We report an experimental observation of superconductivity in Cd3As2 thin films without application of external pressure. The films under study were synthesized by magnetron sputtering. Surface studies suggest that the observed transport characteristics are related to the polycrystalline continuous part of the investigated films with a homogeneous distribution of elements and the Cd-to-As ratio close to stoichiometric Cd3As2. The latter is also supported by Raman spectra of the studied films where two pronounced peaks inherent to Cd3As2 were observed. The obtained x-ray diffraction patterns for studied films also correspond to the Cd3As2 lattice. The formation of a superconducting phase in the films under study is confirmed by the characteristic behavior of the temperature and the magnetic field dependence of the sample resistivity, as well as by the presence of pronounced zero-resistance plateaus in the dV/dI characteristics. The corresponding H-c-T-c plots reveal a clearly pronounced linear behavior within the intermediate temperature range, similar to that observed for bulk Cd3As2 and Bi2Se3 films under pressure, suggesting the possibility of a nontrivial pairing in the films under investigation. We discuss a possible role of the sample inhomogeneities and crystal strains in the observed phenomena.
We consider physical properties of a superconductor with a recently proposed type of odd-frequency pairing that exhibits diamagnetic Meissner response ("odd-dia state"). Such a state was suggested in order to address stability issues arising in an odd-frequency superconducting state with paramagnetic Meissner response ("odd-para state"). Assuming the existence of an odd-dia state (due to a proper retarded interaction), we study its coexistence with an odd-para state. The latter is known to be generated as an induced superconducting component in, e.g., singlet superconductor/ferromagnet proximity structures or triplet superconductor/normal metal systems. Calculating the superfluid density of the mixed odd-para/odd-dia state and the Josephson current between the odd-para and odd-dia states, we find that the expressions for the currents in both cases have non-vanishing imaginary contributions and are therefore unphysical. We show that a realization of the odd-dia state implies the absence of a Hamiltonian description of the system, and suggest that there exists no physically realizable perturbation that could give rise to the spontaneous symmetry breaking necessary for an actual realization of the odd-dia superconducting state.
We consider the optical conductivity of a clean two-dimensional metal near a quantum spin-density-wave transition. Critical magnetic fluctuations are known to destroy fermionic coherence at “hot spots” of the Fermi surface but coherent quasiparticles survive in the rest of the Fermi surface. A large part of the Fermi surface is not really “cold” but rather “lukewarm” in a sense that coherent quasiparticles in that part survive but are strongly renormalized compared to the noninteracting case. We discuss the self-energy of lukewarm fermions and their contribution to the optical conductivity σ(), focusing specifically on scattering off composite bosons made of two critical magnetic fluctuations. Recent study [S. A. Hartnoll et al., Phys. Rev. B 84, 125115 (2011)] found that composite scattering gives the strongest contribution to the self-energy of lukewarm fermions and suggested that this may give rise to a non-Fermi-liquid behavior of the optical conductivity at the lowest frequencies. We show that the most singular term in the conductivity coming from self-energy insertions into the conductivity bubble σ(Omega) ∝ ln^3(Omega) /(Omega)^(1/3) is canceled out by the vertex-correction and Aslamazov-Larkin diagrams. However, the cancellation does not hold beyond logarithmic accuracy, and the remaining conductivity still diverges as 1/(Omega)^(1/3). We further argue that the 1/(Omega)^(1/3) behavior holds only at asymptotically low frequencies, well inside the frequency range affected by superconductivity. At larger Omega, up to frequencies above the Fermi energy, σ(Omega) scales as 1/(Omega), which is reminiscent of the behavior observed in the superconducting cuprates.
We perform a magneto-optical study of a two-dimensional electron systems in the regime of the Stoner ferromagnetic instability for even quantum Hall filling factors on MgxZn1−xO/ZnO heterostructures. Under conditions of Landau-level crossing, caused by enhanced spin susceptibility in combination with the tilting of the magnetic field, the transition between two rivaling phases, paramagnetic and ferromagnetic, is traced in terms of optical spectra reconstruction. Synchronous sharp transformations are observed both in the photoluminescence structure and parameters of collective excitations upon transition from paramagnetic to ferromagnetic ordering. Based on these measurements, a phase diagram is constructed in terms of the two-dimensional electron density and tilt angle of the magnetic field.Apart from stable paramagnetic and ferromagnetic phases, an instability region is found at intermediate parameters with the Stoner transition occurring at ν ≈ 2. The spin configuration in all cases is unambiguously determined by means of inelastic light scattering by spin-sensitive collective excitations. One indicator of the spin ordering is the intra-Landau-level spin exciton, which acquires a large spectral weight in the ferromagnetic phases. The other is an abrupt energy shift of the intersubband charge density excitation due to reconstruction of the many-particle energy contribution. From our analysis of photoluminescence and light scattering data, we estimate the ratio of surface areas occupied by the domains of the two phases in the vicinity of a transition point. In addition, the thermal smearing of a phase transition is characterized.
We consider a bilayer system of two-dimensional Bose-Einstein-condensed dipolar dark excitons (upper layer) and bright ones (bottom layer). We demonstrate that the interlayer interaction leads to a mixing between excitations from different layers. This mixing leads to the appearance of a second spectral branch in the spectrum of bright condensate. The excitation spectrum of the condensate of dark dipolar excitons then becomes optically accessible during luminescence spectra measurements of the bright condensate, which allows one to probe its kinetic properties. This approach is relevant for experimental setups, where detection via conventional techniques remains challenging; in particular, the suggested method is useful for studying dark dipolar excitons in transition metal dichalcogenide monolayers.
Although nanolasers typically have low Q factors and high lasing thresholds, they have been successfully implemented with various gain media. Intuitively, it seems that an increase in the gain coefficient would improve the characteristics of nanolasers. For a plasmonic nanolaser—in particular, a distributed feedback laser—we propose a self-consistent model that takes into account both spontaneous emission and the multimode character of laser generation to show that for a given pumping strength, the gain coefficient has an optimal value at which the radiation intensity is at a maximum and the radiation linewidth is at a minimum.
We calculate Aslamazov-Larkin (AL) paraconductity σAL(T) for a model of strongly disordered superconductors (dimensions d=2,3) with a large pseudogap whose magnitude strongly exceeds transition temperature Tc. We show that, within Gaussian approximation over Cooper-pair fluctuations, paraconductivity is just twice larger that the classical AL result at the same ε=(T−Tc)/Tc. Upon decreasing ε, Gaussian approximation is violated due to local fluctuations of pairing fields that become relevant at ε≤ε1≪1. Characteristic scale ε1 is much larger than the width ε2 of the thermodynamical critical region, that is determined via the Ginzburg criterion, ε2≈εd1. We argue that in the intermediate region ε2≤ε≤ε1, paraconductivity follows the same AL power law, albeit with another (yet unknown) numerical prefactor. At further decrease of the temperature, all kinds of fluctuational corrections become strong at ε≤ε2; in particular, conductivity occurs to be strongly inhomogeneous in real space.
We investigate the effect of interacting quantum phase slips on persistent current and its fluctuations in ultrathin superconducting nanowires and nanorings pierced by the external magnetic flux. We derive the effective action for these systems and map the original problem onto an effective sine-Gordon theory on torus. We evaluate both the flux dependent persistent current and the critical radius of the ring beyond which this current gets exponentially suppressed by quantum fluctuations. We also analyze fluctuations of persistent current caused by quantum phase slips. At low temperatures the supercurrent noise spectrum has the form of coherent peaks which can be tuned by the magnetic flux. Experimental observation of these peaks can directly demonstrate the existence of plasma modes in superconducting nanorings.
We present a detailed theoretical description of quantum coherent electron transport in voltage-biased crosslike Andreev interferometers.Making use of the charge conjugation symmetry encoded in the quasiclassical formalism, we elucidate a crucial role played by geometric and electron-hole asymmetries in these structures. We argue that a nonvanishing Aharonov-Bohm-like contribution to the current IS flowing in the superconducting contour may develop only in geometrically asymmetric interferometers making their behavior qualitatively different from that of symmetric devices. The current I_N in the normal contour—along with I_S—is found to be sensitive to phase-coherent effects thereby also acquiring a 2π-periodic dependence on the Josephson phase. In asymmetric structures this current develops an odd-in-phase contribution originating from electron-hole asymmetry. We demonstrate that both phase-dependent currents I_S and I_N can be controlled and manipulated by tuning the applied voltage, temperature, and system topology, thus rendering Andreev interferometers particularly important for future applications in modern electronics.
We present a detailed analysis of the phase diagram of antiferromagnets with competing exchange-driven and field-induced order parameters. By using the quasi-1D antiferromagnet BaCu2Si2O7 as a test case, we demonstrate that a model based on a Landau type of approach provides an adequate description of both the magnetization process and of the phase diagram. The developed model not only accounts correctly for the observed spin-reorientation transitions, but it predicts also their unusual angular dependence.
We demonstrate that a temperature gradient can strongly stimulate the thermoelectric signal, as well as dc Josephson current, in multiterminal superconducting hybrid nanostructures. At temperatures T sufficiently exceeding the Thouless energy of our device, both the supercurrent and the thermoinduced voltage are dominated by the contribution from nonequilibrium low-energy quasiparticles and are predicted to decay slowly (algebraically rather than exponentially) with increasing T . We also predict a nontrivial current-phase relation and a transition to a π-junction state controlled by both the temperature gradient and the system topology. All these features are simultaneously observable in the same experiment.
Selective excitation of coherent high-amplitude vibrations of atoms in a solid can induce exotic nonequilibrium states, in which the character of interactions between electronic, magnetic and lattice degrees of freedom is considerably altered and the underlying symmetries are broken. Here we use intense single-cycle terahertz pulses to drive coherently the dipole-active E1u phonon mode of a Bi2Se3 crystal. As a result, several Raman-active modes are simultaneously excited in a nonlinear process, while one of them, having the E2g symmetry, experiences dynamical splitting during the first two picoseconds after excitation. The corresponding angular scattering pattern is modified indicating the coexistence of two phonon modes characteristic of a nonequilibrium state with a lower crystal symmetry. We observe also a short-lived frequency splitting of the original E2g mode that immediately after excitation amounts to ∼25% of the unperturbed value. This transient state relaxes with a characteristic time of ∼1 ps, that is close to the decay time of the squared amplitude of the resonantly excited infrared-active E1u mode. We discuss possible mechanisms of the dynamical splitting: nonlinear lattice deformation caused by the intense E1u vibrations and excitation of anisotropic electronic distribution due to nonlinear electron-phonon interaction. Our data also contain an evidence in favor of the sum-frequency Raman mechanism of generation of the coherent E2g phonons in Bi2Se3 excited by terahertz pulses.
We study theoretically the nonequilibrium exciton transport in monolayer transition metal dichalcogenides. We consider the situation where excitons interact with nonequilibrium phonons, e.g., under the conditions of localized excitation where a “hot spot” is formed. We develop the theory of the exciton drag by the phonons and analyze in detail the regimes of diffusive propagation of phonons and ballistic propagation of phonons where the phonon wind is generated. We demonstrate that a halolike spatial distribution of excitons akin observed in [Phys. Rev. Lett. 120, 207401 (2018)] can result from the exciton drag by nonequilibrium phonons.
Electron spin polarization up to 100% has been observed in type-II narrow-gap heterostructures with ultrathin InSb insertions in an InAs matrix via investigation of circularly polarized photoluminescence in an external magnetic field applied in Faraday geometry. The polarization degree decreases drastically, changes its sign, and saturates finally at the value of 10% in the limit of either high temperature or strong excitation. The observed effect is explained in terms of strong Zeeman splitting of the electron conduction band in the InAs matrix and a heavy-hole state confined in the InSb insertion, due to a large intrinsic g-factor of both types of carriers. The hole ground state in a monolayer scale InSb/InAs quantum well, calculated using a tight-binding approach, fits well the observed emission wavelength. Temperature dependence of the emission polarization degree is in good agreement with its theoretical estimation performed in the framework of a proposed phenomenological model.
We address polarization instability in a freely decaying polariton condensate created by 2-ps-long linearly polarized laser pulses in the upper sublevel of the lower-polariton (LP) branch in a GaAs-based microcavity with reduced symmetry. The generated linearly polarized condensate is found to lose its stability at excitation densities above the threshold value: it passes into the regime of inner Josephson oscillations with strongly oscillating circular and diagonal linear polarization degrees, as well as monotonically decreased oscillations in linear polarization accompanied by a gradual increase in the condensate of the low-sublevel component. These phenomena occur with a relatively small decrease in the total polarization and spatial coherence of the spinor condensate. At high LP densities, the LP-LP interaction leads to the nonlinear Josephson effect. All effects are found to be well reproduced by the model based on spinor Gross-Pitaevskii equations. The cause of the instability was clarified by considering a simplified model of the spinor 0D oscillator: it is spin anisotropy of the LP-LP interaction. The threshold density was shown to increase with decreasing difference α of the constants of the interparticle interaction of LPs with identical and opposite spins as δl/α, where δl is the LP level splitting. The reduction in the linear polarization is connected with the fact that the LPs escaping from the condensate with oscillating circular polarization carry off more energy than from the linearly polarized one.
Chiral planar metamaterials are known for their possibility to show strong nonlinear optical effects such as second harmonic generation (SHG) circular dichroism or asymmetric SHG. The underlying mechanisms are commonly discussed in terms of local field effects and formation of localized SHG sources (so called “hotspots”) that are sensitive to the shape and size of meta-atoms. Nevertheless, a full characterization of the polarization state of the nonlinear optical radiation from the hotspots has not been performed until now. Here we present the results of the polarization-resolved second harmonic generation microscopy studies of planar chiral G-shaped metamaterials. We demonstrate that the SHG radiation coming from the hotspots that are localized within a single meta-atom is partially polarized; moreover, the SHG polarization state reveals the chirality of the structure. The observed effects are attributed to the induced plasmonic current oscillations at the fundamental frequency along with the local field distribution.
We have experimentally studied the renormalized effective mass m* and Dingle temperature TD in two spin subbands with essentially different electron populations. Firstly, we found that the product m*T_D that determines the damping of quantum oscillations, to the first approximation, is the same in the majority and minority subbands even at a spin polarization degree as high as 66%. This result confirms the theoretical predictions that the interaction takes place at high energies ~EF rather than within a narrow strip of energies EF ± kBT . Secondly, to the next approximation, we revealed a difference in the damping factor of the two spin subbands, which causes skewness of the oscillation line shape. In the absence of the in-plane magnetic field , the damping factor m*T_D is systematically smaller in the spin-majority subband. The difference, quantified with the skew factor γ = (TD↓ − TD↑)/2T_D0 can be as large as 20%. The skew factor tends to decrease as B or temperature grow, or B⊥ decreases; for low electron densities and high in-plane fields, the skew factor even changes sign. Finally, we compared the temperature and magnetic field dependencies of the magneto-oscillation amplitude with predictions of the interaction correction theory, and found, besides some qualitative similarities, several quantitative and qualitative differences. To explain qualitatively our results, we suggested an empirical model that assumes the existence of easily magnetized triplet scatterers on the Si/SiO2 interface.
We report temperature and density dependences of the spin susceptibility of strongly interacting electrons in Si inversion layers. We measured (i) the itinerant electron susceptibility χ∗ from the Shubnikov-de Haas oscillations in crossed magnetic fields and (ii) thermodynamic susceptibility χTsensitive to all the electrons in the layer. Both χ∗ and χT are strongly enhanced with lowering the electron density in the metallic phase. However, there is no sign of divergency of either quantity at the density of the metal-insulator transition nc. Moreover, the value of χT, which can be measured across the transition down to very low densities deep in the insulating phase, increases with density at n<nc, as expected. In the absence of magnetic field, we found the temperature dependence of χ∗ to be consistent with Fermi-liquid-based predictions, and to be much weaker than the power law predicted by non-Fermi-liquid models. We attribute a much stronger temperature dependence of χT to localized spin droplets. In strong enough in-plane magnetic field, we found the temperature dependence of χ∗to be stronger than that expected for the Fermi liquid interaction corrections.