Kazushi Aoyama

Field-induced superconducting phase of FeSe in the BCS-BEC cross-over.

Significance The BCS-BEC (Bardeen–Cooper–Schrieffer––Bose–Einstein-condensate) cross-over bridges the two important theories of bound particles in a unified picture with the ratio of the attractive interaction to the Fermi energy as a tuning parameter. A key issue is to understand the intermediate regime, where new states of matter may emerge. Here, we show that the Fermi energy of FeSe is extremely small, resulting in that this system can be regarded as an extraordinary “high-temperature” superconductor located at the verge of a BCS-BEC cross-over. Most importantly, we discover the emergence of an unexpected superconducting phase in strong magnetic fields, demonstrating that the Zeeman splitting comparable to the Fermi energy leads to a strong modification of the properties of fermionic systems in such a regime. Fermi systems in the cross-over regime between weakly coupled Bardeen–Cooper–Schrieffer (BCS) and strongly coupled Bose–Einstein-condensate (BEC) limits are among the most fascinating objects to study the behavior of an assembly of strongly interacting particles. The physics of this cross-over has been of considerable interest both in the fields of condensed matter and ultracold atoms. One of the most challenging issues in this regime is the effect of large spin imbalance on a Fermi system under magnetic fields. Although several exotic physical properties have been predicted theoretically, the experimental realization of such an unusual superconducting state has not been achieved so far. Here we show that pure single crystals of superconducting FeSe offer the possibility to enter the previously unexplored realm where the three energies, Fermi energy εF, superconducting gap Δ, and Zeeman energy, become comparable. Through the superfluid response, transport, thermoelectric response, and spectroscopic-imaging scanning tunneling microscopy, we demonstrate that εF of FeSe is extremely small, with the ratio Δ/εF∼1(∼0.3) in the electron (hole) band. Moreover, thermal-conductivity measurements give evidence of a distinct phase line below the upper critical field, where the Zeeman energy becomes comparable to εF and Δ. The observation of this field-induced phase provides insights into previously poorly understood aspects of the highly spin-polarized Fermi liquid in the BCS-BEC cross-over regime.

Antiferromagnetic ordering induced by paramagnetic depairing in unconventional superconductors

Antiferromagnetic (AFM) (or spin-density wave) quantum critical fluctuation enhanced just below H_c2(0) have been often observed in d-wave superconductors with a strong Pauli paramagnetic depairing (PD) including CeCoIn_5. It is shown here that such a tendency of field-induced AFM ordering is a consequence of strong PD and should appear particularly in superconductors with a gap node along the AFM modulation. Two phenomena seen in CeCoIn_5, the anomalous vortex lattice form factor and the AFM order in the Fulde-Ferrell-Larkin-Ovchinnikov state, are explained based on this peculiar PD effect.

Pairing states of superfluid He 3 in uniaxially anisotropic aerogel

Stable pairing states of superfluid 3He in aerogel are examined in the case with a global uniaxial anisotropy which may be created by applying an uniaxial stress to the aerogel. Due to such a global anisotropy, the stability region of an ABM pairing state becomes wider. In an uniaxially stretched aerogel, the pure polar pairing state with a horizontal line node is predicted to occur, as a 3d superfluid phase, over a measurable width just below the superfluid transition at T_c(P). A possible relevance of the present results to the case with no global anisotropy is also discussed.

Stripe order in superfluid 3 He confined in narrow cylinders

We theoretically investigate pairing states of the spin-triplet

Field-induced reentrant superconductivity in thin films of nodal superconductors

p

Strong-coupling effects in superfluid {sup 3}He in aerogel

-wave superfluid

Signatures of the helical phase in the critical fields at twin boundaries of noncentrosymmetric superconductors

{}^{3}

Superfluid H 3 e in globally isotropic random media

He confined in narrow cylinders. The surface-induced distortion and the multiple internal degrees of freedom of the order parameter lead to the occurrence of a stripe structure along the cylinder axis in the superfluid

Flux distribution in vortex lattice states in d-wave and paramagnetic superconductors

{}^{3}

Vortex Core Transitions in Superfluid 3He in Uniaxially Anisotropic Aerogel

He-B phase. We show that in sufficiently small cylinders with an anisotropic surface scattering, the stripe order with broken translational symmetry may be stabilized as the lowest energy state. Periodic spin-current textures caused in this inhomogeneous superfluid phase are also discussed.