Recent superconducting gap classifications based on space group symmetry have revealed nontrivial gap structures that were not shown by point group symmetry. First, we review a comprehensive classification of symmetry-protected line nodes within the range of centrosymmetric space groups. Next, we show an additional constraint; line nodes peculiar to nonsymmorphic systems appear only for primitive or orthorhombic base-centered Bravais lattice. Then, we list useful classification tables of 59 primitive or orthorhombic base-centered space groups for the superconducting gap structures. Furthermore, our gap classification reveals the $j_z$-dependent point nodes (gap opening) appearing on a three- or sixfold axis, which means that the presence (absence) of point nodes depends on the Bloch-state angular momentum $j_z$. We suggest that this unusual gap structure is realized in a heavy-fermion superconductor ${\mathrm{UPt}}_3$, using a group-theoretical analysis and a numerical calculation. The calculation demonstrates that a Bloch phase contributes to $j_z$ as effective orbital angular momentum by site permutation. We also discuss superconducting gap structures in ${\mathrm{MoS}}_2$, SrPtAs, ${\mathrm{UBe}}_{13}$, and ${\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}$.

The recently synthesized ThFeAsN iron pnictide superconductor exhibits a $T_c$ of 30 K, the highest of the 1111-type series in the absence of chemical doping. To understand how pressure affects its electronic properties, we carried out microscopic investigations up to 3 GPa via magnetization, nuclear magnetic resonance, and muon-spin rotation experiments. The temperature dependence of the ${}^{75}\mathrm{As}$ Knight shift, the spin-lattice relaxation rates, and the magnetic penetration depth suggest a multiband $s^{\pm }$-wave gap symmetry in the dirty limit, whereas the gap-to-$T_c$ ratio $\mathrm{\Delta }/k_{\mathrm{B}}{T}_c$ hints at a strong-coupling scenario. Pressure modulates the geometrical parameters, thus reducing $T_c$ as well as $T_m$, the temperature where magnetic-relaxation rates are maximized, both at the same rate of approximately $-1.1\mathrm{K}/\mathrm{GPa}$. This decrease in $T_c$ with pressure is consistent with band-structure calculations, which relate it to the deformation of the Fe $3d_{z^2}$ orbitals.

X-ray powder diffraction experiments using resistively heated diamond anvil cells have been conducted in order to establish the phase behavior of antimony up to 31 GPa and 835 K. The dip in the melting curve at 5.7 GPa and 840 K is identified as the triple point between the Sb-I, incommensurate Sb-II, and liquid phases. No evidence of the previously reported simple cubic phase was observed. Determination of the phase boundary between Sb-II and Sb-III suggests the existence of a second triple point in the region of 13 GPa and 1200 K. The incommensurate composite structure of Sb-II was found to remain ordered to the highest temperatures studies—no evidence of disordering of the guest-atom chains was observed. Indeed, the modulation reflections that arise from interactions between the host and guest subsystems were found to be present to the highest temperatures, suggesting such interactions remain relatively strong in Sb even in the presence of increased thermal motion. Finally, we show that the incommensurately modulated structure recently reported as giving an improved fit to diffraction data from incommensurate Ba-IV can be rejected as the structure of Sb-II using a simple density argument.

We report on a study of the structural, magnetic, and superconducting properties of $\mathrm{Nb}\left(25\mathrm{nm}\right)/\mathrm{Gd}\left(d_f\right)/\mathrm{Nb}\left(25\mathrm{nm}\right)$ hybrid structures of a superconductor/ ferromagnet (S/F) type. The structural characterization of the samples, including careful determination of the layer thickness, was performed using neutron and x-ray scattering with the aid of depth-sensitive mass spectrometry. The magnetization of the samples was determined by superconducting quantum interference device magnetometry and polarized neutron reflectometry, and the presence of magnetic ordering for all samples down to the thinnest Gd(0.8 nm) layer was shown. The analysis of the neutron spin asymmetry allowed us to prove the absence of magnetically dead layers in junctions with Gd interlayer thickness larger than one monolayer. The measured dependence of the superconducting transition temperature $T_c\left(d_f\right)$ has a damped oscillatory behavior with well-defined positions of the minimum at $d_f=3$ nm and the following maximum at $d_f=4$ nm, in qualitative agreement with prior work [J. S. Jiang et al., Phys. Rev. B 54, 6119 (1996)]. We use a theoretical approach based on the Usadel equations to analyze the experimental $T_c\left(d_f\right)$ dependence. The analysis shows that the observed minimum at $d_f=3$ nm can be described by the so-called zero to $\pi$ phase transitions of highly transparent S/F interfaces with a superconducting correlation length ${\xi }_f\approx 4$ nm in Gd. This penetration length is several times higher than for strong ferromagnets like Fe, Co, and Ni, thus simplifying the preparation of S/F structures with $d_f\sim {\xi }_f$ which are of topical interest in superconducting spintronics.

We have used grazing-angle infrared spectroscopy to detect the Berreman effect (BE) in the quasi-two-dimensional electron system (q-2DES) which forms spontaneously at the interface between ${\mathrm{SrTiO}}_3$ (STO) and a thin film of ${\mathrm{LaAlO}}_3$ (LAO). From the BE, which allows one to study longitudinal optical excitations in ultrathin films like the q-2DES, we have extracted at different temperatures its thickness, the charge density and mobility of the carriers under crystalline LAO (sample A), and the charge density under amorphous LAO (sample B). The latter quantity turns out to be higher than in sample A, but a comparison with Hall measurements shows that under amorphous LAO the charges are partly localized at low $T$ with a low activation energy (about 190 K in units of $k_B$) and are thermally activated according to a model for large polarons. The thickness of the q-2DES extracted from our spectra turns out to be $4\pm 1$ nm for crystalline LAO and $7\pm 2$ nm for amorphous LAO.

A complementary metal-oxide semiconductor compatible on-chip light source is the holy grail of silicon photonics and has the potential to alleviate the key scaling issues arising due to electrical interconnects. Despite several theoretical predictions, a sustainable, room temperature laser from a group-IV material is yet to be demonstrated. In this work, we show that a particular loss mechanism, inter-valence-band absorption (IVBA), has been inadequately modeled until now and capturing its effect accurately as a function of strain is crucial to understanding light emission processes from uniaxially strained germanium (Ge). We present a detailed model of light emission in Ge that accurately models IVBA in the presence of strain and other factors such as polarization, doping, and carrier injection, thereby revising the road map toward a room temperature Ge laser. Strikingly, a special resonance between gain and loss mechanisms at 4%–5% $\langle 100\rangle$ uniaxial strain is found resulting in a high net gain of more than $400{\text{cm}}^{-1}$ at room temperature. It is shown that achieving this resonance should be the goal of experimental work rather than pursuing a direct band gap Ge.

In the spirit of recently developed LDA+U and LDA+DMFT methods, we implement a combination of density functional theory in its local density approximation (LDA) with a $k$- and $\omega$-dependent self-energy found from diagrammatic fluctuational exchange (FLEX) approximation. The active Hilbert space here is described by the correlated subset of electrons which allows one to tremendously reduce the sizes of the matrices needed to represent charge and spin susceptibilities. The method is perturbative in nature but accounts for both bubble and ladder diagrams and accumulates the physics of momentum-resolved spin fluctuations missing in such popular approach as GW. As an application, we study correlation effects on band structures in V and Pd. The $d$-electron self-energies emergent from this calculation are found to be remarkably $k$ independent. However, when we compare our calculated electronic mass enhancements against LDA+DMFT, we find that for the longstanding problem of spin fluctuations in Pd, LDA+FLEX delivers a better agreement with experiment, although this conclusion depends on a particular value of the Hubbard $U$ used in the simulation. We also discuss outcomes of a recently proposed combination of $k$-dependent FLEX with dynamical mean-field theory (DMFT).

Single crystal lithium ferrite (LiFe) spheres of sub-mm dimension are examined at mK temperatures, microwave frequencies, and variable dc magnetic field, for use in hybrid quantum systems and condensed matter and fundamental physics experiments. Strong coupling regimes of the photon-magnon interaction (cavity magnon polariton quasiparticles) were observed with coupling strength of up to 250 MHz at 9.5 GHz (2.6%) with magnon linewidths of order 4 MHz (with potential improvement to sub-MHz values). We show that the photon-magnon coupling can be significantly improved and exceed that of the widely used yttrium iron garnet crystal, due to the small unit cell of LiFe, allowing twice the spins per unit volume. Magnon mode softening was observed at low dc fields and, combined with the normal Zeeman effect, creates magnon spin-wave modes that are insensitive to first-order magnetic-field fluctuations. This effect is observed in the Kittel mode at 5.5 GHz (and another higher order mode at 6.5 GHz) with a dc magnetic field close to 0.19 tesla. We show that if the cavity is tuned close to this frequency, the magnon polariton particles exhibit an enhanced range of strong coupling and insensitivity to magnetic field fluctuations with both first-order and second-order insensitivity to magnetic field as a function of frequency (double magic point clock transition), which could potentially be exploited in cavity QED experiments.

We show that the antiferromagnetic state commonly observed in the phase diagrams of the iron-based superconductors necessarily triggers loop currents characterized by charge transfer between different Fe $3d$ orbitals. This effect is rooted on the glide-plane symmetry of these materials and on the existence of an atomic spin-orbit coupling that couples states at the $X$ and $Y$ points of the 1-Fe Brillouin zone. In the particular case in which the magnetic moments are aligned parallel to the magnetic ordering vector direction, which is the moment configuration most commonly found in the iron-based superconductors, these loop currents involve the $d_{xy}$ orbital and either the $d_{yz}$ orbital (if the moments point along the $y$ axis) or the $d_{xz}$ orbitals (if the moments point along the $x$ axis). We show that the two main manifestations of the orbital loop currents are the emergence of magnetic moments in the pnictide/chalcogen site and an orbital-selective band splitting in the magnetically ordered state, both of which could be detected experimentally. Our results highlight the unique intertwining between orbital and spin degrees of freedom in the iron-based superconductors, and reveal the emergence of an unusual correlated phase that may impact the normal state and superconducting properties of these materials.

Motivated by recent terahertz absorption measurements in $\alpha \text{−}{\mathrm{RuCl}}_3$, we develop a theory for the electromagnetic absorption of materials described by the Kitaev model on the honeycomb lattice. We derive a mechanism for the polarization operator at second order in the nearest-neighbor hopping Hamiltonian. Using the exact results of the Kitaev honeycomb model, we then calculate the polarization dynamical correlation function corresponding to electric dipole transitions in addition to the spin dynamical correlation function corresponding to magnetic dipole transitions.

${\mathrm{Ce}}_2{\mathrm{PtIn}}_8$ is a recently discovered heavy-fermion system structurally related to the well-studied superconductor ${\mathrm{CeCoIn}}_5$. Here we report on low-temperature de Haas–van Alphen-effect measurements in high magnetic fields in ${\mathrm{Ce}}_2{\mathrm{PtIn}}_8$ and ${\mathrm{Pr}}_2{\mathrm{PtIn}}_8$. In addition, we performed band-structure calculations for localized and itinerant Ce-$4f$ electrons in ${\mathrm{Ce}}_2{\mathrm{PtIn}}_8$. Comparison with the experimental data of ${\mathrm{Ce}}_2{\mathrm{PtIn}}_8$ and of the $4f$-localized ${\mathrm{Pr}}_2{\mathrm{PtIn}}_8$ suggests the itinerant character of the Ce-$4f$ electrons. This conclusion is further supported by the observation of effective masses in ${\mathrm{Ce}}_2{\mathrm{PtIn}}_8$, which are strongly enhanced with up to 26 bare electron masses.

The microscopic details of the suppression of antiferromagnetic order in the Kondo-lattice series ${\mathrm{Ce}}_{1-x}{\mathrm{La}}_x{\mathrm{Cu}}_2{\mathrm{Ge}}_2$ due to nonmagnetic dilution by La are revealed through neutron diffraction results for $x=0.20$, 0.40, 0.75, and 0.85. Magnetic Bragg peaks are found for $0.20\le x\le 0.75$, and both the Néel temperature $T_{\mathrm{N}}$ and the ordered magnetic moment per Ce $\mu$ linearly decrease with increasing $x$. The reduction in $\mu$ points to strong hybridization of the increasingly diluted Ce $4f$ electrons, and we find a remarkable quadratic dependence of $\mu$ on the Kondo-coherence temperature. We discuss our results in terms of local-moment- versus itinerant-type magnetism and mean-field theory and show that ${\mathrm{Ce}}_{1-x}{\mathrm{La}}_x{\mathrm{Cu}}_2{\mathrm{Ge}}_2$ provides an exceptional opportunity to quantitatively study the multiple magnetic interactions in a Kondo lattice.

Strong interaction of a terahertz (THz) wave with excitons induces nonperturbative optical effects such as Rabi splitting and high-order sideband generation. Here, we investigated coherent properties of THz-induced sideband emissions from GaAs/AlGaAs multiquantum wells. With increasing THz electric field, optical susceptibility of the THz-dressed exciton shows a redshift with spectral broadening and extraordinary phase shift. This implies that the field ionization of the $1s$ exciton modifies the THz-dressed exciton in the nonperturbative regime.

The entanglement negativity is a versatile measure of entanglement that has numerous applications in quantum information and in condensed matter theory. It can not only efficiently be computed in the Hilbert space dimension, but for noninteracting bosonic systems, one can compute the negativity efficiently in the number of modes. However, such an efficient computation does not carry over to the fermionic realm, the ultimate reason for this being that the partial transpose of a fermionic Gaussian state is no longer Gaussian. To provide a remedy for this state of affairs, in this work, we introduce efficiently computable and rigorous upper and lower bounds to the negativity, making use of techniques of semidefinite programming, building upon the Lagrangian formulation of fermionic linear optics, and exploiting suitable products of Gaussian operators. We discuss examples in quantum many-body theory and hint at applications in the study of topological properties at finite temperature.

We investigate the processes of electron capture by a Coulomb impurity center residing in a hybrid system consisting of spatially separated two-dimensional layers of electron and Bose-condensed dipolar exciton gases coupled via the Coulomb forces. We calculate the probability of the electron capture accompanied by the emission of a single Bogoliubov excitation (bogolon), similar to regular phonon-mediated scattering in solids. Furthermore, we study the electron capture mediated by the emission of a pair of bogolons in a single capture event and show that these processes not only should be treated in the same order of the perturbation theory, but also they give a more important contribution than single-bogolon-mediated capture, in contrast with regular phonon scattering.

In a recent paper [Lagos et al., Nature (London) 543, 533 (2017)] we have used electron energy loss spectroscopy with sub-10 meV energy and atomic spatial resolution to map optical and acoustic, bulk and surface vibrational modes in magnesium oxide nanocubes. We found that a local dielectric description works well for the simulation of aloof geometries, similar to related work for surface plasmons and surface plasmon polaritons, while for intersecting geometries such a description fails to reproduce the rich spectral features associated with excitation of bulk acoustic and optical phonons. To account for scatterings with a finite momentum exchange, in this paper we investigate molecular and lattice dynamics simulations of bulk losses in magnesium-oxide nanocubes using a rigid-ion description and investigate the loss spectra for intersecting electron beams. From our analysis we can evaluate the capability of electron energy loss spectroscopy for the investigation of phonon modes at the nanoscale, and we discuss shortcomings of our simplified approach as well as directions for future investigations.

We present observations of highly frustrated quasi-two-dimensional (2D) magnetic correlations in the honeycomb lattice layers of the $S_{\text{eff}} =1/2$ compound $\gamma -{\mathrm{BaCo}}_2{\left({\mathrm{PO}}_4\right)}_2$ ($\gamma$-BCPO). Specific heat shows a broad peak comprised of two weak kink features at $T_{N1}\sim 6$ K and $T_{N2}\sim 3.5$ K, the relative weights of which can be modified by sample annealing. Neutron powder diffraction measurements reveal short range quasi-2D order that is established below $T_{N1}$ and $T_{N2}$, at which two separate, incompatible, short range magnetic orders onset: commensurate antiferromagnetic correlations with correlation length ${\xi }_c=60\pm 2$ Å ($T_{N1}$) and in quasi-2D helical domains with ${\xi }_h=350\pm 11$ Å ($T_{N2}$). The ac magnetic susceptibility response lacks frequency dependence, ruling out spin freezing. Inelastic neutron scattering data on $\gamma$-BCPO is compared with linear spin wave theory, and two separate parameter regions of the XXZ $J_1-J_2-J_3$ model with ferromagnetic nearest-neighbor exchange $J_1$ are favored, both near regions of high classical degeneracy. High energy coherent excitations ($\sim 10$ meV) persist up to at least 40 K, suggesting strong in-plane correlations persist above $T_N$. These data show that $\gamma$-BCPO is a rare highly frustrated, quasi-2D $S_{\text{eff}} =1/2$ honeycomb lattice material which resists long range magnetic order and spin freezing.

While the layered 122 iron arsenide superconductors are highly anisotropic, unconventional, and exhibit several forms of electronic orders that coexist or compete with superconductivity in different regions of their phase diagrams, we find in the absence of iron in the structure that the superconducting characteristics of the end member ${\mathrm{BaPd}}_2{\mathrm{As}}_2$ are surprisingly conventional. Here we report on complementary measurements of specific heat, magnetic susceptibility, resistivity measurements, Andreev spectroscopy, and synchrotron high pressure x-ray diffraction measurements supplemented with theoretical calculations for ${\mathrm{BaPd}}_2{\mathrm{As}}_2$. Its superconducting properties are completely isotropic as demonstrated by the critical fields, which do not depend on the direction of the applied field. Under the application of high pressure, $T_c$ is linearly suppressed, which is the typical behavior of classical phonon-mediated superconductors with some additional effect of a pressure-induced decrease in the electronic density of states and the electron-phonon coupling parameters. Structural changes in the layered ${\mathrm{BaPd}}_2{\mathrm{As}}_2$ have been studied by means of angle-dispersive diffraction in a diamond-anvil cell. At 12 GPa and 24.2 GPa we observed pressure induced lattice distortions manifesting as the discontinuity and, hence discontinuity in the Birch-Murnaghan equation of state. The bulk modulus is $B_0=40\left(6\right)$ GPa below 12 GPa and $B_0=142\left(3\right)$ GPa below 27.2 GPa.

The vortex depinning phenomenon in single crystals of $2H\text{−}\mathrm{Nb}{\mathrm{S}}_2$ superconductors is used as a prototype for investigating properties of the nonequilibrium (NEQ) depinning phase transition. The $2H\text{−}\mathrm{Nb}{\mathrm{S}}_2$ is a unique system as it exhibits two distinct depinning thresholds, viz., a lower critical current $I_c^l$ and a higher one $I_c^h$. While $I_c^l$ is related to depinning of a conventional, static (pinned) vortex state, the state with $I_c^h$ is achieved via a negative differential resistance (NDR) transition where the velocity abruptly drops. Using a generalized finite-temperature scaling ansatz, we study the scaling of current (I)–voltage (V) curves measured across $I_c^l$ and $I_c^h$. Our analysis shows that for $I>I_c^l$, the moving vortex state exhibits Arrhenius-like thermally activated flow behavior. This feature persists up to a current value where an inflexion in the IV curves is encountered. While past measurements have often reported similar inflexion, our analysis shows that the inflexion is a signature of a NEQ phase transformation from a thermally activated moving vortex phase to a free flowing phase. Beyond this inflection in IV, a large vortex velocity flow regime is encountered in the $2H\text{−}\mathrm{Nb}{\mathrm{S}}_2$ system, wherein the Bardeen-Stephen flux flow limit is crossed. In this regime the NDR transition is encountered, leading to the high $I_c^h$ state. The IV curves above $I_c^h$ we show do not obey the generalized finite-temperature scaling ansatz (as obeyed near $I_c^l$). Instead, they scale according to the Fisher's scaling form [Fisher, Phys. Rev. B 31, 1396 (1985)] where we show thermal fluctuations do not affect the vortex flow, unlike that found for depinning near $I_c^l$.

The problem of quantum turbulence in a channel with an inhomogeneous counterflow of superfluid turbulent helium is studied. The counterflow velocity $V_{ns}^x\left(y\right)$ along the channel is supposed to have a parabolic profile in the transverse direction $y$. Such statement corresponds to the recent numerical simulation by Khomenko et al. [Phys. Rev. B 91, 180504 (2015)]. The authors reported about a sophisticated behavior of the vortex-line density (VLD) $\mathcal{L}(\mathbf{r},t)$, different from $\mathcal{L}\propto V_{ns}^x{\left(y\right)}^2$, which follows from the straightforward application of the conventional Vinen theory. It is clear that Vinen theory should be refined by taking into account transverse effects, and the way it ought to be done is the subject of active discussion in the literature. In this work, we discuss several possible mechanisms of the transverse flux of VLD $\mathcal{L}(\mathbf{r},t)$ which should be incorporated in the standard Vinen equation to describe adequately the inhomogeneous quantum turbulence. It is shown that the most effective among these mechanisms is the one that is related to the phase-slippage phenomenon. The use of this flux in the modernized Vinen equation corrects the situation with an unusual distribution of the vortex-line density, and satisfactorily describes the behavior $\mathcal{L}(\mathbf{r},t)$ both in stationary and nonstationary situations. The general problem of the phenomenological Vinen theory in the case of nonuniform and nonstationary quantum turbulence is thoroughly discussed.