arXiv subjects

V. M. Krasnov

Publications and source records attributed to V. M. Krasnov.

At least 37 records · Page 2Linked to original sources

arXiv2013

High bias anomaly in YBa2Cu3O7/LaMnO_{3+delta}/YBa2Cu3O7 Superconductor/Ferromagnetic Insulator/Superconductor junctions: Evidence for a long-range superconducting proximity effect through the conduction band of a ferromagnetic insulator

We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa$_2$Cu$_3$O$_{7-x}$/LaMnO$_{3+δ}$/YBa$_2$Cu$_3$O$_{7-x}$) tunnel junctions. At a large bias voltage $V\sim 1$ V we observe a step-like onset of excess current that occurs below the superconducting transition temperature $T<T_c$ and is easily suppressed by a magnetic field. The phenomenon is attributed to a novel type of the superconducting proximity effect of non-equilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunneling process. The occurrence of a strongly non-equilibrium population is confirmed by the detection of photon emission at large bias voltage. Since the conduction band in our ferromagnetic insulator is strongly spin polarized, the long-range (20 nm) of the observed proximity effect provides evidence for an unconventional spin-triplet superconducting state.

cond-mat.supr-con↗

arXiv2012

Membrane-based nanocalorimeter for high-resolution measurements of low-temperature specific heat

A differential, membrane-based nanocalorimeter for general specific heat studies of very small samples, ranging from 0.5 mg to sub-μg in mass, is described. The calorimeter operates over the temperature range from above room temperature down to 0.5 K. It consists of a pair of cells, each of which is a stack of heaters and thermometer in the center of a silicon nitride membrane, in total giving a background heat capacity less than 100 nJ/K at 300 K, decreasing to 10 pJ/K at 1K. The device has several distinctive features: i) The resistive thermometer, made of a Ge_{1-x}Au_{x} alloy, displays a high dimensionless sensitivity |dlnR/dlnT | \geq 1 over the entire temperature range. ii) The sample is placed in direct contact with the thermometer, which is allowed to self-heat. The thermometer can thus be operated at high dc current to increase the resolution. iii) Data are acquired with a set of eight synchronized lock-in amplifiers measuring dc, 1st and 2nd harmonic signals of heaters and thermometer. This gives high resolution and allows continuous output adjustments without additional noise. iv) Absolute accuracy is achieved via a variable-frequency-fixed-phase technique in which the measurement frequency is automatically adjusted during the measurements to account for the temperature variation of the sample heat capacity and the device thermal conductance. The performance of the calorimeter is illustrated by studying the heat capacity of a small Au sample and the specific heat of a 2.6 μg piece of superconducting Pb in various magnetic fields.

physics.ins-det↗

arXiv2012

Disparity of superconducting and pseudogap scales in low-Tc Bi-2201 cuprates

We experimentally study transport and intrinsic tunneling characteristics of a single-layer cuprate Bi(2+x)Sr(2-y)CuO(6+delta) with a low superconducting critical temperature Tc < 4 K. It is observed that the superconducting energy, critical field and fluctuation temperature range are scaling down with Tc, while the corresponding pseudogap characteristics have the same order of magnitude as for high-Tc cuprates with 20 to 30 times higher Tc. The observed disparity of the superconducting and pseudogap scales clearly reveals their different origins.

cond-mat.supr-con↗

arXiv2011

Temperature dependence of geometrical and velocity matching resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions

We study temperature dependence of geometrical (Fiske) and velocity-matching (Eck) resonances in the flux flow state of small Bi2Sr2CaCu2O8+x mesa structures. It is shown that the quality factor of resonances is high at low T, but rapidly decreases with increasing temperature already at T > 10 K. We also study T-dependencies of resonant voltages and the speed of electromagnetic waves (the Swihart velocity). Surprisingly it is observed that the Swihart velocity exhibits a flat T-dependence at low T, following T-dependence of the c-axis critical current, rather than the expected linear T-dependence of the London penetration depth. Our data indicate that self-heating is detrimental for operation of mesas as coherent THz oscillators because it limits the emission power via suppression of the quality factor. On the other hand, significant temperature dependence of the Swihart velocity allows broad-range tunability of the output frequency.

cond-mat.supr-con↗

arXiv2011

Anomalous Hall effect in NiPt thin films

We study Hall effect in sputtered NixPt1-x thin films with different Ni concentrations. Temperature, magnetic field and angular dependencies are analyzed and the phase diagram of NiPt thin films is obtained. It is found that films with sub-critical Ni concentration exhibit cluster-glass behavior at low temperatures with a perpendicular magnetic anisotropy below the freezing temperature. Films with over-critical Ni concentration are ferromagnetic with parallel anisotropy. At the critical concentration the state of the film is strongly frustrated. Such films demonstrate canted magnetization with the easy axis rotating as a function of temperature. The magnetism appears via consecutive paramagnetic - cluster glass - ferromagnetic transitions, rather than a single second-order phase transition. But most remarkably, the extraordinary Hall effect changes sign at the critical concentration. We suggest that this is associated with a reconstruction of the electronic structure of the alloy at the normal metal - ferromagnet quantum phase transition.

cond-mat.mtrl-sci↗

arXiv2011

Comparative analysis of tunneling magnetoresistance in low-$T_c$ Nb/AlAlOx/Nb and high-$T_c$ Bi$_{2-y}$Pb$_y$Sr$_2$CaCu$_2$O$_{8+δ}$ intrinsic Josephson junctions

We perform a detailed comparison of magnetotunneling in conventional low-$T_c$ Nb/AlAlOx/Nb junctions with that in slightly overdoped Bi$_{2-y}$Pb$_y$Sr$_2$CaCu$_2$O$_{8+δ}$ [Bi(Pb)-2212] intrinsic Josephson junctions and with microscopic calculations. It is found that both types of junctions behave in a qualitatively similar way. Both magnetic field and temperature suppress superconductivity in the state-conserving manner. This leads to the characteristic sign-change of tunneling magnetoresistance from the negative at the sub-gap to the positive at the sum-gap bias. We derived theoretically and verified experimentally scaling laws of magnetotunneling characteristics and employ them for accurate extraction of the upper critical field $H_{c2}$. For Nb an extended region of surface superconductivity at $H_{c2} T_c/2$, but remains negative, although very small, above $T_c$. This may indicate existence of an extended fluctuation region, which, however, does not destroy the general second-order type of the phase transition at $T_c$.

cond-mat.supr-con↗

arXiv2011

Comment on Counterintuitive consequence of heating in strongly driven intrinsic junctions of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ mesas

In a recent paper [Phys.Rev.B 81, 224518 (2010)], C. Kurter et al, analyzed the effect of strong self-heating in large-area Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi-2212) mesa structures. They conclude that $dI/dV$ conductance peaks in their mesas occur when mesas are heated to the superconducting critical temperature $T_c$. Further on they extrapolate this statement for all mesas, including much smaller and much better thermally anchored mesas used in Intrinsic Tunnelling Spectroscopy (ITS). Here I show that their conclusion does not hold neither for previously reported data, nor even for their own mesas; the very remote extrapolation to ITS is invalid. I also point out a number of inconsistencies and misleading references.

cond-mat.supr-con↗

arXiv2010

THz emission from intrinsic Josephson junctions at zero magnetic field via breather auto-oscillations

I propose a new mechanism of intense high-frequency electromagnetic wave generation by spatially uniform stacked Josephson junctions at zero magnetic field. The ac-Josephson effect converts the dc-bias voltage into ac-supercurrent, however, in the absence of spatial variation of Josephson phase difference, does not provide dc-to-ac power conversion, needed for emission of electromagnetic waves. Here I demonstrate that at geometrical resonance conditions, the spatial homogeneity of phase can be spontaneously broken by appearance of breathers (bound fluxon-antifluxon pairs), facilitating effective dc-to-ac power conversion. The proposed mechanism explains all major features of recently observed THz radiation from large area Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ mesa structures.

cond-mat.supr-con↗

arXiv2010

Observation of superluminal geometrical resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions

We study Fiske steps in small Bi2Sr2CaCu2O8+x mesa structures, containing only few stacked intrinsic Josephson junctions. Careful alignment of magnetic field prevents penetration of Abrikosov vortices and facilitates observation of a large variety of high quality geometrical resonances, including superluminal with velocities larger than the slowest velocity of electromagnetic waves. A small number of junctions limits the number of resonant modes and allows accurate identification of modes and velocities. It is shown that superluminal geometrical resonances can be excited by subluminal fluxon motion and that flux-flow itself becomes superluminal at high magnetic fields. We argue that observation of high-quality superluminal geometrical resonances is crucial for realization of the coherent flux-flow oscillator in the THz frequency range.

cond-mat.supr-con↗

arXiv2010

THz emission from a stacked coherent flux-flow oscillator: non-local radiative boundary conditions and the role of geometrical resonances

I derive simple non-local dynamic boundary conditions, suitable for modelling of radiation emission from stacked Josephson junctions, and employ them for analysis of flux-flow emission from intrinsic Josephson junctions in high-$T_c$ superconductors. It is shown that due to the lack of Lorenz contraction of fluxons in stacked junctions, high quality geometrical resonances are prerequisite for high power emission from the stack. This leads to a dual role of the radiative impedance: on the one hand, small impedance increases the efficiency of emission from the stack, on the other hand, enhanced radiative losses reduce the quality factor of geometrical resonances, which may decrease the total emission power. Therefore, the optimal conditions for the coherent flux-flow oscillator are achieved when radiative losses are comparable to resistive losses inside the stack.

cond-mat.supr-con↗

arXiv2009

Nonlinear nonequilibrium quasiparticle relaxation in Josephson junctions

Nonequilibrium electrons in superconductors relax and eventually recombine into Cooper pairs. Relaxation is facilitated by electron-boson interaction and is accompanied by emission of nonequilibrium bosons. Here I solve numerically a full set of nonlinear kinetic balance equations for stacked Josephson junctions, which allows analysis of strongly nonequilibrium phenomena. It is shown that nonlinearity becomes significant already at very small disequilibrium. The following new, essentially nonlinear effects are obtained: (i) At even-gap voltages $V=2n Δ/e$ $(n=2,3,...)$ nonequilibrium bosonic bands overlap. This leads to enhanced emission of $Ω= 2 Δ$ bosons and to appearance of dips in tunnel conductance. (ii) A new type of radiative solution is found at strong disequilibrium. It is characterized by the fast stimulated relaxation of nonequilibrium quasiparticles. A stack in this state behaves as a light emitting diode and directly converts electric power to boson emission, without utilization of the ac-Josephson effect. This leads to very high radiation efficacy and to significant radiative cooling of the stack. The phenomenon can be used for realization of a new type of superconducting cascade laser in the THz frequency range.

cond-mat.supr-con↗

arXiv2009

Stabilization of in-phase fluxon state by geometrical confinement in small Bi-2212 mesa structures

The in-phase (rectangular) fluxon lattice is required for achieving coherent THz emission from stacked Josephson junctions. Unfortunately, it is usually unstable due to mutual repulsion of fluxons in neighbor junctions, which favors the out-of-phase (triangular) lattice. Here we experimentally study magnetic field modulation of the critical current in small Bi-2212 mesa structures with different sizes. A clear Fraunhofer-like modulation is observed when the field is aligned strictly parallel to superconducting CuO planes. For long mesas the periodicity of modulation is equal to half the flux quantum per intrinsic Josephson junction, corresponding to the triangular fluxon lattice. However, the periodicity is changed to one flux quantum, characteristic to the rectangular fluxon lattice, both by decreasing the length of the mesas and by increasing magnetic field. Thus, we demonstrate that the stationary in-phase fluxon state can be effectively stabilized by geometrical confinement in small Bi-2212 mesa structures.

cond-mat.supr-con↗

arXiv2009

Temperature dependence of the bulk energy gap in underdoped Bi-2212: Evidence for the mean-field superconducting transition

Understanding of the puzzling phenomenon of high temperature superconductivity requires reliable spectroscopic information about temperature dependence of the bulk electronic density of states. Here I present a comprehensive analysis of $T-$evolution of bulk electronic spectra in Bi-2212 obtained by Intrinsic Tunneling Spectroscopy on small mesa structures. Unambiguous spectroscopic information is obtained by obviation of self-heating problem and by improving the spectroscopic resolution. The obtained data indicate that the superconducting transition maintains the mean-field character down to moderate underdoping, and is associated with an abrupt opening of the superconducting gap, which is well described by the conventional BCS $T-$dependence. The mean-field critical temperature reaches maximum at the optimal doping and decreases with underdoping. Such behavior is inconsistent with theories assuming intimate connection between superconducting and antiferromagnetic spin gaps, and support proposals associating high temperature superconductivity with the presence of competing ground states and a quantum critical point near optimal doping.

cond-mat.supr-con↗

arXiv2008

Crossover from Thermal Activation to Quantum Interlayer Transport at the Superconducting Transition Temperature of Bi-2212

We perform a detailed study of temperature, bias and doping dependance of interlayer transport in the layered high temperature superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. We observe that the shape of interlayer characteristics in underdoped crystals exhibit a remarkable crossover at the superconducting transition temperature: from thermal activation-type at $T>T_c$, to almost $T-$independent quantum tunneling-type, at $T<T_c$. Our data indicates that the interlayer transport mechanism may change with doping: from the conventional single quasiparticle tunneling in overdoped, to a progressively increasing Cooper pair contribution in underdoped crystals.

cond-mat.supr-con↗

arXiv2007

Anticorrelation between temperature and fluctuations in moderately damped Josephson junctions

We study the influence of dissipation on the switching current statistics of moderately damped Josephson junctions. Different types of both low- and high- $T_c$ junctions with controlled damping are studied. The damping parameter of the junctions is tuned in a wide range by changing temperature, magnetic field, gate voltage, introducing a ferromagnetic layer or in-situ capacitive shunting. A paradoxical collapse of switching current fluctuations occurs with increasing $T$ in all studied junctions. The phenomenon critically depends on dissipation in the junction and is explained by interplay of two counteracting consequences of thermal fluctuations, which on the one hand assist in premature switching into the resistive state and on the other hand help in retrapping back to the superconducting state. This is one of the rare examples of anticorrelation between temperature and fluctuation amplitude of a physically measurable quantity.

cond-mat.supr-con↗

arXiv2006

Quantum cascade phenomenon in natural atomic superlattices formed in Bi2Sr2CaCu2O8 single crystals

Natural atomic superlattices are formed in certain strongly anisotropic layered compound. Here we study interlayer transport in single crystals of Bi2Sr2CaCu2O8 cuprates, which represent stacks of atomic scale intrinsic Josephson junctions. A series of resonant dips in conductance is observed at condition when bremsstrahlung and recombination bands in non-equilibrium spectrum of Josephson junctions overlap. The phenomenon is explained in terms of self-detection of a new type of collective strongly non-equilibrium state in stacked Josephson junctions, bearing certain resemblance with operation of a Quantum Cascade Laser. Conclusions are supported by in-situ generation-detection experiments, providing evidence for cascade amplification of recombination radiation, and by numerical simulations.

cond-mat.supr-con↗

arXiv2006

In-situ measurement of self-heating in intrinsic tunnelling spectroscopy

Using advanced sample engineering we performed simultaneous measurement of intrinsic (interlayer) tunnelling characteristics and in-situ monitoring of temperature in small Bi-2212 mesa structures. Together with a systematic study of size-dependence of intrinsic tunnelling, this allowed an unambiguous discrimination between artifacts of self-heating and gaps in electronic spectrum of Bi-2212 High-$T_c$ superconductor. Finally, we analyze the reliability of intrinsic tunnelling spectroscopy and discuss how it can be improved.

cond-mat.supr-con↗

arXiv2006

Comment: Essence of intrinsic tunneling: Distinguishing intrinsic features from artifacts [Phys.Rev.B 72 (2005) 094503]

In a recent paper V.Zavaritsky has argued that interlayer ($c-$ axis) current-voltage characteristics of high temperature superconductors (HTSC) are Ohmic, and has claimed to disprove all findings of intrinsic tunnelling spectroscopy, as well as existence of the interlayer tunnelling and the intrinsic Josephson effect in HTSC, as such. In this comment I demonstrate, that the genuine interlayer current-voltage characteristics are strongly non-Ohmic, which undermines the basic postulate and the logical construction of the criticized paper.

cond-mat.supr-con↗