On the basis of microscopic theory it is demonstrated how the coupling between the electrons by means of magnetization fluctuations in ferromagnetic metals with orthorhombic symmetry gives rise to an equal-spin pairing superconducting state with the general form of the order parameter dictated by symmetry. The strong upturn of the upper critical field along the $b$ direction above 5 T in UCoGe is explained by the increase of the pairing interaction caused by the suppression of the Curie temperature by a magnetic field parallel to the $b$ axis. It is proposed that a similar phenomenon at a much higher field must take place also for a field directed along the magnetically hardest $a$ direction.

We develop a general Ginzburg-Landau theory that describes the effect of a Zeeman field on the superconducting order parameter in two-dimensional triplet superconductors. Starting from Ginzburg-Landau theories that describe fully gapped time-reversal symmetric triplet superconductors, we show that the Zeeman field has dramatic effects on the topological properties of the superconductors. In particular, in the vicinity of a critical chemical potential separating two topologically distinct phases, it is possible to induce a phase transition to a topologically nontrivial phase which supports chiral edge modes. Moreover, for specific directions of the Zeeman field, we obtain nodal superconducting phases with an emerging chiral symmetry, and with Majorana flat bands at the edge. The Ginzburg-Landau theory is microscopically supported by a self-consistent mean-field theory of the doped Kitaev-Heisenberg model.

We study a microscopic model of a thermocouple device with two connected correlated quantum wires driven by a constant electric field. In such a closed system we follow the time and position dependence of the entropy density using the concept of the reduced density matrix. At weak driving, the initial changes of the entropy at the junctions can be described by the linear Peltier response. At longer times the quasiequilibrium situation is reached with well defined local temperatures which increase due to an overall Joule heating. On the other hand, a strong electric field induces a nontrivial nonlinear thermoelectric response, e.g., the Bloch oscillations of the energy current. Moreover, we show for the doped Mott insulators that strong driving can reverse the Peltier effect.

The redistribution of electrons in an ultrafast pump-probe experiment causes significant changes to the spectral distribution of the retarded interaction between electrons and bosonic modes. We study the influence of these changes on pump-probe photoemission spectroscopy for a model electron-phonon coupled system using the nonequilibrium Keldysh formalism. We show that spectral rearrangement due to the driving field preserves an overall sum rule for the electronic self-energy, but modifies the effective electron-phonon scattering as a function of energy. Experimentally, this pump-modified scattering can be tracked by analyzing the fluence or excitation energy dependence of population decay rates and transient changes in dispersion kinks.

Electrolyte gating using ionic liquid electrolytes has recently generated considerable interest as a method to achieve large carrier density modulations in a variety of materials. In noble metal thin films, electrolyte gating results in large changes in sheet resistance. The widely accepted mechanism for these changes is the formation of an electric double layer with a charged layer of ions in the liquid and accumulation or depletion of carriers in the thin film. We report here a different mechanism. In particular, we show using x-ray absorption near edge structure (XANES) that the previously reported large conductance modulation in gold films is due to reversible oxidation and reduction of the surface rather than the charging of an electric double layer. We show that the double layer capacitance accounts for less than 10% of the observed change in transport properties. These results represent a significant step towards understanding the mechanisms involved in electrolyte gating.

We consider the nonequilibrium dynamics of a system of interacting massless fermions in a ring threaded by a magnetic flux. We focus on the quench where the flux is initially vanishing and is then turned on. We show that the definition of the limit of abrupt quench is problematic due to the presence of gauge invariance that has to be taken into account. We then propose a specific protocol where the dynamics is nontrivial. Employing techniques coming from the algebraic Bethe ansatz, we present an exact formula for the Loschmidt echo valid at all times as a Fredholm determinant at the free fermionic point. From the analysis of the asymptotic behavior of the Fredholm determinant, we show that the distribution of work done at small energies presents an edge singularity whose exponent can be explicitly computed. Using the correspondence between the edge singularity and the decay of the fidelity at finite size we propose a general formula for the exponent valid also in the interacting case.

We present a theoretical study of the structural, electronic, and transport properties of bulk ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ within density functional theory taking into account the van der Waals interactions (vdW) and the quasiparticle self-energy corrections. It is found that the optB86b-vdW functional can well reproduce the experimental lattice constants and interlayer distances for ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. Based on the fully optimized structure, the band structure of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ is obtained from first-principles calculations with the GW approximation and the Wannier function interpolation method. The global band extrema are found to be off the high-symmetry lines, and the real energy band calculated is in good agreement with that measured experimentally. In combination with the Boltzmann theory, the GW calculations also give accurate prediction of the transport properties, and the calculated thermoelectric coefficients of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ almost coincide with the experimental data.

K-TCNQ crystals show photoinduced phase transition (PIPT) between a Mott insulator and a spin Peierls (SP) phase, having $T_c=395$ K (= and the crystal has $T_c=395$ K of the PIPT). We report the intra-molecular vibrations in the excited K-TCNQ crystal after impulsive excitation using a sub 9.4 fs pulse laser in the reflection spectrum. The frequencies of the vibrations are close to the Raman frequencies of molecules. The frequencies increase with melting of the SP phase, and then decrease with rearrangement of the dimerization. In addition, the frequencies of the vibrations are time-resolved to be modulated by the intermolecular optical phonon with 510 fs period corresponding to 65 $\mathrm{cm}{}^{-1}$. This modulation is associated with the periodical change of coupling strengths of intermolecular charge transfer and/or the dipolar coupling between the counterpart molecules in the neighboring chains.

Recent experiments on nanoscale conductors coupled to microwave cavities showed the prospect of transport investigations of electron-photon interplay in the deep quantum regime. Here we propose a pump-probe scheme to investigate the transient dynamics of individual electron-photon excitations in a double-quantum-dot-cavity system. Excitations pumped into the system decay via charge tunneling at the double dot, probed in real time. We investigate theoretically the short-time charge-transfer statistics at the dot for periodic pumping and show that this gives access to vacuum Rabi oscillations as well as excitation dynamics in the presence of double-dot dephasing and relaxation.

Quantum mechanics allows the existence of “virtual states” that have no classical analog. Such virtual states defy direct observation through strong measurement, which would destroy the volatile virtual state. Here, we show how a virtual state of an interacting many-body system can be detected employing a weak measurement protocol with post-selection. We employ this protocol for the measurement of the time it takes an electron to tunnel through a virtual state of a quantum dot (cotunneling). Contrary to classical intuition, this cotunneling time is independent of the strength of the dot-lead coupling and may deviate from that predicted by time-energy uncertainty relation. Our approach, amenable to experimental verification, may elucidate an important facet of quantum mechanics which hitherto was not accessible by direct measurements.

The ${180}^{\circ }$ ferroelectric domain walls (FDWs) have long been regarded as purely Ising type in ferroelectrics, but recent theoretical works suggested that they can also have Néel- and/or Bloch-like rotations. Using a combination of first-principles calculations with phase-field simulations, we studied the ${180}^{\circ }$ FDWs on different crystallographic planes in prototypical ferroelectric perovskite ${\mathrm{BaTiO}}_3$. The polarization profiles of ${180}^{\circ }$ FDWs on (100) and (410) planes revealed that the (100)- and (410)-FDWs both exhibit Néel-like character besides their intrinsic Ising character, while the (410)-FDW also simultaneously shows a Bloch-like oblique of $\sim 6$ nm, as a consequence of the deviation of polarization gradient from the high symmetry direction. Due to the existence of the Néel-like component of polarization, ${180}^{\circ }$ FDWs in ${\mathrm{BaTiO}}_3$ exhibit a multilayer charge redistribution and thus may strongly trap charged defects.

The structural behavior of ${\mathrm{Np}}_2{\mathrm{Co}}_{17}$ is investigated by means of high-pressure diamond-anvil compression measurements and is compared with that of the isostructural compounds ${\mathrm{Lu}}_2{\mathrm{Co}}_{17}$ and ${\mathrm{Lu}}_2{\mathrm{Ni}}_{17}$. The ${\mathrm{Th}}_2{\mathrm{Ni}}_{17}$-type hexagonal crystal structure is preserved with no measurable discontinuous volume collapses up to the highest achieved pressure, $p=43$ GPa. For ${\mathrm{Np}}_2{\mathrm{Co}}_{17}$, fits to the Birch-Murnaghan and Vinet equations of state give values of the isothermal bulk modulus and its pressure derivative of $B_0=286$ GPa and $B_0^{\prime }=3$, revealing that this Np compound is a highly incompressible solid with stiffness comparable to that of superhard covalently bonded materials. For the ${\mathrm{Lu}}_2{\mathrm{T}}_{17}$ (T = Co, Ni) compounds, the measured bulk modulus changes from $B_0=137$ GPa for T = Co to $B_0=257$ GPa for T = Ni. The isothermal equation of state for the studied compounds are in excellent agreement with the results of ab initio fully relativistic, full-potential local spin-density functional calculations. Theoretical estimates of the bulk modulus are given also for ${\mathrm{Np}}_2{\mathrm{Ni}}_{17}$, for which $B_0$ is predicted to assume values intermediate between those measured for ${\mathrm{Lu}}_2{\mathrm{Ni}}_{17}$ and ${\mathrm{Np}}_2{\mathrm{Co}}_{17}$.

We report the study of interactions between cavity photons and paramagnetic ${\mathrm{Cr}}^{3+}$ spins in a ruby ${(\mathrm{Cr}}^{3+}:{\mathrm{Al}}_2{\mathrm{O}}_3)$ whispering gallery mode (WGM) resonator. Examining the system at microwave frequencies and millikelvin temperatures, spin-photon couplings up to 610 MHz or about 5% of photon energy are observed between the impurity spins and high quality factor $\left(Q>{10}^5\right)$ WGM. Large tunability and spin-spin interaction allows operation in the strong coupling regime. The system exhibits behavior not predicted by the usual Tavis-Cummings model because of interactions within the two-level spin bath, and the existence of numerous photonic modes.

It has been understood that short range interactions can reduce the classification of topological superconductors in all dimensions. In this Rapid Communication we demonstrate by explicit calculations that when the topological phase transition between two distinct phases in the noninteracting limit is gapped out by interaction, the bulk fermion Green's function $G\left(i\omega \right)$ at the “transition” approaches zero as $G\left(i\omega \right)\sim \omega$ at certain momentum $\vec{k}$ in the Brillouin zone.

${\mathrm{EuTiO}}_3$, which is a G-type antiferromagnet below $T_{\mathrm{N}}=5.5$ K, has some fascinating properties at high temperatures, suggesting that macroscopically hidden dynamically fluctuating weak magnetism exists at high temperatures. This conjecture is substantiated by magnetic field dependent magnetization measurements, which exhibit pronounced anomalies below 200 K becoming more distinctive with increasing magnetic field strength. Additional results from muon spin rotation experiments provide evidence for weak fluctuating bulk magnetism induced by spin-lattice coupling which is strongly supported in increasing magnetic field.

We report the low-temperature properties of ${}^{11}\mathrm{B}$-enriched single-crystal ${\mathrm{CrB}}_2$ as prepared from high-purity Cr and B powder by a solid-state reaction and optical float zoning. The electrical resistivity, ${\rho }_{\mathrm{xx}}$, Hall effect, ${\rho }_{\mathrm{xy}}$, and specific heat, $C$, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap $\Delta \approx 220\mathrm{K}$ below $T_{\mathrm{N}}=88\mathrm{K}$. This observation is corroborated by the absence of a Curie dependence in the magnetization for $T\to 0$ reported in the literature. Comparison of $C$ with $d{\rho }_{\mathrm{xx}}/dT$, where we infer lattice contributions from measurements of ${\mathrm{VB}}_2$, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature $T_{\mathrm{sf}}\approx 257\mathrm{K}$ in the paramagnetic state, followed by a pronounced second-order mean-field transition at $T_{\mathrm{N}}$, and unusual excitations around $\approx T_{\mathrm{N}}/2$. The pronounced anisotropy of ${\rho }_{\mathrm{xx}}$ above $T_{\mathrm{N}}$ is characteristic of an easy-plane anisotropy of the spin fluctuations consistent with the magnetization. The ratio of the Curie-Weiss to the $\mathrm{N}\acute{\mathrm{e}}\mathrm{el}$ temperatures, $f=-{\Theta }_{\mathrm{CW}}/T_{\mathrm{N}}\approx 8.5$, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to 14 T, the highest field studied. In contrast to earlier suggestions of local-moment magnetism our study identifies ${\mathrm{CrB}}_2$ as a weak itinerant antiferromagnet par excellence with strong geometric frustration.

Determinant quantum Monte Carlo (DQMC) is used to determine the pairing and magnetic response for a Hubbard model built up from four-site clusters—a two-dimensional square lattice consisting of elemental $2\times 2$ plaquettes with hopping $t$ and on-site repulsion $U$ coupled by an interplaquette hopping $t^{\prime }\le t$. Superconductivity in this geometry has previously been studied by a variety of analytic and numeric methods, with differing conclusions concerning whether the pairing correlations and transition temperature are raised near half filling by the inhomogeneous hopping or not. For $U/t=4$, DQMC indicates an optimal $t^{\prime }/t\approx 0.4$ at which the pairing vertex is most attractive. The optimal $t^{\prime }/t$ increases with $U/t$. We then contrast our results for this plaquette model with a hamiltonian which instead involves a regular pattern of site energies whose large site energy limit is the three-band ${\mathrm{CuO}}_2$ model; we show that there the inhomogeneity rapidly, and monotonically, suppresses pairing.