^{1}and J. Wosnitza

^{1,a)}

### Abstract

In this short review, we report on the recently found growing experimental evidence for the existence of Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) states in quasi-two-dimensional organic superconductors. At high magnetic fields aligned parallel to the conducting organic layers, we observe an upturn of the upper critical field beyond the Pauli limit, as evidenced by specific-heat and torque-magnetization measurements. Inside the superconducting state a second thermodynamic transition emerges. These features appear only in a very narrow angular region close to parallel-field orientation.

This article is dedicated to Igor Yanson whom J.W had the pleasure to meet at many occasions. We thank our coworkers B. Bergk, R. Lortz, A. Demuer, I. Sheikin, S. Yasin, Y. Wang, Y. Nakazawa, J. A. Schlueter, and G. Zwicknagl which contributed substantially to the work presented here. Part of this work was supported by EuroMagNET II (EU contract No. 228043).

I. Introduction

II. Experimental

III. Results and discussion

IV. Summary

### Key Topics

- Superconductivity
- 27.0
- Magnetic fields
- 26.0
- Critical fields
- 16.0
- Organic superconductors
- 16.0
- Superconductors
- 15.0

##### H01F13/00

## Figures

Schematic presentation of (left) the usual BCS pairing state with zero resulting momentum and spin and (right) the FFLO pairing state with a finite center-of-mass momentum, *q*. The circles represent the Fermi surfaces for spin-up and spin-down bands.

Schematic presentation of (left) the usual BCS pairing state with zero resulting momentum and spin and (right) the FFLO pairing state with a finite center-of-mass momentum, *q*. The circles represent the Fermi surfaces for spin-up and spin-down bands.

Temperature dependence of the specific-heat difference between the superconducting and normal state, Δ*C*/*T*, of κ-(BEDT-TTF)_{2}Cu(NCS)_{2} in magnetic fields applied parallel to the superconducting layers.

Temperature dependence of the specific-heat difference between the superconducting and normal state, Δ*C*/*T*, of κ-(BEDT-TTF)_{2}Cu(NCS)_{2} in magnetic fields applied parallel to the superconducting layers.

(a) High-field data shown in Fig. 2 in an expanded scale. (b) Specific-heat data measured at 22 T during warming and cooling.

(a) High-field data shown in Fig. 2 in an expanded scale. (b) Specific-heat data measured at 22 T during warming and cooling.

(a) Magnetic-torque data of κ-(BEDTTTF)_{2}Cu(NCS)_{2} at various temperatures for in-plane magnetic fields. For *T* = 1.7 K, data for both up and down field sweeps, otherwise only down sweeps are shown. The vertical arrows mark the small dip-like features, which are associated with the transition into the FFLO state. (b) Schematic phase diagram deduced from the data shown in (a) and Fig. 2 . ^{ 18 }

(a) Magnetic-torque data of κ-(BEDTTTF)_{2}Cu(NCS)_{2} at various temperatures for in-plane magnetic fields. For *T* = 1.7 K, data for both up and down field sweeps, otherwise only down sweeps are shown. The vertical arrows mark the small dip-like features, which are associated with the transition into the FFLO state. (b) Schematic phase diagram deduced from the data shown in (a) and Fig. 2 . ^{ 18 }

(a) Temperature dependence of the specific heat of β″(BEDT-TTF)_{2}SF_{5}CH_{2}CF_{2}SO_{3} in a double-logarithmic scale. Data in the superconducting state in zero field and in the normal state at 10 T applied perpendicular to the BEDT-TTF planes are shown. The solid line is a fit to the 10 T data below 2 K using a linear and cubic term. (b) Temperature dependence of the specific-heat difference between the superconducting and normal state. The lines show the BCS behavior for weak coupling (dashed) ^{ 30 } and moderately strong coupling (solid). ^{ 31 } The inset shows the electronic part of the specific heat, *C _{e} *, divided by γ

*T*as function of

_{c}*T*/

_{c}*T*. The solid line shows the exponential vanishing of

*C*towards low

_{e}*T*.

(a) Temperature dependence of the specific heat of β″(BEDT-TTF)_{2}SF_{5}CH_{2}CF_{2}SO_{3} in a double-logarithmic scale. Data in the superconducting state in zero field and in the normal state at 10 T applied perpendicular to the BEDT-TTF planes are shown. The solid line is a fit to the 10 T data below 2 K using a linear and cubic term. (b) Temperature dependence of the specific-heat difference between the superconducting and normal state. The lines show the BCS behavior for weak coupling (dashed) ^{ 30 } and moderately strong coupling (solid). ^{ 31 } The inset shows the electronic part of the specific heat, *C _{e} *, divided by γ

*T*as function of

_{c}*T*/

_{c}*T*. The solid line shows the exponential vanishing of

*C*towards low

_{e}*T*.

Difference between the specific heat for selected in-plane magnetic fields and the normal-state specific heat divided by temperature as a function of temperature for β″-(BEDT-TTF)_{2}SF_{5}CH_{2}CF_{2}SO_{3}. The inset shows thefield-dependent evolution of the electronic part of the specific heat, *C _{e} *, divided by temperature.

Difference between the specific heat for selected in-plane magnetic fields and the normal-state specific heat divided by temperature as a function of temperature for β″-(BEDT-TTF)_{2}SF_{5}CH_{2}CF_{2}SO_{3}. The inset shows thefield-dependent evolution of the electronic part of the specific heat, *C _{e} *, divided by temperature.

Superconducting phase diagram of β″-(BEDTTTF)_{2}SF_{5}CH_{2}CF_{2}SO_{3} for fields aligned parallel to and by 0.23 and 0.31° out of the superconducting layers. The data of the second anomaly observed at 0.23° [Fig. 9b ] are labeled by *T** (open blue triangles). The dashed line is a rough extrapolation of the data between 2 and 3 K to the Pauli limit of 9.73 T. The dotted line represents the calculated *H _{c} *

_{2}(see text for details). The inset shows the angular dependence of

*T*and

_{c}*T** at 9.5 and 10 T.

Superconducting phase diagram of β″-(BEDTTTF)_{2}SF_{5}CH_{2}CF_{2}SO_{3} for fields aligned parallel to and by 0.23 and 0.31° out of the superconducting layers. The data of the second anomaly observed at 0.23° [Fig. 9b ] are labeled by *T** (open blue triangles). The dashed line is a rough extrapolation of the data between 2 and 3 K to the Pauli limit of 9.73 T. The dotted line represents the calculated *H _{c} *

_{2}(see text for details). The inset shows the angular dependence of

*T*and

_{c}*T** at 9.5 and 10 T.

Specific-heat differences divided by temperature of β″-(BEDT-TTF)_{2} SF_{5}CH_{2}CF_{2}SO_{3} measured in different magnetic fields aligned parallel (open symbols) and 0.31° out of the conducting plane (closed symbols). The data are plotted offset for clarity.

Specific-heat differences divided by temperature of β″-(BEDT-TTF)_{2} SF_{5}CH_{2}CF_{2}SO_{3} measured in different magnetic fields aligned parallel (open symbols) and 0.31° out of the conducting plane (closed symbols). The data are plotted offset for clarity.

Specific-heat differences, Δ*C*/*T*, of β″-(BEDTTTF)_{2}SF_{5}CH_{2}CF_{2}SO_{3} measured (a) at 10 T for different angles close to in-plane field orientation and (b) in different magnetic fields aligned 0.23° out of the conducting plane.

Specific-heat differences, Δ*C*/*T*, of β″-(BEDTTTF)_{2}SF_{5}CH_{2}CF_{2}SO_{3} measured (a) at 10 T for different angles close to in-plane field orientation and (b) in different magnetic fields aligned 0.23° out of the conducting plane.

Article metrics loading...

Full text loading...

Commenting has been disabled for this content