Upper Critical Field of Organic Superconductor κ-(ET)2Cu(NCS)2 Determined by Specific Heat Measurements

Both specific heat and resistance measurements of organic superconductor -(ET)₂Cu(NCS)₂ under magnetic field were performed. Upper critical fields, obtained from the specific heat data, are significantly less than those from the magnetoresistance measurements. At temperatures below 2 K and magnetic fields below 1 T, we observed additional power dissipation due to the field sweep. This effect may be attributed to the existence of irreversibility line.     

Magnetic torque of κ-(BEDT-TTF)2Cu(NCS)2 single crystals

The magnetic torque ofκ-(BEDT-TFF)₂Cu(NCS)₂ was measured as a function of field directionθ with respect to thea ^*-axis under constant magnetic fields,H, up to 8 kOe in the temperature range from 1.3 to 8 K. A sharp cusp, C₁, in the irreversible region was found atθ _c1 near theH∥bc-plane between 1.3 and 7 K. In addition, extra cusps, C₂ and C₃, were observed atθ _c2 andθ _c3, respectively, between 2.5 and 6 K. At each temperature, the perpendicular component ofH giving each cusp is kept constant as $H\cos \theta _{cn} = const \equiv H_{cp_n } (n = 1,2,3),$ i.e., cusps C₁, C₂, and C₃ are ruled by the characteristic field perpendicular to thebc-planeH _cp1,H _cp2, andH _cp3, respectively. These behaviors are almost the same as those we found in the oxide superconductor Bi₂Sr₂CaCu₂O₈. These results suggest that the cusps are intrinsic for irreversible vortex states of these layered superconductors.     

Probing the Upper Critical Field of κ-(BEDT-TTF)2Cu(NCS)2

Studies of heat conduction and microwave absorption in the vortex state of -(BEDT-TTF)₂Cu(NCS)₂ reveal some of the specific aspects of superconductivity in this system. Moreover they allow us to probe the destruction of bulk superconductivity by a magnetic field. The two set of results yield identical magnitudes for the upper critical field with a temperature dependence distinctly different from the one suggested by the onset of resistive transition.     

Magnetization anomaly and mixed state phase diagram of κ-(BEDT-TTF)2Cu(NCS)2

Local magnetization measurements using micro Hall probes were carried out on a quasi two-dimensional organic superconductor -(BEDT-TTF)₂Cu(NCS)₂. Peak structures in the local magnetization hysteresis loops were found at around 50 G. We ascribe this anomaly to the field-induced decomposition of vortex lines into pancake vortices, which is found in a high temperature superconductor Bi₂Sr₂CaCu₂O_8+y. Mixed state phase diagram in -(BEDT-TTF)₂Cu(NCS)₂ is compared with that in Bi₂Sr₂CaCu₂O_8+y.     

Critical Field and Shubnikov-de Haas Oscillations of κ-(BEDT-TTF)2Cu(NCS)2under Pressure

We have used a novel experimental method to study the crossover of an anisotropic superconductor from a possible Pauli limited superconducting state to a vortex limited superconducting state by applying pressure. The new apparatus combined a tuned tank circuit with a nonmetallic diamond anvil cell to measure the change in critical field with angle in -(BEDT-TTF)₂Cu(NCS)₂ at pressures up to 1.75 kbar and at temperatures down to 70 mK. The critical fields (in the perpendicular or parallel orientation to the conducting planes) have been found to decrease by more than 90% within less than 2 kbar of pressure. In the parallel orientation, at 1.75 kbar, we have seen a clear change from the ambient pressure behavior of the critical field with temperature at low temperatures. Up to P=1.75 kbar, the H_c2 phase diagram is in good agreement with the theoretical prediction for weakly coupled layered superconductors. We have also succeeded in measuring oscillations in the resistivity of the normal state at higher magnetic field which could be used to find the effective quasi-particle mass. The -orbit Shubnikov-de Haas frequency was found to increase at a rate of 44T/kbar. Our experiment opens the possibility for further investigations of the effective mass with pressure, especially because the setup is suitable for pulsed fields as well.     

First Order Vortex Phase Transition in the Organic Superconductor κ-(BEDT-TTF)2Cu(NCS)2

The local magnetic induction is measured in the quasi-two dimensional organic superconductor -(BEDT-TTF) ₂ Cu(NCS) ₂ by using a micro Hall probe. The small step is observed on the temperature dependence of the local magnetic induction. This result implies the existence of the first-order phase transition of the vortex system in this organic superconductor as is already reported in high-T _c oxide and other organic superconductors.     

Ultrasonic investigations on the flux state of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

The flux state of the organic superconductor -(BEDT-TTF)₂Cu(NCS)₂ has been investigated by ultrasonic measurements. The magnetic field dependence in the longitudinal sound velocity, shows a remarkable dip. The amount of the dip is proportional to cos, where is the angle between the propagation direction of the sound and the magnetic field. The magnetoresistance anomaly in this salt, which is seen in the same magnetic field range as for the sound velocity, has been interpreted by the thermal fluctuation of the Josephson coupling between the superconducting layers. The sound velocity anomaly is interpreted by this mechanism. It would be concluded that such anomaly is caused by a large Grüneisen parameter in this salt.     

Microwave response of Josephson plasma in the mixed state of κ-(BEDT-TTF)2Cu(NCS)2

A cavity perturbation technique is used to measure the magnetic-field dependence of surface impedance at 24 and 41 GHz in superconducting -(BEDT-TTF)₂Cu(NCS)₂ single crystals. A peak structure in the surface resistance R_s(H) is observed (1) below T_c, (2) when microwave electric field is perpendicular to the superconducting layer, and (3) at higher magnetic field for the lower measurement frequency (anticyclotronic behavior). These observations are identified as the Josephson plasma mode. The peak field increases as the temperature is lowered even below the irreversibility line in contrast to the case of Bi₂Sr₂CaCu₂O_8+y.     

Anomalous hall conductivity in the vortex state of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2

Hall effect measurements in the vortex state of the quasi two-dimensional (Q2D) organic superconductor -(BEDT-TTF)₂Cu(NCS)₂ are reported. The sign of the Hall conductivity changes with field and temperature in the vortex state. The Hall conductivity is described as the sum of the two terms resulting from the quasiparticle ( H) and the vortex motion ( 1/H). The sign of the latter term depends on the current direction in Q2D plane. The behavior can be explained by the anisotropic motion of the charged vortex in the anisotropic Q2D plane.The authors would like to thank Professor K. Takanaka and Professor H. Ebisawa for stimulating discussions. This work was carried out at HFLSM, IMR, Tohoku University and partially supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.     

Vibrating reed study of the flux line dynamics of κ-(ET)2Cu[N(CN)2]Br

Vibrating reed measurements were performed on single-crystal samples of-(ET)₂- Cu[N(CN)₂]Br in an applied magnetic fieldH oriented either parallel (longitudinal) or perpendicular (transverse) to the highly conducting ac-plane. Field-cooling data taken for the longitudinal orientation andH<0.7 T revealed a peak with a low-temperature shoulder in the reed dissipation 1/Q located at temperatureT _Q below the superconducting transition temperatureT _c (11.6 forH=0). The shoulder disappeared nearH0.7 T, accompanied by an abrupt change in the slope ofT _Q (H), corresponding to a similar change in the slope of the upper critical magnetic fieldH _c2 measured by Kwoket al. The existence of the shoulder in the dissipation peak bears on a number of current explanations for the exotic superconducting properties of-(ET)₂Cu[N(CN)₂]Br.T _Q data taken for the transverse orientation fell far below estimates ofH _c2., indicating the existence of a substantial region of flux line (FL) mobility belowH _c2(T). The location of a peak in NMR relaxation observed by De Sotoet al. lies close toT _Q for the transverse orientation.     

The mixed state of theκ-(BEDT-TTF)2Cu(NCS)2 organic superconductor

The quasi-2D organic superconductor -(BEDT-TTF)₂Cu(NCS)₂ presents properties in the superconducting state which are similar to most anisotropic high-T_c compounds. The smaller upper critical fields (100kOe) enable us to study the entire (H, T) phase diagram by electrical transport measurements: large superconducting fluctuations lead to a broadening of the normal-superconducting transition as well as to the presence of a phase transition line between a solid and a viscous liquid phase. In low magnetic fields, the displacement of the vortices in the liquid phase is due to the creation of double kinks in the flux lines.     

Magnetoresistivity in the vortex liquid phase ofκ-(BEDT-TTF)2Cu(NCS)2

We report single crystal magnetoresistivity measurements of the organic superconductor -(BEDT-TTF)₂Cu(NCS)₂ in magnetic fields applied perpendicularly to the superconducting planes. We present an analysis of the behavior of the resistivity in the vortex liquid phase. It turns out, that at high temperatures and sufficiently low fields the vortex liquid phase can be described as a very viscous vortex liquid of 3D vortices with activation energies arising from generations of vortex-doublekinks. Quantitatively a value of the superconducting anisotropy parameter = _ab/_c 80 can be derived from the measurements presented here.     

Magnetic torque of κ-(BEDT-TTF)2Cu(NCS)2 single crystals

The magnetic torque of-(BEDT-TFF)₂Cu(NCS)₂ was measured as a function of field direction with respect to thea ^*-axis under constant magnetic fields,H, up to 8 kOe in the temperature range from 1.3 to 8 K. A sharp cusp, C₁, in the irreversible region was found at _c1 near theHbc-plane between 1.3 and 7 K. In addition, extra cusps, C₂ and C₃, were observed at _c2 and _c3, respectively, between 2.5 and 6 K. At each temperature, the perpendicular component ofH giving each cusp is kept constant as i.e., cusps C₁, C₂, and C₃ are ruled by the characteristic field perpendicular to thebc-planeH _cp1,H _cp2, andH _cp3, respectively. These behaviors are almost the same as those we found in the oxide superconductor Bi₂Sr₂CaCu₂O₈. These results suggest that the cusps are intrinsic for irreversible vortex states of these layered superconductors.     

Magnetic torque ofκ-(BEDT-TTF)2Cu[N(CN)2]Br single crystal

Magnetic torque of the single crystal of a layered superconductor -(BEDT-TTF)₂Cu[N(CN)₂]Br has been measured as a function of field direction with respect to the b-axis under several magnetic fields H up to 8 kOe in the temperature range from 1.5 to 20 K. Smaller value of the anisotropy parameter = 180 is obtained for Cu[N(CN)₂]Br salt than our previous report = 350 for -(BEDT-TTF)₂Cu(NCS)₂. The irreversible region in H — T phase diagram for Cu[N(CN)₂]Br salt with smaller is larger than that for Cu(NCS)₂ salt. A sharp cusp due to the intrinsic pinning is found near Hac-plane as in the case of our previous report on other layered superconductors.     

Superconducting State of Layered Superconductor κ-(ET)4Hg2.89Br8 Under Magnetic Field Parallel to the Conducting Plane

The upper critical field of -(ET)₄Hg_2.89Br₈ was measured under a parallel magnetic field up to 24 T in the temperature region down to 0.5 K. The data obtained at 0.5 K indicate that the upper critical field tends to saturate toward 0 K but exceeds the Pauli limit by more than two times. The anisotropy of the upper critical field within the superconducting plane was not observed. The implication of the high upper critical field is discussed.     

Microstructure and superconductivity of highly ordered YBa(2)Cu(3)O(7-δ) nanowire arrays

In order to explore the fundamental properties of one-dimensional nanostructured high-temperature superconductors and enhance their promising applications, a universal and general method for the synthesis of high-quality YBa(2)Cu(3)O(7-δ) (YBCO) nanowire arrays is developed, which involves the combination of a novel sol-gel process to lower the crystallization temperature of YBCO, and porous anodic alumina (PAA) as an effective morphology-directing hard template. Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) results indicate that the as-prepared YBCO nanowires have average diameters of about 50?nm and lengths up to several microns. The structures of the samples were analysed by x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), energy-dispersive x-ray spectroscopy (EDX) and inductively coupled plasma (ICP) analysis, which indicate that the nanowires are well crystallized with orthorhombic YBCO-123 structure. The magnetization measurement under zero-field-cooled (ZFC) mode indicates that the superconducting transition temperature (T(c)) of the nanowires is about 92?K, which is in agreement with that of a bulk YBCO sample.     

The Angular Dependence of the Superconductivity Fluctuation Effects near the Upper Critical Field Observed Through Damping of the de Haas–van Alphen Oscillation of κ-(BEDT-TTF)2Cu(NCS)2

The de Haas–van Alphen (dHvA) oscillation was observed in the superconductivity mixed state near the upper critical field H _c2 of the two-dimensional organic superconductor -(BEDT-TTF)₂Cu(NCS)₂. By analyzing an additional damping of the oscillation, the superconductivity fluctuation effects near H _c2 are elucidated quantitatively as the magnetic field dependence of the mean square of the superconducting gap. By rotating the external magnetic field, the angular dependence of the fluctuation effects and probable values of the H _c2 are studied.     

Systematic investigation of electronic structure in BEDT-TTF based organic superconductors withTc above 10 K; κ-(BEDT-TTF)2X (X=Cu(NCS)2, Cu[N(CN)2]Br, and Cu(CN)[N(CN)2])

The electronic structure of the title superconductors has been investigated by electrical resistivity, complex susceptibility, and electron paramagnetic resonance (EPR) measurements. The superconducting properties (pressure dependence ofT _c , magnetic penetration depth, upper critical field, and so on) of these three salts are similar to each other, while transport properties in the normal state have shown a large variety in the temperature dependence. In order to clarify the electronic structure in the normal state, the EPR parameters, the spin susceptibility ( _spin), and the linewidth (H _pp), are compared. An anomalous temperature dependence of theg-value has been observed below 150 K in the Cu(NCS)₂ and Cu(CN)[N(CN)₂] salts.     

Cu(InAl)Se2薄膜太阳电池的研究进展

具有黄铜矿结构的CuInSe2 (CIS)类薄膜太阳电池随着其转换效率的进一步提高,受到日益广泛的关注.相对于CIS材料,Cu(InAl) Se2(CIAS)具有更高的禁带宽度,可用比Ga合金更小的相对浓度来实现更可观的带隙,CIAS有望取代CIS成为薄膜太阳电池的新型材料.结合最新研究进展论述了CIAS薄膜材料的不同制备方法及合成机理,并讨论了目前CIAS太阳电池研究中存在的问题和今后的发展方向.     

Transport and Cu K-XANES Studies of (Hg,Cr)Sr2(Ca,Y)Cu2O6 + δ

The nominal (Hg_0.7Cr_0.3)Sr₂(Ca_0.7Y_0.3)Cu₂O_{6 +} , referred to as (Hg,Cr)-1212, were successfully synthesized at high temperature in partial vacuum. The samples were subsequently annealed in Ar or O₂ annealed at 300°C under ambient pressure. No T _c was observed in any of these samples down to 12 K which is in contrast to high pressure synthesized Y-free (Hg,Cr)-1212 phase where the latter exhibits a superconductivity at 80 K. The divalent state of Cu, as shown by XANES in the as-prepared (Hg,Cr)-1212, is not affected either by Ar or O₂ annealing. Depletion of holes due to trivalent Y substitution at the Ca-site, and a relatively higher 'a' lattice parameter (3.851 Å) are the apparent reasons for the non-superconducting nature of the samples.