We consider a Fermi gas with short-range attractive interactions that is confined along one direction by a tight harmonic potential. For this quasi-two-dimensional (quasi-2D) Fermi gas, we compute the pressure equation of state, radiofrequency spectrum, and the superfluid critical temperature $T_c$ using a mean-field theory that accounts for all the energy levels of the harmonic confinement. Our calculation for $T_c$ provides a natural generalization of the Thouless criterion to the quasi-2D geometry, and it correctly reduces to the 3D expression derived from the local density approximation in the limit where the confinement frequency ${\omega }_z\to 0$. Furthermore, our results suggest that $T_c$ can be enhanced by relaxing the confinement and perturbing away from the 2D limit.

We present the derivation of an ab initio and parameter-free effective electron-electron interaction that goes beyond the screened random phase approximation and accounts for superconducting pairing driven by spin fluctuations. The construction is based on many-body perturbation theory and relies on the approximation of the exchange-correlation part of the electronic self-energy within time-dependent density functional theory. This effective interaction is included in an exchange-correlation kernel for superconducting density functional theory in order to achieve a completely parameter free superconducting gap equation. First results from applying the new functional to a simplified two-band electron gas model are consistent with experiments.

Motivated by ideas of fractionalization and intrinsic topological order in bosonic models with short-range interactions, we consider similar phenomena in formal lattice gauge theory models. Specifically, we show that a compact quantum electrodynamics (CQED) can have, besides the familiar Coulomb and confined phases, additional unusual confined phases where excitations are quantum lines carrying fractions of the elementary unit of electric field strength. We construct a model that has $N$-tupled monopole condensation and realizes $1/N$ fractionalization of the quantum Faraday lines. This phase has another excitation which is a $Z_N$ quantum surface in spatial dimensions five and higher, but can be viewed as a quantum line or a quantum particle in four or three spatial dimensions, respectively. These excitations have statistical interactions with the fractionalized Faraday lines; for example, in three spatial dimensions, the particle excitation picks up a Berry phase of $e^{i2\pi /N}$ when going around the fractionalized Faraday line excitation. We demonstrate the existence of this phase by Monte Carlo simulations in (3+1) space-time dimensions.

Many applications of ferroic materials, such as data storage and spintronics, are achieved through the control and manipulation of their domain wall (DW) orientations and configurations. Here we propose a rotational compatibility condition to identify low-energy DWs in perovskites with oxygen octahedral tilt instability. It is derived from the strong DW energy anisotropy arising from the rigidity and corner-sharing feature of the octahedral network. We analyze quantitatively the DWs in $\mathrm{SrTi}{\mathrm{O}}_3$ and explain successfully the unusual ferroelectric DW width and energy in $\mathrm{BiFe}{\mathrm{O}}_3$.

We introduce general principles for achieving maximal violation of detailed balance in thermal radiation. We validate these principles by direct numerical calculations, based on fluctuational electrodynamics, on thermal emitters constructed from magneto-optical photonic crystals. Such a capability to maximally violate the detailed balance provides different opportunities for the design of thermal absorbers and emitters.

In thin-film ferromagnets with perfect easy-plane anisotropy, the component of total spin perpendicular to the easy plane is a good quantum number and the corresponding spin supercurrent can flow without dissipation. Here we explain how spin supercurrents couple spatially remote spin-mixing vertical transport channels, even when easy-plane anisotropy is imperfect, and discuss the possibility that this effect can be used to fabricate new types of electronic devices.

Spin accumulation in a paramagnetic semiconductor due to voltage-biased current tunneling from a polarized ferromagnet is experimentally manifest as a small additional spin-dependent resistance. We describe a rigorous model incorporating the necessary self-consistency between electrochemical potential splitting, spin-dependent injection current, and applied voltage that can be used to simulate this so-called “$3T$” signal as a function of temperature, doping, ferromagnet bulk spin polarization, tunnel barrier features and conduction nonlinearity, and junction voltage bias.

The formation of electron pairs is a prerequisite of superconductivity. The fermionic nature of electrons yields four classes of superconducting correlations with definite symmetry in spin, space, and time. Here, we suggest double quantum dots coupled to conventional $s$-wave superconductors in the presence of inhomogeneous magnetic fields as a model system exhibiting unconventional pairing. Due to their small number of degrees of freedom, tunable by gate voltages, quantum-dot systems are ideal to gain fundamental insight into unconventional pairing. We propose two detection schemes for unconventional superconductivity, based on either Josephson or Andreev spectroscopy.

The magnetization and magnetic torque of a high-quality single crystal of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ have been measured down to 0.1 K under precise control of the magnetic-field orientation. When the magnetic field is applied exactly parallel to the $ab$ plane, a sharp magnetization jump $4\pi \delta M$ of $(0.74\pm 0.15)$ G at the upper critical field $H_{\mathrm{c}2,ab}\sim 15$ kOe with a field hysteresis of 100 Oe is observed at low temperatures, evidencing a first-order superconducting-normal transition. A strong magnetic torque appearing when $H$ is slightly tilted away from the $ab$ plane confirms an intrinsic anisotropy $\Gamma ={\xi }_a/{\xi }_c$ of as large as 60 even at 100 mK, in contrast with the observed $H_{\mathrm{c}2}$ anisotropy of $\sim 20$. The present results raise fundamental issues in both the existing spin-triplet and spin-singlet scenarios, providing, in turn, crucial hints toward the resolution of the superconducting nature of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

We report detailed ${}^{75}\mathrm{As}$ nuclear quadrupole resonance investigations of the locally noncentrosymmetric superconductor SrPtAs. The spin-lattice relaxation studies prove weakly coupled multigap superconductivity. A retardation of the decay in $1/T_{1}T$ evidences a nodeless (fully gapped) superconducting state on the complex multipocket Fermi surface, which is consistent with an anisotropic $s$-wave order parameter and with proposed unconventional $f$-wave and chiral $d$-wave symmetries. A quantitative analysis of these models favors the unconventional $f$-wave state.

Topological superconductors (TSCs) are of significant current interest because they offer promising platforms for finding Majorana fermions. Here we report on a superconductor synthesized by intercalating Cu into a naturally formed topological insulator (TI) heterostructure consisting of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ TI units separated by nontopological PbSe units. Interestingly, in this TI-based superconductor, the specific-heat behavior suggests the occurrence of unconventional superconductivity with gap nodes. The existence of gap nodes in a strongly spin-orbit coupled superconductor would give rise to spin-split Andreev bound states that are the hallmark of topological superconductivity. Hence, this superconductor emerges as an intriguing candidate TSC.

In many cases the standard perturbation approach appears to be too simple to describe precisely the angle-resolved photoemission spectrum of a strongly correlated electron system. In particular, to describe the momentum asymmetry observed in the photoemission spectra of high-$T_c$ cuprates, a phenomenological approach based on an extremely correlated Fermi-liquid model has been recently introduced. Here we analyze the general structure of the Green's function of quasiparticles in strongly correlated electron systems and stress that it is defined not only by the self-energy of Hubbard quasiparticles but also by a strength operator. The latter leads to an additional odd momentum contribution to the spectral function and alone can explain the observed asymmetry. So, the asymmetry of the angle-resolved photoemission spectra can be a measure of the strength of electron correlations in materials.

We report heat capacity measurements of solid ${}^4\mathrm{He}$ and ${}^3\mathrm{He}-{}^4\mathrm{He}$ solid mixture samples grown in silica aerogel. In addition to the Debye $T^3$ term, the heat capacity of solid ${}^4\mathrm{He}$ samples includes $T$ linear and $T^2$ terms which are attributed to the amorphous and two-dimensional helium layers confined near the silica strands of aerogel. Beside these regular polynomial terms, a broad heat capacity peak is found between 0.1 and 0.35 K. The peak probably has its origin in the vibration of the dislocation lines pinned by the silica strands. Compared to ${}^4\mathrm{He}$ samples, the heat capacity of solid ${}^3\mathrm{He}-{}^4\mathrm{He}$ mixture samples shows evidence of phase separation below a characteristic temperature $T_c$ that increases with ${}^3\mathrm{He}$ concentration $\left(X_3\right)$. Below this temperature the heat capacity shows $X_3^{1/2}$ dependence suggesting that the ${}^3\mathrm{He}$ atoms are phase separated from being uniformly distributed in the pore space into two-dimensional patches enfolding the silica strands that are themselves coated with a strongly bound amorphous ${}^4\mathrm{He}$ layer.

A wide range of disordered materials contain electronic states that are spatially well localized. In this work, we investigated the electrical response of such systems in nonequilibrium conditions to external electromagnetic field. We obtained the expression for optical conductivity valid for any nonequilibrium state of electronic subsystem. In the case of incoherent nonequilibrium state, this expression contains only the positions of localized electronic states, Fermi's golden rule transition probabilities between the states, and the populations of electronic states. The same form of expression is valid both in the case of weak electron-phonon interaction and weak electron-impurity interaction that act as perturbations of electronic Hamiltonian. The derivation was performed by expanding the general expression for ac conductivity in powers of small electron-phonon interaction or electron-impurity interaction parameter. Applications of the expression to two model systems, a simple one-dimensional Gaussian disorder model, and the model of a realistic three-dimensional organic polymer material, were presented as well.

We consider multiple noninteracting quantum mechanical two-level systems coupled to a common bosonic bath and study its quantum phase transition with Monte Carlo simulations using a continuous imaginary time cluster algorithm. The common bath induces an effective ferromagnetic interaction between the otherwise independent two-level systems, which can be quantified by an effective interaction strength. For degenerate energy levels above a critical value of the bath coupling strength $\alpha$ all two-level systems freeze into the same state and the critical value ${\alpha }_c$ decreases asymptotically as $1/N$ with increasing $N$. For a finite number $N$ of two-level systems the quantum phase transition (at zero temperature) is in the same universality class as the single spin-boson model; in the limit $N\to \infty$ the system shows mean-field critical behavior independent of the power of the spectral function of the bosonic bath. We also study the influence of a spatial separation of the spins in a bath of bosonic modes with linear dispersion relation on the location and characteristics of the phase transition as well as on correlations between the two-level systems.

We have performed magnetic, electric, thermal, and neutron powder diffraction (NPD) experiments as well as density functional theory (DFT) calculations on ${\mathrm{Ba}}_3{\mathrm{MnNb}}_2{\mathrm{O}}_9$. All results suggest that ${\mathrm{Ba}}_3{\mathrm{MnNb}}_2{\mathrm{O}}_9$ is a spin-5/2 triangular lattice antiferromagnet (TLAF) with weak easy-axis anisotropy. At zero field, we observed a narrow two-step transition at $T_{\mathrm{N}1}=3.4$ K and $T_{\mathrm{N}2}=3.0$ K. The neutron diffraction measurement and the DFT calculation indicate a ${120}^{\circ }$ spin structure in the $ab$ plane with out-of-plane canting at low temperatures. With increasing magnetic field, the ${120}^{\circ }$ spin structure evolves into up-up-down ($uud$) and oblique phases showing successive magnetic phase transitions, which fits well to the theoretical prediction for the 2D Heisenberg TLAF with classical spins. Multiferroicity is observed when the spins are not collinear but suppressed in the $uud$ and oblique phases.

In thin magnetic layers with structural inversion asymmetry and spin-orbit coupling, the Dzyaloshinskii-Moriya interaction arises at the interface. When a spin-wave current ${\mathbf{j}}_m$ flows in a system with a homogeneous magnetization m, this interaction produces an effective fieldlike torque of the form ${\mathbf{T}}_{\mathrm{FL}}\propto \mathbf{m}\times (\mathbf{z}\times {\mathbf{j}}_m)$ as well as a dampinglike torque, ${\mathbf{T}}_{\mathrm{DL}}\propto \mathbf{m}\times [(\mathbf{z}\times {\mathbf{j}}_m)\times \mathbf{m}]$, the latter only in the presence of spin-wave relaxation $(\mathbf{z}$ is normal to the interface). These torques mediated by the magnon flow can reorient the time-averaged magnetization direction and display a number of similarities with the torques arising from the electron flow in a magnetic two-dimensional electron gas with Rashba spin-orbit coupling. This magnon-mediated spin-orbit torque can be efficient in the case of magnons driven by a thermal gradient.

We study in-plane magnetic tunnel junctions with additional perpendicular polarizer for subnanosecond-current-induced switching memories. The spin-transfer-torque switching dynamics was studied as a function of the cell aspect ratio both experimentally and by numerical simulations using the macrospin model. We show that the anisotropy field plays a significant role in the dynamics, along with the relative amplitude of the two spin-torque contributions. This was confirmed by micromagnetic simulations. Real-time measurements of the reversal were performed with samples of low and high aspect ratio. For low aspect ratios, a precessional motion of the magnetization was observed and the effect of temperature on the precession coherence was studied. For high aspect ratios, we observed magnetization reversals in less than 1 ns for high enough current densities, the final state being controlled by the current direction in the magnetic tunnel junction cell.

The doping dependence of the critical current density $J_c$ was measured in a series of high quality $c$-axis-oriented ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ thin films. For each doping level we measured the temperature dependence of $J_c$. We find that all samples have the same temperature dependence, but the critical current at zero temperature $J_c\left(0\right)$ has a nontrivial doping dependence with a maximum at a unique doping level in the overdoped side of the superconducting dome.

Oscillations on free surfaces of superfluids at the inviscid limit are damped by quasiparticle scattering. We study this effect in both superfluid ${}^3\mathrm{He}$ and superfluid ${}^4\mathrm{He}$, deep below the respective critical temperatures. Surface oscillators offer several benefits over immersed mechanical oscillators traditionally used for similar purposes. Damping is modeled as specular scattering of ballistic quasiparticles from the moving free surface. The model is in reasonable agreement with our measurements for superfluid ${}^4\mathrm{He}$ but significant deviation is found for ${}^3\mathrm{He}$.