Anisotropy of the Upper Critical Field of the Organic Superconductor λ-(BETS)2GaCl4

The upper critical field B _c2 of the organic superconductor -(BETS)₂GaCl₄ has been determined from resistance measurements for orientation of the magnetic field along three perpendicular crystallographic directions. The Ginzburg–Landau coherence length was estimated from the slope of B _c2(T) curve near T _c as 12.5 nm, 1.6 nm, and 12.5 nm for the a*-, b*-, and c-directions. Angular dependence of the critical field within highly conducting ac plane at 1.5 K was shown to possess two-fold symmetry, which can be related to the band structure anisotropy.     

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.     

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.     

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.     

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.     

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.     

Theoretical aspects of charge correlations in θ-(BEDT-TTF)2X

Abstract

A review is given on the theoretical studies of charge correlations in θ-(BEDT-TTF)₂X. Various studies show that within a purely electronic model on the θ-type lattice with on-site U and nearest neighbor V_p and V_c interactions, the diagonal stripe, c-axis three-fold, and the vertical stripe charge correlations are favored in the regime V_p<V_c, V_p～V_c, and V_p>V_c, respectively. In the realistic parameter regime of V_p～V_c, there is competition between the c-axis three fold state and diagonal stripe state. Since these are different from the experimentally observed a-axis three fold and the horizontal stripe charge correlations, additional effects have to be included in order to understand the experiments. The electron–lattice coupling, which tends to distort the lattice into the θ_d-type, is found to favor the horizontal stripe state, suggesting that the occurrence of this stripe ordering in the actual materials may not be of purely electronic origin. On the other hand, distant electron–electron interactions have to be considered in order to understand the a-axis three fold correlation, whose wave vector is close to the nesting vector of the Fermi surface. These studies seem to suggest that the minimal model to understand the charge correlation in θ-(BEDT-TTF)₂X may be more complicated than expected. Future problems regarding the competition between different types of charge correlations are discussed.     

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.     

Experimental study of organic zero-gap conductor α-(BEDT-TTF)2I3

Abstract

A zero-gap state with a Dirac cone type energy dispersion was discovered in the organic conductor α-(BEDT-TTF)₂I₃ under high hydrostatic pressures. This is the first two-dimensional (2D) zero-gap state discovered in bulk crystals with a layered structure. In contrast to the case of graphene, the Dirac cone in this system is highly anisotropic. The present system, therefore, provides a new type of massless Dirac fermion system with anisotropic Fermi velocity. This system exhibits remarkable transport phenomena characteristic to electrons on the Dirac cone type energy structure. The carrier density, written as n∝T², is a characteristic feature of the 2D zero-gap structure. On the other hand, the resistivity per layer (sheet resistance R_S) is given as R_S=h/e² and is independent of temperature. The effect of a magnetic field on samples in the zero-gap system was examined. The difference between zero-gap conductors and conventional conductors is the appearance of a Landau level called the zero mode at the contact points when a magnetic field is applied normal to the conductive layer. Zero-mode Landau carriers give rise to strong negative out-of-plane magnetoresistance.     

π-d Interaction in the Field Induced Superconductor λ-(BETS)2FeCl4: Studied by 77Se NMR

⁷⁷Se NMR measurements have been performed in the normal state of the organic, magnetic field induced superconductor λ(BETS)₂FeCl₄, to examine the interaction between Fe local spins and π conduction electrons. Large, negative Knight shift values were observed at fields lower than the one corresponding to onset of field induced superconductivity. This evidences that the conduction electrons feel a negative internal field coming from the Fe spins through the exchange interaction { } · { }. This suggests that the superconductivity induced by high magnetic fields has its origin in the Jaccarino-Peter compensation mechanism. The estimated value of the exchange interaction J, is 30% smaller than that predicted by the resistivity measurements.     

Theoretical study of the zero-gap organic conductor α-(BEDT-TTF)2I3

Abstract

The quasi-two-dimensional molecular conductor α-(BEDT-TTF)₂I₃ exhibits anomalous transport phenomena where the temperature dependence of resistivity is weak but the ratio of the Hall coefficient at 10 K to that at room temperature is of the order of 10⁴. These puzzling phenomena were solved by predicting massless Dirac fermions, whose motions are described using the tilted Weyl equation with anisotropic velocity. α-(BEDT-TTF)₂I₃ is a unique material among several materials with Dirac fermions, i.e. graphene, bismuth, and quantum wells such as HgTe, from the view-points of both the structure and electronic states described as follows. α-(BEDT-TTF)₂I₃ has the layered structure with highly two-dimensional massless Dirac fermions. The anisotropic velocity and incommensurate momenta of the contact points, ±k₀, originate from the inequivalency of the BEDT-TTF sites in the unit cell, where ±k₀ moves in the first Brillouin zone with increasing pressure. The massless Dirac fermions exist in the presence of the charge disproportionation and are robust against the increase in pressure. The electron densities on those inequivalent BEDT-TTF sites exhibit anomalous momentum distributions, reflecting the angular dependences of the wave functions around the contact points. Those unique electronic properties affect the spatial oscillations of the electron densities in the vicinity of an impurity. A marked behavior of the Hall coefficient, where the sign of the Hall coefficient reverses sharply but continuously at low temperatures around 5 K, is investigated by treating the interband effects of the magnetic field exactly. It is shown that such behavior is possible by assuming the existence of the extremely small amount of electron doping. The enhancement of the orbital diamagnetism is also expected. The results of the present research shed light on a new aspect of Dirac fermion physics, i.e. the emergence of unique electronic properties owing to the structure of the material.     

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.     

Spatial mapping of electronic states in κ-(BEDT-TTF)2X using infrared reflectivity

Abstract

We review our recent work on spatial inhomogeneity of the electronic states in the strongly correlated molecular conductors κ-(BEDT-TTF)₂X. Spatial mapping of infrared spectra (SMIS) is used for imaging the distribution of the local electronic states. In molecular materials, the infrared response of the specific molecular vibration mode with a strong electron–molecular vibration coupling can reflect the electronic states via the change in the vibration frequency. By spatially mapping the frequency shift of the molecular vibration mode, an electronic phase separation has been visualized near the first-order Mott transition in the bandwidth-controlled organic conductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br. In addition to reviewing SMIS of the phase separation, we briefly mention the electronic and optical properties of κ-(BEDT-TTF)₂X.     

Phase-matched Optical Second-harmonic Generation in the Organic Crystal MBA-NP, (−)2-(α-methylbenzylamino)-5-nitropyridine

Abstract

Large (5 × 3 × 3 cm³) crystals of the organic nonlinear optical material MBA-NP have been prepared by growth from solution, in an optically and structurally highly perfect state. Oriented cut and polished specimens were examined by the Maker fringe technique at a wavelength of 1·064 µm. Analysis of the data yielded d ₂₂ as 69 × d₁₁ quartz with a corresponding coherence length of 2 µm. A large rotation of the X and Z dielectric axes with wavelength (30° over 200 nm) was observed. Type I phase-matched second-harmonic generation (SHG) was recorded. For the 1·064 µm dielectric axis set and a (010) faced crystal, phase matched SHG was observed for rotation about X to an angle of incidence of 45°. This angle is very sensitive to the correct alignment of dielectric axes and the effect of its misalignment was investigated.     

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.     

Low Temperature Giant Magnetocaloric Effect and Critical Behavior in Amorphous Co100−xEr x (x = 55, 65) Alloys

We report on the magnetic properties, magnetocaloric effect (MCE) and critical exponents in amorphous Co_100?xEr _x (x = 55 and 65), prepared by liquid quenching technique. The transition temperature from ferromagnetic to paramagnetic state has been evaluated according to M(T) measurements, and it is found to be 26 and 15 K for Co₄₅Er₅₅ and Co₃₅Er₆₅, respectively. The magnetization dependence M(H, T) on the temperature T and magnetic field H was measured carefully in the critical region. Magnetic entropy change (– ΔS _M ) allowing estimation of the MCE was determined based on thermodynamic Maxwell’s relation. The magnetocaloric study exposes a quite large value of the magnetic entropy change, which decreases when increasing Er concentration. For an applied magnetic field of 5 T,the values of (– ΔS _Max) peak are about 10.8 and 9.8 J kg ^?1 K ^?1 for Co₄₅Er₅₅ and Co₃₅Er₆₅, respectively. From the field dependence of the magnetic entropy ΔS _M (ΔS _M α H ⁿ ) and the relative cooling power (RCP) (RCP α H ^1+1/δ), it was possible to evaluate the critical exponents of the magnetic phase transitions. Their values are in good agreement with those obtained from the critical exponents using a modified Arrott method.     

Room Temperature Ferromagnetism in Dual Doped (Mn 2+ , Ni 2+ ) ZnO Codoped with Li 1+ Prepared Using EDTASintered at Low Temperature

Nanopowders of Znl_x_y_zMnxNiyLizO [（x = 0.04, y = 0, z = 0）, （x = 0.04, y = 0.03, z = 0） and （x = 0.04, y = 0.03, z = 0.03）] have been synthesized by sol-gel precursor route using ethylene diamine tetraacetic acid （EDTA） as a metal chelating agent. X-ray diffraction analysis confirms the formation of wurtzite hexagonal structure for all the three compositions. Mn2＋ doped ZnO exhibits room temperature ferromagnetism （RTFM）, and it is found that further Ni2＋ doping has decreased Ms because of limit of solid solubility of transition metal in ZnO. But codoping of monovalent Li1＋, further increases the ferromagnetism （FM） value, due to introduction of free carriers compared to the dual doped samples. Photoluminescence （PL） spectra of the system, exhibit near band edge （NBE） emission peak at --464 nm due to the electron transition from interstitials to the valence band. Recombination of conduction electron with hole trapped at oxygen vacancy, leads to prominent defect emission peaks at --482 nm and 532 nm. The evidence of the formation of metaI-EDTA complexes are found from the Fourier transform infrared spectra at 2800-3800 cm-1 with shifting, splitting of the peak and also drastic variations in the intensity.     

High Field Magnetoresistance and Magnetic Torque in One-Dimensional Organic Conductor TPP[Fe(Pc)(CN)2]2

Magnetoresistance (MR) and magnetic torque measurements are performed for a magnetic organic conductor TPP[Fe(Pc)(CN)₂]₂. The results suggest that the large negative MR is associated with a magnetic transition. This magnetic transition is considered as a metamagnetic transition of the localized Fe moments, and the MR effect is qualitatively explained by an analogy with the double exchange interaction system.     

Carrier Relaxation Dynamics in the Organic Superconductor <Emphasis Type="Italic">κ</Emphasis>-(BEDT-TTF)<Subscript>2</Subscript>Cu(NCS)<Subscript>2</Subscript> Under Pressure

Photo-induced carrier relaxation dynamics have been investigated for the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂ with different probe polarization under 1.3 kbar at low temperatures. We successfully observed the isotropic and anisotropic responses for the probe polarization, which were found to appear at 56 K. By comparing the responses with and without applying pressure, we found that those were slightly changed, indicating that the application of pressure has effect on the carrier relaxation dynamics.     

Superplasticity in Aluminium Brass(HAl 66-6-3-2)

1.IntroductionSuperplasticity is not merely a specialphenomenon for certain specific alley,but isone of the intrinsic properties of metallicmaterials[1].When the internal and externalconditions are suitable,superplasticity ofmetal materials will be presented.