Zero-dimensional dissipative action possesses nontrivial minima known as Korshunov instantons. They have been known so far only for imaginary time representation that is limited to equilibrium systems. In this work we reconstruct and generalise Korshunov instantons using real-time Keldysh approach. This allows us to formulate the dissipative action theory for generic nonequilibrium conditions. Possible applications of the theory to transport in strongly biased quantum dots are discussed.

Ultrafast transient pump-probe measurements of thin ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ perovskite films over a wide spectral range from 350 to 800 nm reveal a family of photoinduced bleach (PB) and absorption (PA) features unequivocally pointing to the fundamentally multiband character of the underlying electronic structure. Excitation pump-energy dependent kinetics of three long-lived PB peaks at 1.65, 2.55, and 3.15 eV along with a broad PA band shows the involvement of band-edge thermalized carriers in all transitions and at least four, possibly more, electronic bands. The evolution of the transient signatures is described in terms of the redistribution of the conserved oscillator strength of the whole system. The multiband perspective opens up different directions for understanding and controlling photoexcitations in hybrid perovskites.

We investigate the optical conductivity of ${\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{RuO}}_3$ across the ferromagnetic to paramagnetic transition that occurs at $x=0.8$. The thin films were grown by metalorganic aerosol deposition with $0\le x\le 1$ onto ${\mathrm{NdGaO}}_3$ substrates. We performed terahertz frequency domain spectroscopy in a frequency range from 3 to $40 {\mathrm{cm}}^{-1}$ (100 GHz to 1.4 THz) and at temperatures ranging from 5 to 300 K, measuring transmittivity and phase shift through the films. From this we obtained the real and imaginary parts of the optical conductivity. The end-members, ferromagnetic ${\mathrm{SrRuO}}_3$ and paramagnetic ${\mathrm{CaRuO}}_3$, show a strongly frequency dependent metallic response at temperatures below 20 K. Due to the high quality of these samples we can access pronounced intrinsic electronic contributions to the optical scattering rate, which at 1.4 THz exceeds the residual scattering rate by more than a factor of three. Deviations from a Drude response start at about 0.7 THz for both end-members in a remarkably similar way. For the intermediate members a higher residual scattering originating in the compositional disorder leads to a featureless optical response instead. The relevance of low-lying interband transitions is addressed by a calculation of the optical conductivity within density functional theory in the local-density approximation.

We suggest an inverse dispersion method for calculating a photonic band diagram for materials with arbitrary frequency-dependent dielectric functions. The method is able to calculate the complex wave vector for a given frequency by solving the eigenvalue problem with a non-Hermitian operator. The analogy with $\mathcal{PT}$-symmetric Hamiltonians reveals that the operator corresponds to the momentum as a physical quantity, and the singularities at the band edges are related to the branch points and responses for the features on the band edges. The method is realized using a plane wave expansion technique for a two-dimensional periodic structure in the case of TE and TM polarizations. We illustrate the applicability of the method by the calculation of the photonic band diagrams of an infinite two-dimensional square lattice composed of dielectric cylinders using the measured frequency-dependent dielectric functions of different materials (amorphous hydrogenated carbon, silicon, and chalcogenide glass). We show that the method allows one to distinguish unambiguously between Bragg and Mie gaps in the spectra.

Photoinduced carrier dynamics in a correlated electron system on a coupled two-leg ladder lattice are studied. The two-leg ladder Hubbard model is analyzed utilizing the exact diagonalization method based on the Lanczos algorithm in finite-size clusters. In order to reveal the transient carrier dynamics after photoirradiation, we calculate the low-energy components of the hole kinetic energy, the pair-field correlation function, the optical conductivity spectra, and other characteristics. It is shown that the photoinduced metallic-like state appears in a half-filled Mott insulating state, while the low-energy carrier motion is suppressed by photoirradiation in the case of hole-doped metallic states. These photoinduced changes in the electron dynamics are associated with changes in the carrier-pair coherence; they are not attributed to a naive thermalization, and are associated with a ladder-lattice effect instead. Based on the numerical results, optical control of hole pairs using double-pulse pumping is demonstrated. Implications for recent optical pump-probe experiments are presented.

Quaternary chalcogenide materials offer a wide variety of chemical and physical properties, and hence, those compounds have been widely studied for several technological applications. Recently, experimental studies have found that the chalcogenide ${\mathrm{Cs}}_2{\mathrm{M}}^{\mathrm{II}}{\mathrm{M}}_3^{\mathrm{IV}}{\mathrm{Q}}_8$ family (${\mathrm{M}}^{\mathrm{II}}$ = $\mathrm{Mg}, \mathrm{Zn}, \mathrm{Cd}, \mathrm{Hg}, {\mathrm{M}}^{\mathrm{IV}}$ = $\mathrm{Ge}, \mathrm{Sn}$ and $\mathrm{Q}$ = $\mathrm{S}, \mathrm{Se}, \mathrm{Te}$), which includes 24 compounds, yields a wide range of band gaps, namely, from 1.07 to 3.4 eV, and hence, they have attracted great interest. To obtain an improved atomistic understanding of the role of the cations and anions on the physical properties, we performed a first-principles investigation of the 24 ${\mathrm{Cs}}_2{\mathrm{M}}^{\mathrm{II}}{\mathrm{M}}_3^{\mathrm{IV}}{\mathrm{Q}}_8$ compounds employing density functional theory within semilocal and hybrid exchange-correlation energy functionals and the addition of van der Waals corrections to improve the description of the weakly interacting layers. Our lattice parameters are in good agreement with the available experimental data (i.e., 11 compounds), and the equilibrium volume increases linearly by increasing the atomic number of the chalcogen, which can be explained by the increased atomic radius of the chalcogen atoms from $\mathrm{S}$ to $\mathrm{Te}$. We found that van der Waals corrections play a crucial role in the lattice parameter in the stacking direction of the ${\mathrm{Cs}}_2{\mathrm{M}}^{\mathrm{II}}{\mathrm{M}}_3^{\mathrm{IV}}{\mathrm{Q}}_8$ layers, while the binding energy per unit area has similar magnitude as obtained for different layered materials. We obtained that the band gaps follow a linear relation as a function of the unit cell volume, which can be explained by the atomic size of the chalcogen atom and the relative position of the $\mathrm{Q} p$ states within the band structure. The fundamental and optical band gaps differ by less than 0.1 eV. The band gaps obtained with the hybrid functional are in good agreement with the available experimental data. Furthermore, we found from the Bader analysis, that the Coulomb interations among the cations and anions play a crucial role on the energetic properties.

We extend the electrodynamics of two-dimensional electron gases to account for the extrinsic spin Hall effect (SHE). The theory is applied to doped graphene decorated with a random distribution of absorbates that induce spin-orbit coupling (SOC) by proximity. The formalism extends previous semiclassical treatments of the SHE to the nonlocal dynamical regime. Within a particle-number conserving approximation, we compute the conductivity, dielectric function, and spin Hall angle in the small frequency and wave vector limit. The spin Hall angle is found to decrease with frequency and wave number, but it remains comparable to its zero-frequency value around the frequency corresponding to the Drude peak. The plasmon dispersion and linewidth are also obtained. The extrinsic SHE affects the plasmon dispersion in the long wavelength limit, but not at large values of the wave number. This result suggests an explanation for the rather similar plasmonic response measured in exfoliated graphene, which does not exhibit the SHE, and graphene grown by chemical vapor deposition, for which a large SHE has been recently reported. Our theory also lays the foundation for future experimental searches of SOC effects in the electrodynamic response of two-dimensional electron gases with SOC disorder.

We present a theoretical analysis of the normal and superfluid fractions of quantum fluids described by a nonequilibrium extension of the Gross-Pitaevskii equation in the presence of an external potential. Both disordered and regular potentials are considered. The normal and superfluid fractions are defined by the response of the nonequilibrium quantum fluid to a vector potential, in analogy with the equilibrium case. We find that the physical meaning of these definitions breaks down out of equilibrium. The normal and superfluid fractions no longer add up to one and for some types of external potentials they can even become negative.

We analyze the superconducting instabilities in the vicinity of the quantum-critical point of an inversion symmetry breaking order. We first show that the fluctuations of the inversion symmetry breaking order lead to two degenerate superconducting (SC) instabilities, one in the $s$-wave channel, and the other in a time-reversal invariant odd-parity pairing channel (the simplest case being the same as the of ${}^3\mathrm{He}-\mathrm{B}$ phase). Remarkably, we find that unlike many well-known examples, the selection of the pairing symmetry of the condensate is independent of the momentum-space structure of the collective mode that mediates the pairing interaction. We found that this degeneracy is a result of the existence of a conserved fermionic helicity $\chi$, and the two degenerate channels correspond to even and odd combinations of SC order parameters with $\chi =\pm 1$. As a result, the system has an enlarged symmetry $U\left(1\right)\times U\left(1\right)$, with each $U\left(1\right)$ corresponding to one value of the helicity $\chi$. Because of the enlarged symmetry, this system admits exotic topological defects such as a fractional quantum vortex, which we show has a Majorana zero mode bound at its core. We discuss how the enlarged symmetry can be lifted by small perturbations, such as the Coulomb interaction or Fermi surface splitting in the presence of broken inversion symmetry, and we show that the resulting superconducting state can be topological or trivial depending on parameters. The $U\left(1\right)\times U\left(1\right)$ symmetry is restored at the phase boundary between the topological and trivial SC states, and allows for a transition between topologically distinct SC phases without the vanishing of the order parameter. We present a global phase diagram of the superconducting states and discuss possible experimental implications.

We study the dynamics of the equation obtained by Schryer and Walker for the motion of domain walls. The reduced equation is a reaction diffusion equation for the angle between the applied field and the magnetization vector. If the hard-axis anisotropy $K_d$ is much larger than the easy-axis anisotropy $K_u$, there is a range of applied fields where the dynamics does not select the Schryer-Walker solution. We give an analytic expression for the speed of the domain wall in this regime and the conditions for its existence.

We investigate the photon-mediated heat flow between two Josephson-coupled Bardeen-Cooper-Schrieffer (BCS) superconductors. We demonstrate that in standard low temperature experiments involving temperature-biased superconducting quantum interference devices (SQUIDs), this radiative contribution is negligible if compared to the direct galvanic one, but it largely exceeds the heat exchanged between electrons and the lattice phonons. The corresponding thermal conductance is found to be several orders of magnitude smaller, for real experiments setup parameters, than the universal quantum of thermal conductance, ${\kappa }_0\left(T\right)=\pi k_B^{2}T/6\hslash$.

We develop a variant of the density matrix renormalization group (DMRG) algorithm for two-dimensional cylinders that uses a real space representation in the direction along the axis of the cylinder and a momentum space representation in the direction around the circumference. The mixed representation allows us to use the momentum around the cylinder as a conserved quantity in the DMRG algorithm. Compared with the traditional purely real-space approach, we find a significant speedup in computation time and a considerable reduction in memory usage. Applying the method to the interacting fermionic Hofstadter model, we demonstrate a reduction in computation time by over 20-fold, in addition to a sixfold memory reduction.

Numerous efforts have been devoted to reveal exotic semimetallic phases with topologically nontrivial bulk and/or surface states in materials with strong spin-orbit coupling. In particular, semimetals with nodal line Fermi surface (FS) exhibit novel properties, and searching for candidate materials becomes an interesting research direction. Here we provide a generic condition for a fourfold degenerate nodal line FS in nonsymmorphic crystals with inversion and time-reversal symmetry (TRS). When there are two glide planes or screw axes perpendicular to each other, a pair of Bloch bands related by nonsymmorphic symmetry become degenerate on a Brillouin zone (BZ) boundary. There are two pairs of such bands, and they disperse in a way that the partners of two pairs are exchanged on other BZ boundaries. This enforces a nodal line FS on a BZ boundary plane protected by nonsymmorphic symmetries. When TRS is broken, fourfold degenerate Dirac points or Weyl ring FS could occur depending on a direction of the magnetic field. On a certain surface double helical surface states exist, which become double Fermi arcs as TRS is broken.

We investigate the competition between electron-solid and quantum-liquid phases in graphene, which arise in partially filled Landau levels. The differences in the wave function describing the electrons in the presence of a perpendicular magnetic field in graphene with respect to the conventional semiconductors, such as GaAs, can be captured in a form factor which carries the Landau-level index. This leads to a quantitative difference in the electron-solid and -liquid energies. For the lowest Landau level, there is no difference in the wave function of relativistic and nonrelativistic systems. We compute the cohesive energy of the solid phase analytically using a Hartree-Fock Hamiltonian. The liquid energies are computed analytically as well as numerically, using exact diagonalization. We find that the liquid phase dominates in the $n=1$ Landau level, whereas the Wigner crystal and electron-bubble phases become more prominent in the $n=2$ and 3 Landau level.

We study the formation of Majorana states in quasi-one-dimensional (quasi-1D) and two-dimensional square lattices with open boundary conditions, with general anisotropic Rashba coupling, in the presence of an applied Zeeman field and in the proximity of a superconductor. For systems in which the length of the system is very large (quasi-1D) we calculate analytically the exact topological invariant, and we find a rich corresponding phase diagram which is strongly dependent on the width of the system. We compare our results with previous results based on a few-band approximation. We also investigate numerically open two-dimensional systems of finite length in both directions. We use the recently introduced generalized Majorana polarization, which can locally evaluate the Majorana character of a given state. We find that the formation of Majoranas depends strongly on the geometry of the system: for a very elongated wire the finite-size numerical phase diagram reproduces the analytical phase diagram for infinite systems, while if the length and the width are comparable, no Majorana states can form; however, one can show the formation of “quasi-Majorana” states that have a local Majorana character but no global Majorana symmetry.

Hyperthermal ion implantation offers a controllable method of producing high-quality substitutionally doped graphene with nitrogen, an $n$-type dopant that has great potential for graphene electronics and spintronics applications where high carrier concentration, uniform doping, and minimal vacancy defect concentration is desired. Here we examine the transport properties of monolayer graphene sheets as a function of implantation beam energy and dose. We observe a transition from weak to strong localization that varies as a function of carrier concentration. For nominally equivalent doses, increased N ion energy results in an increasing magnetoresistance magnitude, reaching a value of approximately −5.5% at 5000 Oe, which we discuss in the context of dopant concentration and defect formation. We use a model for the temperature dependence of the conductivity that takes into account both temperature activation, due to the formation of a transport gap, and Mott variable-range hopping, due to the formation of defects, to further study the electronic properties of the doped films as a function of dose and N ion energy. We find that the temperature activation component dominates the behavior.

The effect of external mechanical fields on relaxor $0.94\left(\mathrm{N}{\mathrm{a}}_{1/2}\mathrm{B}{\mathrm{i}}_{1/2}\right)\mathrm{Ti}{\mathrm{O}}_3\text{−}0.06\mathrm{BaTi}{\mathrm{O}}_3$ was investigated by means of temperature- and stress-dependent dielectric constant measurements between 223 and 673 K. Analogous to previous investigations that showed an electric-field-induced ferroelectric long-range order in relaxor ferroelectrics, we show that compressive stress can also result in the transition to the long-range ferroelectric order, marked by the formation of an anomaly in the permittivity-temperature curves and a nonlinear, remanent change in permittivity during mechanical loading. In situ stress-dependent high-energy x-ray diffraction experiments were performed at room temperature and reveal an apparent phase transition during mechanical loading, consistent with previous macroscopic electrical measurements. The transition lines between the relaxor states and the stress-induced ferroelectric state were determined at constant temperatures with stress-dependent dielectric constant measurements, providing a stress-temperature phase diagram.

We studied the microscopic mechanism of the paraelectric-ferroelectric (PE-FE) phase transition of ${\mathrm{CsH}}_2{\mathrm{PO}}_4$ (CDP) by means of first-principles electronic structure calculations. The calculated structural parameters in the PE and FE phases as well as the total spontaneous polarization $P_s$ obtained with the Berry phase formalism for CDP are in good agreement with experiments. The main contribution to $P_s$ originates from a large $yx$ component of the calculated Born effective-charge tensor for the disordered protons. Moreover, this component is $\approx 2.7$ times larger than the $zx$ component of the proton effective-charge tensor relevant to the polarization in the H-bonded FE ${\mathrm{KH}}_2{\text{PO}}_4$ (KDP). This is the main feature that compensates the different number of protons per formula unit involved in the phase transitions for CDP and KDP to give close values for their measured $P_s$. Correlations among protons and heavy atoms along chains in the $b$ direction lead to larger instabilities for the global and local FE distortions in CDP and its deuterated counterpart DCDP. We conclude that the tunneling particle is a dressed proton (deuteron) in agreement with experiments and with recent ab initio results for KDP.

The zero-temperature phase diagram of the monoaxial chiral helimagnet in the magnetic-field plane formed by the components parallel and perpendicular to the helical axis is thoroughly analyzed. The nature of the transition to the commensurate state depends on the angle between the field and the helical axis. For field directions close to the directions parallel or perpendicular to the helical axis the transition is continuous, while for intermediate angles the transition is discontinuous and the incommensurate and commensurate states coexist on the transition line. The continuous and discontinuous transition lines are separated by two tricritical points with specific singular behavior. The location of the continuous and discontinuous lines and of the tricritical points depend strongly on the easy-plane anisotropy, the effect of which is analyzed. For high anisotropy the conical approximation locates the transition line very accurately, although it does not predict the continuous transitions and the tricritical behavior. It is shown that for high anisotropy, as in ${\mathrm{CrNb}}_3{\mathrm{S}}_6$, the form of the transition line is universal, that is, independent of the sample, and obeys a simple equation. The position of the tricritical points, which is not universal, is theoretically estimated for a sample of ${\mathrm{CrNb}}_3{\mathrm{S}}_6$.