Superconducting phase diagram and FFLO signature inλ-(BETS)2GaCl4

Abstract

We report the phase diagram of λ-(BETS)2GaCl4 from rf penetration depth measurements with a tunnel diode oscillator in a pulsed magnetic field. We examined four samples with 1100 field sweeps in a range of angles with the magnetic field parallel and perpendicular to the conducting planes. In the parallel direction, Hc2 appears to include a tricritical point at 1.6 K and 10 T with a phase line that increases to 11 T as the temperature is decreased to 500 mK. The second phase line forms a clearly defined high field low temperature region satisfying several of the conditions of the FuldeFerrell-Larkin-Ovchinnikov (FFLO) state. We show remarkably good fits of Hc2 to WHH in the reentrant α > 1, λso = 0 regime. We also note a sharp angle dependence of the phase diagram about the field parallel orientation that characterizes Pauli paramagnetic limiting and further supports the possibility of FFLO behavior. Unrelated to the FFLO study, at fields and temperatures below Hc2 and Tc, we find rich structure in the penetration depth data that we attribute to impurities at the surface altering the superconducting properties while maintaining the same crystallographic axes as Hc2.

In Search of Unambiguous Evidence of the Fulde–Ferrell–Larkin–Ovchinnikov State in Quasi-Low Dimensional Superconductors

“…There have been observed several signatures indicating the experimental realization of the FFLO state in organic superconductors for in-plane external magnetic field when the flux penetrates between the layers in the form of Josephson vortices, thus limiting orbital suppression [19][20][21]. In particular, (i) the anomaly in the thermal conductivity [22], and fine details in the phase diagram obtained by the tunnel diode oscillator (rf)-penetration depth measurements and pulsed field techniques [23] for the clean organic sample λ-(BETS) 2 GaCl 4 ; (ii) the magnetic torque evidence for the appearance of an additional first-order phase transition line within the superconducting phase in the in-plane high field regime for organic sample κ-(BEDT-TTF) 2 Cu(NCS) 2 [24,25]; (iii) evidence for phase transition within the superconducting phase obtained from the local electronic spin polarization in 13 C NMR spectroscopy experiment [26], NMR detection of spin-polarized quasiparticles [27] forming the Andreev bound states spatially localized in the nodes of the order parameter [5,28,29], and phase transitions in the (H − T) phase diagram that are consistent with the FFLO phase obtained from rf-penetration depth measurements [30] for organic sample κ-(BEDT-TTF) 2 Cu(NCS) 2 ; (iv) NMR relaxation rate evidence for an additional phase transition line [31] and the clear upturn beyond the Pauli limit in the magnetic-field and angular-dependent high-resolution specific-heat measurements for the organic materials κ-(BEDT-TTF) 2 Cu(NCS) 2 and β -(ET) 2 SF 5 CH 2 CF 2 SO 3 [32,33], (v) magnetic torque evidence for the tricritical point between the FFLO, homogeneous superconducting, and paramagnetic metallic phases in the 2D magnetic-field-induced organic superconductor λ-(BETS) 2 FeCl 4 [34]; (vi) measurements of the temperature and angular dependencies of H c2 in pnictide superconductor LiFeAs [35] and KFe 2 As 2 [36] as well as a dip in the interlayer resistance in parallel fields observed in the compound λ-(BETS) 2 Fe 1−x Ga x Cl 4 for x = 0.37 [37] (for all types of salts, (x = 1.0, x = 0.37, x = 0) signs for the FFLO state have been observed [23,38]) probably suggest that these sys...…”

Spin density wave and superconducting properties of nanoparticle organic conductor assemblies

“…(The superconducting magnet was swept through zero magnetic field to eliminate remnant field effects due to trapped flux in the solenoid.) The inductive method involves a tunnel diode oscillator (TDO) self-resonating LC circuit (L = inductor, C = capacitor) operating with a frequency f and driven by a tunnel diode biased in the negative resistance region 17,18 . Changes in the resonant frequency are directly proportional to any mechanism that changes the effective inductance of a coil in which the sample is placed.…”

Calorimetric Measurements of Magnetic-Field-Induced Inhomogeneous Superconductivity Above the Paramagnetic Limit

“…As a consequence, the vortices become Josephson coupled [25] and only weakly interact with the superconducting layers [26]. Phase diagrams suggesting the existence of a higher field superconducting phase in κ -(BEDT-TTF) 2 Cu(NCS) 2 (T c of 9.5 K) and other superconductors have been established using rf penetration depth [27], tunnel diode oscillator (TDO) rf penetration depth [24,[28][29][30], resistivity [31], thermal conductivity [32], heat capacity [33], torque magnetometry [31,34], and NMR [35,36], but the claimed location, slope, and curvature of the phase boundary between the two superconducting states varies dramatically.…”