arXiv subjects

A. Rydh

Publications and source records attributed to A. Rydh.

32 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

Absolute accuracy in membrane-based ac nanocalorimetry

To achieve accurate results in nanocalorimetry a detailed analysis and understanding of the behavior of the calorimetric system is required. There are especially two system-related aspects that should be taken in consideration: the properties of the empty cell and the effect of the thermal link between sample and cell. Here we study these two aspects for a membrane-based system where heater and thermometer are both in good contact with each other and the center of the membrane. Practical, analytical expressions for describing the frequency dependence of heat capacity, thermal conductance, and temperature oscillation of the system are formulated and compared with measurements and numerical simulations. We finally discuss the experimental conditions for an optimal working frequency, where high resolution and good absolute accuracy are combined.

cond-mat.other↗

arXiv2012

Absolute specific heat measurements of a microgram Pb crystal using ac nanocalorimetry

Heat capacity measurements using the ac steady state method are often considered difficult to provide absolute accuracy. By adjusting the working frequency to maintain a constant phase and using the phase information to obtain the heat capacity, we have found that it is possible to achieve good absolute accuracy. Here we present a thermodynamic study of a ~ 2.6 μg Pb superconducting crystal to demonstrate the newly opened capabilities. The sample is measured using a differential membrane-based calorimeter. The custom-made calorimetric cell is a pile of thin film Ti heater, insulation layer and Ge_{1-x}Au_{x} thermometer fabricated in the center of two Si_{3}N_{4} membranes. It has a background heat capacity < 100 nJ/K at 300K, decreasing to 9 pJ/K at 1 K. The sample is characterized at temperatures down to 0.5 K. The zero field transition at Tc = 7.21 K has a width \approx 20 mK and displays no upturn in C. From the heat capacity jump at Tc and the extrapolated Sommerfeld term we find \DeltaC/γTc = 2.68. The latent heat curve obtained from the zero field heat capacity measurement, and the deviations of the thermodynamic critical field from the empirical expression Hc =Hc(0)[1-(T/Tc)^2] are discussed. Both analyses give results in good agreement with literature.

cond-mat.supr-con↗

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

Doping dependence of the specific heat of single crystal BaFe2(As1-xPx)2

We present specific heat measurements on a series of BaFe2(As1-xPx)2 single crystals with phosphorous doping ranging from x = 0.3 to 0.55. Our results reveal that BaFe2(As1-xPx)2 follows the scaling Delta_C/Tc ~ Tc^2 remarkably well. The clean-limit nature of this material imposes new restraints on theories aimed at explaining the scaling. Furthermore, we find that the Ginzburg-Landau parameter decreases significantly with doping whereas the superconducting anisotropy is gamma~2.6, independent of doping.

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

Anisotropic Phase Diagram and Superconducting Fluctuations in SmFeAsO0.85F0.15

We report on the specific heat determination of the anisotropic phase diagram of single crystals of optimally doped SmFeAsO1-xFx. In zero-field, the optimally doped compound displays a clear cusp-like anomaly in C/T with ΔC/Tc = 24 mJ/molK2 at Tc = 49.5 K. In magnetic fields applied along the c-axis, we find pronounced superconducting fluctuations induced broadening and suppression of the specific heat anomaly which can be described using three-dimensional lowest-Landau-level scaling with an upper critical field slope of -3.5 T/K and an anisotropy of Γ = 8. The small value of ΔC/Tc yields a Sommerfeld coefficient γ ~ 8 mJ/molK2 indicating that SmFeAsO1-xFx is characterized by a modest density of states and strong coupling.

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↗

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↗

arXiv2004

Surface plasmons at single nanoholes in Au-films

The generation of surface plasmon polaritons (SPP's) at isolated nanoholes in 100 nm thick Au films is studied using near-field scanning optical microscopy (NSOM). Finite-difference time-domain calculations, some explicitly including a model of the NSOM tip, are used to interpret the results. We find the holes act as point-like sources of SPP's and demonstrate that interference between SPP's and a directly transmitted wave allows for determination of the wavelength, phase, and decay length of the SPP. The near-field intensity patterns can be manipulated by varying the angle and polarization of the incident beam.

cond-mat.mtrl-sci↗

arXiv2003

Two-band Effects in the Angular Dependence of Hc2 of MgB2 Single Crystals

The angular dependence of the upper critical field Hc2 of MgB2 single crystals is studied at various temperatures by means of specific heat and transport measurements in magnetic fields up to 17 T. Clear deviations from Ginzburg-Landau behavior are observed at all temperatures and are explained by two-band effects. The angular- and temperature dependence of the deviations are in qualitative agreement with theoretical predictions based on band-structure calculations. Quantitative agreement is obtained with an interband coupling slightly stronger than the calculated, enabling band-structure anisotropies and interband coupling strength to be experimentally estimated. This provides a new pathway to the study of disorder and doping effects in MgB2.

cond-mat.supr-con↗

arXiv2003

Surface Contribution to the Superconducting Properties of MgB2 Single Crystals

We demonstrate direct evidence of possible surface superconductivity on small, well-shaped MgB2 single crystals. Transport measurements in the range H < 1.6 Hc2^c, where Hc2^c is the bulk upper critical field for the c axis, show non-Ohmic and strongly angular dependent resistivity. Studies of the alignment of H with selected crystal surfaces, transport and specific heat measurements on the same crystal, and a physical sculpturing of the crystal surfaces using a focused ion beam all support the conclusion. Similar, albeit less pronounced results are obtained for fields in the basal plane.

cond-mat.supr-con↗