Spectroscopy of bulk and few-layer superconducting NbSe2 with van der Waals tunnel junctions.

Tunnel junctions, an established platform for high resolution spectroscopy of superconductors, require defect-free insulating barriers; however, oxides, the most common barrier, can only grow on a limited selection of materials. We show that van der Waals tunnel barriers, fabricated by exfoliation and transfer of layered semiconductors, sustain stable currents with strong suppression of sub-gap tunneling. This allows us to measure the spectra of bulk (20 nm) and ultrathin (3- and 4-layer) NbSe2 devices at 70 mK. These exhibit two distinct superconducting gaps, the larger of which decreases monotonically with thickness and critical temperature. The spectra are analyzed using a two-band model incorporating depairing. In the bulk, the smaller gap exhibits strong depairing in in-plane magnetic fields, consistent with high out-of-plane Fermi velocity. In the few-layer devices, the large gap exhibits negligible depairing, consistent with out-of-plane spin locking due to Ising spin-orbit coupling. In the 3-layer device, the large gap persists beyond the Pauli limit.

Emergence of domains and nonlinear transport in the zero-resistance state.

We study transport in the domain state, the so-called zero-resistance state, that emerges in a two-dimensional electron system in which the combined action of microwave radiation and magnetic field produces a negative absolute conductivity. We show that the voltage-biased system has a rich phase diagram in the system size and voltage plane, with second- and first-order transitions between the domain and homogeneous states for small and large voltages, respectively. We find the residual negative dissipative resistance in the stable domain state.

Enhancement of the London penetration depth in pnictides at the onset of spin-density-wave order under superconducting dome.

Recent measurements of the doping dependence of the London penetration depth λ(x) at low T in clean samples of isovalent BaFe2(As(1-x)P(x))2 at T≪T(c) [Hashimoto et al., Science 336, 1554 (2012)] revealed a peak in λ(x) near optimal doping x=0.3. The observation of the peak at T≪T(c), points to the existence of a quantum critical point beneath the superconducting dome. We associate such a quantum critical point with the onset of a spin-density-wave order and show that the renormalization of λ(x) by critical magnetic fluctuations gives rise to the observed feature. We argue that the case of pnictides is conceptually different from a one-component Galilean invariant Fermi liquid, for which correlation effects do not cause the renormalization of the London penetration depth at T=0.

Interpocket pairing and gap symmetry in Fe-based superconductors with only electron pockets.

We analyze the pairing symmetry in Fe-based superconductors AFe2Se2 (A=K, Rb, Cs) which contain only electron pockets. We argue that the pairing condensate in such systems contains not only intrapocket component but also interpocket component, made of fermions belonging to different electron pockets. We analyze the interplay between intrapocket and interpocket pairing, depending on the ellipticity of electron pockets and the strength of their hybridization. We show that with increasing hybridization, the system undergoes a transition from a d-wave state to an s+- state, in which the gap changes sign between hybridized pockets. This s+- state has the full gap and at the same time supports spin resonance, in agreement with the data. Near the boundary between d and s+- states, we found a long-sought s+id state which breaks time-reversal symmetry.

Quantum-fluctuation effects in the transport properties of ultrathin films of disordered superconductors above the paramagnetic limit.

We study the transport in ultrathin disordered film near the quantum critical point induced by the Zeeman field. We calculate corrections to the normal state conductivity due to quantum pairing fluctuations. The fluctuation-induced transport is mediated by virtual rather than real quasiparticle excitations. We find that at zero temperature, where the corrections come from purely quantum fluctuations, the Aslamazov-Larkin paraconductivity term, the Maki-Thompson interference contribution, and the density of states effects are all of the same order. The total correction leads to the negative magnetoresistance. This result is in qualitative agreement with the recent transport observations in the parallel magnetic field of the homogeneously disordered amorphous films and superconducting two-dimensional electron gas realized at the oxide interfaces.

Nonlinear magnetoresistance oscillations in intensely irradiated two-dimensional electron systems induced by multiphoton processes.

We report on magneto-oscillations in differential resistivity of a two-dimensional electron system subject to intense microwave radiation. The period of these oscillations is determined not only by microwave frequency but also by its intensity. A theoretical model based on quantum kinetics at high microwave power captures all important characteristics of this phenomenon which is strongly nonlinear in microwave intensity. Our results demonstrate a crucial role of the multiphoton processes near the cyclotron resonance and its harmonics in the presence of strong dc electric field and offer a unique way to reliably determine the intensity of microwaves acting on electrons.

Metal to insulator transition on the N=0 Landau level in graphene.

The magnetotransport in single layer graphene has been experimentally investigated in magnetic fields up to 18 T as a function of temperature. A pronounced T dependence is observed for T≲50 K, which is either metallic, or insulating, depending on the filling factor ν. The metal-insulator transition (MIT) occurs at |ν{c}|∼0.65 and in the regime of the dissipative transport, where the longitudinal resistance Rxx>1/2R{K}. The critical resistivity (Rxx per square) is ρ{xx}(ν{c})≈1/2R{K} and is correlated with the appearance of zero plateau in Hall conductivity σ{xy}(ν) and peaks in σ{xx}(ν). This leads us to construct a universal low-T (n, B) phase diagram of this quantum phase transition.

Dynamics of excitations in a one-dimensional bose liquid.

We show that the dynamic structure factor of a one-dimensional Bose liquid has a power-law singularity defining the main mode of collective excitations. Using the Lieb-Liniger model, we evaluate the corresponding exponent as a function of the wave vector and the interaction strength.

Dynamic response of one-dimensional interacting fermions.

We evaluate the dynamic structure factor S(q, omega) of interacting one-dimensional spinless fermions with a nonlinear dispersion relation. The combined effect of the nonlinear dispersion and of the interactions leads to new universal features of S(q, omega). The sharp peak S(q, omega) approximately q(delta(omega -uq), characteristic for the Tomonaga-Luttinger model, broadens up; for a fixed becomes finite at arbitrarily large . The main spectral weight, however, is confined to a narrow frequency interval of the width deltaomega approximately q(2)/m. At the boundaries of this interval the structure factor exhibits power-law singularities with exponents depending on the interaction strength and on the wave number q.

Spin polarization of electrons by nonmagnetic heterostructures: the basics of spin optics.

We propose to use the lateral interface between two regions with different strengths of the spin-orbit interaction(s) to spin polarize the electrons in gated two-dimensional semiconductor heterostructures. For a beam with a nonzero angle of incidence, the transmitted electrons will split into two spin polarization components propagating at different angles. We analyze the refraction at such an interface and outline the basic schemes for filtration and control of the electron spin.

Relaxation to the invariant density for the kicked rotor

The relaxation rates to the invariant density in the chaotic phase space component of the kicked rotor (standard map) are calculated analytically for a large stochasticity parameter K. These rates are the logarithms of the poles of the matrix elements of the resolvent, Rinsertion mark(z)=(z-Uinsertion mark)(-1), of the classical evolution operator Uinsertion mark. The resolvent poles are located inside the unit circle. For hyperbolic systems this is a rigorous result, but little is known about mixed systems such as the kicked rotor. In this work, the leading relaxation rates of the kicked rotor are calculated in the presence of noise, to the leading order in 1/sqrt[K]. Then the limit of vanishing noise is taken and the relaxation rates are found to be finite, corresponding to poles lying inside the unit circle. It is found that the slow relaxation rates, in essence, correspond to diffusion modes in the momentum direction. Faster relaxation modes intermix the motion in the momentum and the angle space. The slowest relaxation rate of distributions in the angle space is calculated analytically by studying the dynamics of inhomogeneities projected down to this space. The analytical results are verified by numerical simulations.

Relaxation and diffusion for the kicked rotor

The dynamics of the kicked rotor, which is a paradigm for a mixed system, where the motion in some parts of phase space is chaotic and in other parts is regular, is studied statistically. The evolution operator of phase space densities in the chaotic component is calculated in the presence of noise, and the limit of vanishing noise is taken in the end. The relaxation rates to the equilibrium density are calculated analytically within an approximation that improves with increasing stochasticity. The results are tested numerically. A global picture is presented of relaxation to the equilibrium density in the chaotic component when the system is bounded and to diffusive behavior when it is unbounded.

[Prognostic significance of lactate concentration and indicators of acid-base balance in air embolism of cerebral vessels]. / Prognosticheskoe znachenie kontsentratsii laktata i pokazatelei kislotno-osnovnogo ravnovesiia spinomozgovoi zhidkosti pri vozdushnoi émbolii sosudov golovnogo mozga.

Acid-base balance and lactate concentration in the arterial blood and spinal liquor have been studied in 32 patients after air embolism of the brain vessels developed during cardiopulmonary bypass surgery. It has been shown that an increase of lactate concentration in the spinal liquor over 3.5 mmol/l after air embolism of the brain vessels is indicative of severe hypoxic brain damage, in which the prospect of stable consciousness recovery is doubtful. In lactate concentration 4.5-5 mmol/l and higher as well as in metabolic acidosis of liquor (sodium hydrocarbonate concentration less than 20 mmol/l) the intensive care procedures used were not effective. It is believed that these parameters are of prognostic value in the condition under study.

[Practical aspects of cardioplegia]. / Nekotorye prakticheskie voprosy kardioplegii.

Concentrations of the ingredients have been analysed in a nonstandard cardioplegia solution (CS) and CS from St. Thomas Hospital at the outset before CS introduction into the coronary vessels of patients under cardioplegia. It has been shown that the use of nonstandard CS was accompanied by undesirable variability in ingredient concentration, which to some extent may be accounted for by the use of incompletely unfrozen CS. The use of CS from St. Thomas Hospital could be accompanied by moderate hypermagnesiumemia, which did not lead to the onset of vascular insufficiency. It is stressed that plasma magnesium content should be controlled when this CS is used.

[New parameters of the oxygen transport function of arterial blood]. / O novykh parametrakh kislorodtransportnoi funktsii arterial'noi krovi.

New criteria of the arterial blood oxygen transport function suggested by O. Siggaard-Andersen et al. are reviewed. The examples from clinical practice show the possibility to evaluate the capacity of the arterial blood to transport O2 during general anesthesia and intensive care.

[The Soviet colloid modegel as a component of a perfusion medium used in surgery with extracorporeal circulation]. / Otechestvennyi kolloid modezhel' kak komponent perfuzionnoi sredy dlia operatsii s iskusstvennym krovoobrashcheniem.

The results of the application of a new Soviet colloid solution modegel in cardiopulmonary bypass surgery in 50 patients have been analysed. The data obtained have been compared to those of control patients who were administered gelatinol instead of modegel. Modegel ensures optimal basic plasma electrolyte content, colloid-osmotic pressure and osmolality during perfusion.

[Regulation of blood oxygen transport function in patients after open-heart surgery]. / Reguliatsiia kislorodtransportnoi funktsii krovi u bol'nykh posle operatsii na otkrytom serdtse.

The oxygen transport function of the blood was studied in 223 patients in the early postoperative period after open-heart surgery, which included correction of acquired cardiac valve defects and aortocoronary bypass. The studied parameters comprised central hemodynamics, pulmonary gas exchange, oxygen supply and utilization, acid-base status, blood gas and lactate content, as well as erythrocytic 2,3-diphosphoglycerate (DPG); Boehringer Mannheim reagents (FRG) were used. The venous blood P50 was calculated using a Severinghaus device. It was found, that the Bohr effect and temperature changes are the major factors controlling the Hb affinity to oxygen. No coefficient modifications in pH and T were detected. Patients with cardiac and pulmonary disorders showed, along with the Bohr effect and temperature influences, an additional shift of the oxyhemoglobin dissociation curve. A positive relationship was revealed between the 2,3-DPG level and standard P50 value. However, the coefficient was low (0.010) at 2,3-DPG and insufficiently stable. It is most probable, that 2,3-DPG mediates the buffer effect eliminating the drastic P50 deviations and is not completely responsible for additional ODC shifts in cardiopulmonary disorders. A quantitative relationship between hypoxia and Hb affinity to oxygen was revealed. The coefficient at SO2 Y varied from -0.0025 to -0.0030 by one percent of changes in the latter. Reduction in SO2 by 15 percent in cardiopulmonary patients was equivalent to the Bohr effect at a pH fall by 0.1 unit.

[Transcutaneous measurement of gas exchange parameters in anesthesiologic practice]. / Transkutannoe izmerenie parametrov gazoobmena v anesteziologicheskoi praktike.

pO2 and pCO2 were studied transcutaneously (tcpO2 and tcpCO2) in 36 patients during reconstructive surgery performed under various multicomponent balanced anesthesia techniques. The results, in the absence of hemodynamic disturbances, serve as a reliable indication of changes in arterial blood pO2 and pCO2 (PaO2 and PaCO2), however, it is impossible to identify transcutaneous PaO2 and PaCO2 values. A decrease in tcpO2 and an increase in tcpCO2 during their dynamic study may give evidence of the inadequacy of controlled lung ventilation. At the same time, with normal PaO2 and PaCO2 values, hypoxia and hypercarbia, recorded, using transcutaneous transducers, may serve as an early index of central and peripheral hemodynamic disturbances. It is advisable to use the results of transcutaneous pO2 and pCO2 measurements as additional tests, characterizing O2 and CO2 transport.

[Activated coagulation time and the concentration of heparin in assessing the level of blood heparinization during operations using artificial blood circulation]. / Aktivirovannoe vremia svertyvaniia i kontsentratsiia geparina v otsenki urovnia geparinizatsii krovi pri operatsiiakh s iskusstvennym krovoobrashcheniem.

Blood heparinization was assessed, using two techniques (activated coagulation time--ACT and heparin concentration) in 31 patients during cardiopulmonary bypass surgery. It has been shown that ACT-technique, not always revealing heparin concentration, may serve as a criterion for blood heparinization adequacy during cardiopulmonary bypass, as in every case it reflects individual changes in anticoagulant blood activity.