I.J. Lee

Coexistence of superconductivity and antiferromagnetism probed by simultaneous nuclear magnetic resonance and electrical transport in (TMTSF)2PF6 system

We report simultaneous NMR and electrical transport experiments in the pressure range near the boundary of the antiferromagnetic spin density wave (SDW) insulator and the metallic/superconducting (SC) phase in (TMTSF)2PF6. Measurements indicate a tricritical point separating a line of second-order SDW/metal transitions from a line of first-order SDW/metal(SC) transitions with coexistence of macroscopic regions of SDW and metal(SC) order, with little mutual interaction but strong hysteretic effects. NMR results quantify the fraction of each phase.

Unconventional superconductivity in (TMTSF)2ClO4

Abstract We have performed detailed measurements to characterize the superconducting (T c ≈1.1 K ) state of (TMTSF)2ClO4 using both normal and deuterated samples. Using ZF-μSR measurements to search for spontaneous magnetic fields below Tc, we see no evidence for such fields within our resolution, thus we conclude the time reversal symmetry is likely not broken. We have also performed precession measurements in the vortex state of (TMTSF)2ClO4. The superconducting relaxation rate decreases strongly with increasing field, which may indicate a dimensional crossover in the nature of the vortex lattice. We estimate that the penetration depth is roughly 535 nm .

Interlayer decoupling, Lebed magic angle magnetoresistance and triplet superconductivity in (TMTSF)2PF6

We present transport and NMR studies of the normal and superconducting states of (TMTSF) 2 PF 6 in precisely aligned magnetic fields. For the normal state the anomalous angular dependent magnetoresistance as a function of field and temperature is shown to be reasonably fit as a sum of conductivity contributions arising from interchain hopping integrals which are suppressed by a perpendicular magnetic field In the superconducting state NMR Knight shift measurements support a triplet state as do critical fields which exceed the Pauli limit by more than a factor of four. An anomalous upward curvature in H c2c (Perpendicular to the most conducting planes) is observed for pressures near the critical pressure which separates the SDW and superconducting phases.

Field-induced electronic phase transitions in high magnetic fields

We report on recent experiments on (TMTSF) 2 PF 6 under high and precisely aligned magnetic fields, moderate pressure and low temperature. The primary effect of the magnetic field is to reduce the electronic dimensionality of the system. This leads to some novel states which are now understood (a cascade of field-induced spin-density-wave states which exhibit the quantum Hall effect) and to many phenomena which suggest even more exotic phases. In particular, at low temperature the compound shows quantum phase transitions between metallic and nonmetallic behaviour as the magnetic field is rotated through alignment with the crystallographic axes, and the superconducting critical field greatly exceeds the Pauli limit, indicating triplet superconductivity.

Triplet superconductivity and stripes? In (TMTSF)2PF6

Abstract We report on 77 Se NMR studies of (TMTSF)2PF6 with field aligned along the a and b axes. We observe no change in the Knight shift on cooling well into the superconducting state, consistent with an equal spin paired triplet state. Nuclear spin lattice relaxation measurements show an increase just below Tc which we associate with a Hebel–Slichter (HS) peak and suggest P wave pairing that is nodeless. An additional study of the critical field as a function of pressure shows a strong enhancement as pressure is reduced toward the critical pressure, Pc, for spin density wave formation. We find that the superconducting Tc is not reduced as the SDW phase is entered. However, Hc2 is increased and there is a strong upward curvature of Hc2 along all axes as pressure is decreased toward Pc. We suggest that near Pc, there is a coexistence region consisting of stripes (slabs) of commingled SDW and superconducting regions and that the application of a magnetic field further finely divides the superconductor.

The superconducting state in (TMTSF)2PF6 in high magnetic field

The resistive upper critical field along the b-axis (H c2 //b) in (TMTSF) 2 PF 6 is investigated in detail in fields to 17.5 T and temperatures to T/T c ∼ 1/60, at various pressures near the border between spin density wave and metallic states. Previously, we had obtained a novel crossover where H c2 //b exceeded both H c2 //a, in contrast to band structure, and the Pauli paramagnetic limit μ o H p 2 T. Remarkably, in an optimum pressure setting reported here, it is found that the onset of superconductivity persists up to 9 T (> 4 H p ), which strongly suggests triplet Cooper pairing. Strong upward curvature with nearly diverging behaviour of the critical field also seem to suggest a field-dependent dimensional crossover, although the H c2 enhancement is considerably more than that predicted by recent theories.