of those photoinduced phase transitions are discussed in terms of the magnitudes of the anisotropic bandwidths and molecular dimerizations along two different directions of the molecular stacks.

Solid solutions of the “soft” quantum spin ice pyrochlore magnets ${\mathrm{Tb}}_2{\mathrm{B}}_2{\mathrm{O}}_7$ with B = Ti and Sn display a novel magnetic ground state in the presence of strong B-site disorder, characterized by a low susceptibility and strong spin fluctuations to temperatures below 0.1 K. These materials have been studied using ac susceptibility and $\mu \mathrm{SR}$ techniques to very low temperatures, and time-of-flight inelastic neutron scattering techniques to 1.5 K. Remarkably, neutron spectroscopy of the ${\mathrm{Tb}}^{3+}$ crystal-field levels appropriate to high B-site mixing ($0.5<x<1.5$ in ${\mathrm{Tb}}_2{\mathrm{Sn}}_{2-x}{\mathrm{Ti}}_x{\mathrm{O}}_7$) reveal that the doublet ground and first excited states present as continua in energy, while transitions to singlet excited states at higher energies simply interpolate between those of the end members of the solid solution. The resulting ground state suggests an extreme version of a random-anisotropy magnet, with many local moments and anisotropies, depending on the precise local configuration of the six B sites neighboring each magnetic ${\mathrm{Tb}}^{3+}$ ion.

We report on specific heat measurements on clean overdoped ${\text{BaFe}}_2{\left({\text{As}}_{1-x}{\text{P}}_x\right)}_2$ single crystals performed with a high resolution membrane-based nanocalorimeter. A nonzero residual electronic specific heat coefficient at zero temperature ${\gamma }_{\mathrm{r}}{=C/T|}_{T\to 0}$ is seen for all doping compositions, indicating a considerable fraction of the Fermi surface ungapped or having very deep minima. The remaining superconducting electronic specific heat is analyzed through a two-band $s$-wave $\alpha$ model in order to investigate the gap structure. Close to optimal doping we detect a single zero-temperature gap of ${\mathrm{\Delta }}_0\sim 5.3\mathrm{meV}$, corresponding to ${\mathrm{\Delta }}_0/k_{\mathrm{B}}{T}_{\mathrm{c}}\sim 2.2$. Increasing the phosphorus concentration $x$, the main gap reduces till a value of ${\mathrm{\Delta }}_0\sim 1.9\mathrm{meV}$ for $x=0.55$ and a second weaker gap becomes evident. From the magnetic field effect on ${\gamma }_{\mathrm{r}}$, all samples however show similar behavior $[{\gamma }_{\mathrm{r}}\left(H\right)-{\gamma }_{\mathrm{r}}(H=0)\propto H^n$, with $n$ between 0.6 and 0.7]. This indicates that, despite a considerable redistribution of the gap weights, the total degree of gap anisotropy does not change drastically with doping.

We report a study of the normal- and superconducting-state electronic properties of the centrosymmetric compound ${\mathrm{SrPt}}_3\mathrm{P}$ via ${}^{31}\mathrm{P}$ nuclear-magnetic-resonance (NMR) and magnetometry investigations. Essential features such as a sharp drop of the Knight shift at $T<T_c$ and an exponential decrease of the NMR spin-lattice relaxation ratio $1/\left(T_{1}T\right)$ below $T_c$ are consistent with an $s$-wave electron pairing in ${\mathrm{SrPt}}_3\mathrm{P}$, although a direct confirmation in the form of a Hebel-Slichter-type peak is lacking. Normal-state NMR data at $T<50$ K indicate conventional features of the conduction electrons, typical of simple metals such as lithium or silver. Our data are finally compared with available NMR results for the noncentrosymmetric superconductors ${\mathrm{LaPt}}_3\mathrm{Si}$ and ${\mathrm{CePt}}_3\mathrm{Si}$, which adopt similar crystal structures.

We report a gate-controlled transition of a semimetallic InAs/GaSb heterostructure to a topological insulator. The transition is induced by decreasing the degree of band inversion with front- and back-gate voltages. Temperature dependence of the longitudinal resistance peak shows the energy gap opening in the bulk region with increasing gate electric field. The suppression of bulk conduction and the transition to a topological insulator are confirmed by nonlocal resistance measurements using a dual lock-in technique, which allows us to rigorously compare the voltage distribution in the sample for different current paths without the influence of time-dependent resistance fluctuations.

The structure of a graphene monolayer on Ir(111) has been investigated in situ in the growth chamber by surface x-ray diffraction including the specular rod, which allows disentangling the effect of the sample roughness from that of the nanorippling of graphene and iridium along the moiré-like pattern between graphene and Ir(111). Accordingly, we are able to provide precise estimates of the undulation associated with this nanorippling, which is small in this weakly interacting graphene-metal system and thus has proved difficult to assess in the past. The nanoripplings of graphene and iridium are found in phase, i.e., the in-plane positions of their height maxima coincide, but the amplitude of the height modulation is much larger for graphene $\left(0.379\pm 0.044\text{Å}\right)$ than, e.g., for the topmost Ir layer $\left(0.017\pm 0.002\text{Å}\right)$. The average graphene-Ir distance is found to be $3.38\pm 0.04\text{Å}$.

The half-integer quantum Hall effect (QHE) is often suppressed in graphene grown by chemical vapor deposition on metals. The reason behind the suppression is unclear, and we hypothesize that it might be connected to extended defects in the material. In this paper we present results for the quantum Hall effect in graphene with [0001] tilt grain boundaries connecting opposite sides of Hall bar devices. Such grain boundaries contain 5-7 ring complexes that host defect states that hybridize to form bands with varying degree of metallicity depending on grain-boundary defect density. In a magnetic field, edge states on opposite sides of the Hall bar can be connected by the defect states along the grain boundary. This destroys Hall resistance quantization and leads to nonzero longitudinal resistance. Anderson disorder can partly recover quantization, where current instead flows along returning paths along the grain boundary depending on defect density in the grain boundary and on disorder strength. Since grain sizes in graphene made by chemical vapor deposition are usually small, this may help explain why the quantum Hall effect is usually poorly developed in devices made of this material.

The electronic structure of warm dense silicon dioxide has been investigated by x-ray absorption near-edge spectroscopy. An ultrafast optical laser pulse was used to isochorically heat a thin silicon dioxide sample, and measured spectra were compared with simulations generated by molecular dynamics and density functional theory. In comparison with the room temperature spectrum, two features were observed: a peak below the band gap and absorption within the band gap. This behavior was also observed in the simulations. From consideration of the calculated spectra, the peak below the gap is attributed to valence electrons that have been promoted to the conduction band, while absorption within the gap is attributed to broken Si-O bonds.

We examine the adsorption of a single Ni atom on a monolayer of MgO on a Ag substrate using DFT and $\mathrm{DFT}+U$ computational approaches. We find that the electronic and magnetic properties vary considerably across the three binding sites of the surface. Two of the binding sites are competitive in energy, and the preferred site depends on the strength of the on-site Coulomb interaction $U$. These results can be understood in terms of the competition between bonding and magnetism for surface adsorbed transition metal atoms. Comparisons are made with a recent experimental and theoretical study of Co on MgO/Ag, and implications for scanning tunneling microscopy experiments on the Ni system are discussed.

In a ferromagnet/heavy-metal bilayer device with strong spin Hall effect an in-plane current excites magnetic dynamics through spin torque. We analyze bilayers with perpendicular magnetization and calculate three-dimensional phase diagrams describing switching by external magnetic field at a fixed current. We then concentrate on the case of a field applied in the plane formed by the film normal and the current direction. Here we analytically study the evolution of both the conventional “up”/“down” magnetic equilibria and the additional equilibria created by the spin torque. Expressions for the stability regions of all equilibria are derived, and the nature of switching at each critical boundary is discussed. The qualitative picture obtained this way predicts complex hysteresis patterns that should occur in bilayers. Analyzing the phase portraits of the system we find regimes where switching between the up and down states proceeds through the current-induced state as an intermediate. The first step of such two-step process is fast and resembles ballistic switching for the reasons discussed in the paper. Using numeric simulations we analyze the switching time and compare it to that of a conventional spin torque device with collinear magnetizations of the polarizer and the free layer.