We analyze the quantum dynamics of two electromagnetic oscillators coupled in series to a voltage-biased Josephson junction. When the applied voltage leads to a Josephson frequency across the junction which matches the sum of the two mode frequencies, tunneling Cooper pairs excite photons in both modes simultaneously leading to far-from-equilibrium states. These states display highly nonclassical features including strong antibunching, violation of Cauchy-Schwartz inequalities, and number squeezing. We obtain approximate analytic results for both the regimes of low and high photon occupancies which are supported by a full numerical treatment. The impact of asymmetries between the two modes is explored, revealing a pronounced enhancement of number squeezing when the modes are damped at different rates.

We applied cluster density matrix embedding theory, with some modifications, to a spin lattice system. The reduced density matrix of the impurity cluster is embedded in the bath states, which are a set of block-product states. The ground state of the impurity model is formulated using a variational wave function. We tested this theory in a two-dimensional spin-1/2 $J_1\text{−}{J}_2$ model for a square lattice. The ground-state energy (GSE) and the location of the phase boundaries agree well with the most accurate previous results obtained using the quantum Monte Carlo and coupled cluster methods. Moreover, this cluster density matrix embedding theory is cost effective and convenient for calculating the von Neumann entropy, which is related to the quantum phase transition.

We investigate a longitudinal conductivity of a two-dimensional relativistic electron gas with a tilted Dirac cone in magnetic field. It is demonstrated that the conductivity behaves differently in the directions parallel and perpendicular to the tilting of the cone. At high magnetic fields, the conductivity at nonzero Landau levels in the direction perpendicular to the tilting modifies nontrivially, in contrast to the parallel case. At zero temperature, the crossover of the conductivity at the Dirac point from high to low magnetic field is studied numerically. It is found that the tilting produces anisotropy of the conductivity which changes with the magnetic field which is different from the anisotropy coming from the Fermi velocity. We also discuss the conductivity at finite temperatures and finite magnetic fields which can be directly compared with the experiments in $\alpha$-(BEDT-TTF)${}_2{\mathrm{I}}_3$ organic conductor. We find that the tilting does not affect so much the magnetic-field dependence of the conductivity except for the prefactor. We discuss the interpretation of recent experimental data and make some proposals to detect the effect of the tilting in future experiments.

We investigate nonequilibrium transport in the reentrant integer quantum Hall phases of the second Landau level. At high currents, we observe a transition from the reentrant integer quantum Hall phases to classical Hall conduction. Surprisingly, this transition is markedly different for the hole- and electron sides of each spin branch. While the hole bubble phases exhibit a sharp transition to an isotropic compressible phase, the transition for the electron side occurs via an intermediate phase. This might indicate a more complex structure of the bubble phases than currently anticipated, or a breaking of the particle-hole symmetry. Such a symmetry breaking in the second Landau level might also have consequences for the physics at filling factor $\nu =5/2$.

The topological properties of the bulk band structure of a three-dimensional topological insulator (TI) manifest themselves in the form of metallic surface states. In this paper, we propose a probe which directly couples to an exotic property of these surface states, namely, the spin-momentum locking. We show that the information regarding the spin textures, so extracted, for different surfaces can be put together to reconstruct the parameters characterizing the bulk band structure of the material, hence acting as a hologram. For specific TI materials such as ${\text{Bi}}_2{\text{Se}}_3,{\text{Bi}}_2{\text{Te}}_3$, and ${\text{Sb}}_2{\text{Te}}_3$, the planar surface states are distinct from one another with regard to their spectrum and the associated spin texture for each angle $\theta$ which the normal to the surface makes with the crystal-growth axis. We develop a tunnel Hamiltonian between such arbitrary surfaces and a spin-polarized scanning tunneling microscope (STM) which provides a unique fingerprint of the dispersion and the associated spin texture corresponding to each $\theta$. Additionally, the theory presented in this paper can be used to extract the value of $\theta$ for a given arbitrary planar surface from the STM spectra itself, hence effectively mimicking x-ray spectroscopy.

Topological quantum information processing relies on adiabatic braiding of non-Abelian quasiparticles. Performing the braiding operations in finite time introduces transitions out of the ground-state manifold and deviations from the non-Abelian Berry phase. We show that these errors can be eliminated by suitably designed counterdiabatic correction terms in the Hamiltonian. We implement the resulting shortcuts to adiabaticity for simple protocols of non-Abelian braiding and show that the error suppression can be substantial even for approximate realizations of the counterdiabatic terms.

A series of recent magneto-optical studies pointed to contradicting values of the $s\text{−}d$ exchange energy $N_0\alpha$ in Mn-doped GaAs and GaN as well as in Fe-doped GaN. Here, a strong sensitivity of weak-localization phenomena to symmetry-breaking perturbations (such as spin-splitting and spin-disorder scattering) is exploited to evaluate the magnitude of $N_0\alpha$ for $n$-type wurtzite (Ga,Mn)N:Si films grown by metalorganic vapor phase epitaxy. Millikelvin magnetoresistance studies and their quantitative interpretation point to $N_0\alpha <40$ meV, a value at least 5 times smaller than the one found with similar measurements on, e.g., $n$-(Zn,Mn)O. It is shown that this striking difference in the values of the $s\text{−}d$ coupling between $n$-type III-V and II-VI dilute magnetic semiconductors can be explained by a theory that takes into account the acceptor character of Mn in III-V compounds.

By combining angle-resolved photoemission spectroscopy and first-principles calculations, we systematically studied the electronic structures of ultrathin Bi(111) films ($\le 5$ bilayers) epitaxially grown on ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. High-resolution low-energy band dispersions and Fermi surfaces of ultrathin $\mathrm{Bi}\left(111\right)/{\mathrm{Bi}}_2{\mathrm{Te}}_3$ films as a function of thickness were experimentally determined. Our results also indicate that the electronic structures of epitaxial Bi films are strongly influenced by the substrate compared with freestanding films. The substrate effects mainly include two aspects. First, the in-plane lattice constant of Bi(111) films is compressed, which increases the bandwidth of the surface-state-like bands. Furthermore, the band dispersion near the $\overline{\Gamma }$ point is significantly modified as well. Second, there exists a strong hybridization at the $\mathrm{Bi}/{\mathrm{Bi}}_2{\mathrm{Te}}_3$ interface, and the hybridization effects spatially extend to three Bi bilayers.

We study the second-order Raman process of mono- and few-layer ${\mathrm{MoTe}}_2$, by combining ab initio density functional perturbation calculations with experimental Raman spectroscopy using 532, 633, and 785 nm excitation lasers. The calculated electronic band structure and the density of states show that the resonance Raman process occurs at the $M$ point in the Brillouin zone, where a strong optical absorption occurs due to a logarithmic Van Hove singularity of the electronic density of states. The double resonance Raman process with intervalley electron-phonon coupling connects two of the three inequivalent $M$ points in the Brillouin zone, giving rise to second-order Raman peaks due to the $M$-point phonons. The calculated vibrational frequencies of the second-order Raman spectra agree with the observed laser-energy-dependent Raman shifts in the experiment.

We demonstrate experimentally the possibility of revealing fluctuations in the eigenfrequency of a resonator when the frequency noise is of the telegraph type. Using a resonantly driven micromechanical resonator, we show that the time-averaged vibration amplitude spectrum exhibits two peaks. They merge with an increasing rate of frequency switching and the spectrum displays an analog of motional narrowing. We also show that the moments of the complex amplitude depend strongly on the frequency noise characteristics. This dependence remains valid even when strong thermal noise or detector noise is present.

There are ample experimental evidences indicating that the ferroelastic domain walls of incipient ferroelectrics, such as $\mathrm{SrTi}{\mathrm{O}}_3$ and $\mathrm{CaTi}{\mathrm{O}}_3$, are polar. The emergence of such interfacial polar order at a domain wall is exciting and believed to arise from the coupling between a primary order parameter, such as a strain or an antiferrodistortive (AFD) order parameter, and polarization. There have been several mechanisms proposed to explain the emergence of interfacial polar order, including biquadratic coupling, AFD-antiferroelectric coupling, and flexoelectric coupling. Using $\mathrm{CaTi}{\mathrm{O}}_3$ as an example, we demonstrate, using both asymptotic analytics and numerical calculation, that the flexoelectric coupling is likely the dominant mechanism leading to the interfacial polar order.

We report on a heat-capacity study of high-quality single-crystal samples of ${\mathrm{LiCuVO}}_4$—a frustrated spin $S=\frac{1}{2}$ chain system—in a magnetic field amounting to 3/4 of the saturation field. A detailed examination of magnetic phase transitions observed in this field range shows that although the low-field helical state clearly has three-dimensional properties, the field-induced spin-modulated phase turns out to be quasi-two-dimensional. The model proposed in this paper allows one to qualitatively understand this crossover, thus eliminating the presently existing contradictions in the interpretations of NMR and neutron-scattering measurements.

Here we present our experimental and theoretical study of the effects of pressure on the transport properties of the heavy-fermion alloy ${\mathrm{Ce}}_{1-x}{\mathrm{Yb}}_x{\mathrm{CoIn}}_5$ with actual concentration $x\approx 0.07$. We specifically choose this value of ytterbium concentration because the magnetic-field-induced quantum critical point, which separates the antiferromagnetic and paramagnetic states at zero temperature, approaches zero, as has been established in previous studies. Our measurements show that pressure further suppresses quantum fluctuations in this alloy, just as it does in the parent compound ${\mathrm{CeCoIn}}_5$. In contrast, the square-root temperature-dependent part of resistivity remains insensitive to pressure, indicating that the heavy quasiparticles are not involved in the inelastic scattering processes leading to such a temperature-dependent resistivity. We demonstrate that the growth of the coherence temperature with pressure, as well as the decrease of the residual resistivity, can be accurately described by employing the coherent potential approximation for a disordered Kondo lattice.

We present neutron-scattering studies of the interplanar magnetic correlations in the high-temperature superconductor ${\mathrm{La}}_{1.88}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_4$ ($T_c=27$ K). The correlations are studied both in a magnetic field applied perpendicular to the ${\mathrm{CuO}}_2$ planes, and in zero field under different cooling conditions. We find that the effect of the magnetic field is to increase the magnetic scattering signal at all values of the out-of-plane wave vector $L$, indicating an overall increase of the magnetic moments. In addition, weak correlations between the copper oxide planes develop in the presence of a magnetic field. This effect is not taken into account in previous reports on the field effect of magnetic scattering, since usually only $L\approx 0$ is probed. Interestingly, the results of quench-cooling the sample are similar to those obtained by applying a magnetic field. Finally, a small variation of the incommensurate peak position as a function of $L$ provides evidence that the incommensurate signal is twinned with the magnetic scattering from the dominant and subdominant structural twin displaying peaks at even and odd values of $L$, respectively, in our crystal.

In Fe pnictide (Pn) superconducting materials, neither Mn nor Cr doping to the Fe site induces superconductivity, even though hole carriers are generated. This is in strong contrast with the superconductivity appearing when holes are introduced by alkali-metal substitution on the insulating blocking layers. We investigate in detail the effects of Mn doping on magnetotransport properties in $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}{)}_2$ for elucidating the intrinsic reason. The negative Hall coefficient for $x=0$ estimated in the low magnetic field $\left(B\right)$ regime gradually increases as $x$ increases, and its sign changes to a positive one at $x=0.020$. Hall resistivities as well as simultaneous interpretation using the magnetoconductivity tensor including both longitudinal and transverse transport components clarify that minority holes with high mobility are generated by the Mn doping via spin-density wave transition at low temperatures, while original majority electrons and holes residing in the paraboliclike Fermi surfaces of the semimetallic $\mathrm{Ba}(\mathrm{FeAs}{)}_2$ are negligibly affected. Present results indicate that the mechanism of hole doping in $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x\mathrm{As}{)}_2$ is greatly different from that of the other superconducting FePn family.

We measured the optical conductivity of superconducting LiFeAs. In the superconducting state, the formation of the condensate leads to a spectral-weight loss and yields a penetration depth of 225 nm. No sharp signature of the superconducting gap is observed. This suggests that the system is likely in the clean limit. A Drude-Lorentz parametrization of the data in the normal state reveals a quasiparticle scattering rate supportive of spin fluctuations and proximity to a quantum critical point.

Substantial amounts of the transition metals Mn, Fe, Co, and Ni can be substituted for Li in single crystalline ${\mathrm{Li}}_2\left({\mathrm{Li}}_{1-x}{T}_x\right)\mathrm{N}$. Isothermal and temperature-dependent magnetization measurements reveal local magnetic moments with magnitudes significantly exceeding the spin-only value. The additional contributions stem from unquenched orbital moments that lead to rare-earth-like behavior of the magnetic properties. Accordingly, extremely large magnetic anisotropies have been found. Most notably, the magnetic anisotropy alternates as $\text{easy plane}\to \text{easy axis}\to \text{easy plane}\to \text{easy axis}$ when progressing from $T=\mathrm{Mn}\to \mathrm{Fe}\to \mathrm{Co}\to \mathrm{Ni}$. This behavior can be understood based on a perturbation approach in an analytical, single-ion model. The calculated magnetic anisotropies show surprisingly good agreement with the experiment and capture the basic features observed for the different transition metals.