Superconductivity in an organic insulator at very high magnetic fields

Strain-switchable field-induced superconductivity

Field-induced superconductivity is a rare phenomenon where an applied magnetic field enhances or induces superconductivity. Here, we use applied stress as a control switch between a field-tunable superconducting state and a robust non-field-tunable state. This marks the first demonstration of a strain-tunable superconducting spin valve with infinite magnetoresistance. We combine tunable uniaxial stress and applied magnetic field on the ferromagnetic superconductor Eu(Fe0.88Co0.12)2As2 to shift the field-induced zero-resistance temperature between 4 K and a record-high value of 10 K. We use x-ray diffraction and spectroscopy measurements under stress and field to reveal that strain tuning of the nematic order and field tuning of the ferromagnetism act as independent control parameters of the superconductivity. Combining comprehensive measurements with DFT calculations, we propose that field-induced superconductivity arises from a novel mechanism, namely, the uniquely dominant effect of the Eu dipolar field when the exchange field splitting is nearly zero.

A review of UTe2at high magnetic fields

Uranium ditelluride (UTe2) is recognized as a host material to unconventional spin-triplet superconductivity, but it also exhibits a wealth of additional unusual behavior at high magnetic fields. One of the most prominent signatures of the unconventional superconductivity is a large and anisotropic upper critical field that exceeds the paramagnetic limit. This superconductivity survives to 35 T and is bounded by a discontinuous magnetic transition, which itself is also field-direction-dependent. A different, reentrant superconducting phase emerges only on the high-field side of the magnetic transition, in a range of angles between the crystallographicbandcaxes. This review discusses the current state of knowledge of these high-field phases, the high-field behavior of the heavy fermion normal state, and other phases that are stabilized by applied pressure.

Sequence of superconducting states in field cooled FeCr2S4

In the present article we discuss theoretically the emergence of superconductivity in field cooled FeCr₂S₄. The chromium electrons form a triplett_2gstates and due to antiferromagnetic exchange with the iron spins have Zeeman splitting. Applied, during preparation, magnetic field along the moment of iron ions, successively compensates the Zeeman splittings. The chromium electrons with zero Zeeman energy form Cooper pairs induced by iron magnons. In that way, we predict theoretically the existence of sequence of superconducting states in field cooled FeCr₂S₄. Actually there are three different superconductors prepared applying, during preparation, different magnetic fields. In these compounds superconductivity coexist with the saturated magnetism of iron ions.

Pauli-limit violation and re-entrant superconductivity in moiré graphene

Moiré quantum matter has emerged as a materials platform in which correlated and topological phases can be explored with unprecedented control. Among them, magic-angle systems constructed from two or three layers of graphene have shown robust superconducting phases with unconventional characteristics^1-5. However, direct evidence of unconventional pairing remains to be experimentally demonstrated. Here we show that magic-angle twisted trilayer graphene exhibits superconductivity up to in-plane magnetic fields in excess of 10 T, which represents a large (2-3 times) violation of the Pauli limit for conventional spin-singlet superconductors^6,7. This is an unexpected observation for a system that is not predicted to have strong spin-orbit coupling. The Pauli-limit violation is observed over the entire superconducting phase, which indicates that it is not related to a possible pseudogap phase with large superconducting amplitude pairing. Notably, we observe re-entrant superconductivity at large magnetic fields, which is present over a narrower range of carrier densities and displacement fields. These findings suggest that the superconductivity in magic-angle twisted trilayer graphene is likely to be driven by a mechanism that results in non-spin-singlet Cooper pairs, and that the external magnetic field can cause transitions between phases with potentially different order parameters. Our results demonstrate the richness of moiré superconductivity and could lead to the design of next-generation exotic quantum matter.

Modern History of Organic Conductors: An Overview

This short review article provides the reader with a summary of the history of organic conductors. To retain a neutral and objective point of view regarding the history, background, novelty, and details of each research subject within this field, a thousand references have been cited with full titles and arranged in chronological order. Among the research conducted over ~70 years, topics from the last two decades are discussed in more detail than the rest. Unlike other papers in this issue, this review will help readers to understand the origin of each topic within the field of organic conductors and how they have evolved. Due to the advancements achieved over these 70 years, the field is nearing new horizons. As history is often a reflection of the future, this review is expected to show the future directions of this research field.

Magnetic-Field-Induced Re-entrance of Superconductivity in Ta2PdS5 Nanostrips

The motion of Abrikosov vortices is the dominant origin of dissipation in type II superconductors subjected to a magnetic field, which leads to a finite electrical resistance. It is generally believed that the increase in the magnetic field results in the aggravation of energy dissipation through the increase in vortex density. Here, we show a distinctive re-entrance of the dissipationless state in quasi-one-dimensional superconducting Ta₂PdS₅ nanostrips. Utilizing magnetotransport measurements, we unveil a prominent magnetoresistance drop with the increase in the magnetic field below the superconducting transition temperature, manifesting itself as a giant re-entrance to the superconducting phase. Time-dependent Ginzburg-Landau calculations show that this is originated from the suppression of the vortex motion by the increased energy barrier on the edges. Interestingly, both our experiments and simulations demonstrate that this giant re-entrance of superconductivity occurs only in certain geometrical regimes because of the finite size of the vortex.

Extreme magnetic field-boosted superconductivity

Applied magnetic fields underlie exotic quantum states, such as the fractional quantum Hall effect¹ and Bose-Einstein condensation of spin excitations². Superconductivity, however, is inherently antagonistic towards magnetic fields. Only in rare cases^3-5 can these effects be mitigated over limited fields, leading to re-entrant superconductivity. Here, we report the coexistence of multiple high-field re-entrant superconducting phases in the spin-triplet superconductor UTe₂ (ref. ⁶). We observe superconductivity in the highest magnetic field range identified for any re-entrant superconductor, beyond 65 T. Although the stability of superconductivity in these high magnetic fields challenges current theoretical models, these extreme properties seem to reflect a new kind of exotic superconductivity rooted in magnetic fluctuations⁷ and boosted by a quantum dimensional crossover⁸.

Topological quantum phase transitions and edge states in spin-orbital coupled Fermi gases

We study superconducting states in the presence of spin-orbital coupling and Zeeman field. It is found that a phase transition from a Fulde-Ferrell-Larkin-Ovchinnikov state to the topological superconducting state occurs upon increasing the spin-orbital coupling. The nature of this topological phase transition and its critical property are investigated numerically. Physical properties of the topological superconducting phase are also explored. Moreover, the local density of states is calculated, through which the topological feature may be tested experimentally.

Magnetic-field-induced superconducting state in Zn nanowires driven in the normal state by an electric current

Four-terminal resistance measurements have been carried out on Zn nanowires formed using electron-beam lithography. When driven resistive by current, these wires reenter the superconducting state upon application of small magnetic fields. The data are qualitatively different from those of previous experiments on superconducting nanowires, which revealed either negative magnetoresistance near T_{c} or high-magnetic-field-enhanced critical currents.

Directional field-induced metallization of quasi-one-dimensional Li0.9Mo6O17

We report a detailed magnetotransport study of the highly anisotropic quasi-one-dimensional oxide Li(0.9)Mo(6)O(17) whose in-chain electrical resistivity diverges below a temperature T_{min} approximately 25 K. For T < T_{min}, a magnetic field applied parallel to the conducting chain induces a large negative magnetoresistance and, ultimately, the recovery of a metallic state. We show evidence that this insulator-metal crossover is a consequence of field-induced suppression of a density-wave gap in a highly one-dimensional conductor. At the highest fields studied, there is evidence for the possible emergence of a novel superconducting state with an onset temperature T_{c} > 10 K.

Pi-donor BETS based bifunctional superconductor with polymeric dicyanamidomanganate(II) anion layer: kappa-(BETS)2Mn[N(CN)2]3

Crystals of the bis(ethylenedithio)tetraselenafulvalene (BETS) radical cation salt with dicyanamidomanganate(II) anion, kappa-(BETS)2Mn[N(CN)2]3, were synthesized, which combine conducting and magnetic properties at ambient pressure and are superconducting (Tc approximately/= 5 K) at a moderate pressure of 0.3 kbar.

Antiferromagnetic d-electron exchange via a spin-singlet pi-electron ground state in an organic conductor

Electron spin resonance reveals the spin behavior of conduction (pi) and localized (d) electrons in beta-(BDA-TTP)2MCl4 (M=Fe, Ga). Both the Ga3+(S=0) and Fe3+(S=5/2) compounds exhibit a metal-insulator transition at 113 K with the simultaneous formation of a spin-singlet ground state in the pi electron system of the donor molecules. The behavior is consistent with charge ordering in beta-(BDA-TTP)2MCl4 at the metal-insulator transition. At 5 K, the Fe3+ compound orders antiferromagnetically, even though the pi electrons, which normally would facilitate magnetic exchange, are localized nonmagnetic singlets.

Crystal Structure and Physical Properties of Conducting Molecular Antiferromagnets with a Halogen-Substituted Donor: (EDO-TTFBr2)2FeX4 (X = Cl, Br)

The crystal structure and physical properties of radical ion salts (EDO-TTFBr2)2FeX4 (X = Cl, Br) based on halogen-substituted organic donor and magnetic anions are investigated, including the comparison with the isomorphous compounds (EDO-TTFBr2)2GaX4 with nonmagnetic anions. The crystal structure of these four salts consists of uniformly stacked donor molecules and tetrahedral counter anions, and the Br substituents of the donor molecules are connected to halide ligands of anions with remarkably short intermolecular atomic distances. These salts show metallic behavior around room temperature and undergo a spin-density-wave transition in the low-temperature range, as confirmed with the divergence of the electron spin resonance (ESR) line width. Although close anion-anion contacts are absent in these salts, the FeCl4 salt undergoes an antiferromagnetic transition at TN = 4.2 K, and the FeBr4 salt shows successive magnetic transitions at TN = 13.5 K and TC2 = 8.5 K with a helical spin structure as a candidate for the ground state of the d-electron spins. The magnetoresistance of the FeCl4 salt shows stepwise anomalies, which are explained qualitatively using a pi-d interaction-based frustrated spin system model composed of the donor pi-electron and the anion d-electron spins. Although on the ESR spectra of the FeX4 salts signals from the pi- and d-electron spins are separately observed, the line width of the pi-electron spins broadens under the temperature where the susceptibility deviates from the Curie-Weiss behavior, showing the presence of the pi-d interaction.

Vortex dynamics and the Fulde-Ferrell-Larkin-Ovchinnikov state in a magnetic-field-induced organic superconductor

Under special conditions, a superconducting state where the order parameter oscillates in real space, the so-called FFLO state, is theoretically predicted to exist near the upper critical field, as first proposed by Fulde and Ferrell, and Larkin and Ovchinnikov. We report systematic measurements of the interlayer resistance in high magnetic fields to 45 T in the two-dimensional magnetic-field-induced organic superconductor lambda-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene. The resistance is found to show characteristic dip structures in the superconducting state. The results are consistent with pinning interactions between the vortices penetrating the insulating layers and the order parameter of the FFLO state. This gives strong evidence for an oscillating order parameter in real space.

Compensation of effective field in the field-induced superconductor kappa-(BETS)2FeBr4 observed by 77Se NMR

We report results of 77Se NMR frequency shift in the normal state of the organic charge-transfer salt kappa-(BETS)2FeBr4 which shows magnetic field-induced superconductivity (FISC). From a simple mean-field analysis, we determined the field and the temperature dependences of the magnetization m(pi) of the pi conduction electrons on BETS molecules. We found that the Fe spins are antiferromagnetically coupled to the pi electrons and determined the exchange field to be J = -2.3T/microB. The exchange field from the fully saturated Fe moments (5 microB) is compensated by an external field of 12 T. This is close to the central field of the FISC phase, consistent with the Jaccarino-Peter local field-compensation mechanism for FISC [Phys. Rev. Lett. 9, 290 (1962)].

Local tunneling spectroscopy as a signature of the Fulde-Ferrell-Larkin-Ovchinnikov state in s- and d-wave superconductors

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states for two-dimensional s- and d-wave superconductors (s- and d-SCs) are self-consistently studied under an in-plane magnetic field. While the stripe solution of the order parameter is found to have lower free energy in s-SCs, a square lattice solution appears to be energetically more favorable in the case of d-SCs. At certain symmetric sites, we find that the features in the local density of states (LDOS) can be ascribed to two types of bound states. We also show that the LDOS maps for d-SCs exhibit bias-energy-dependent checkerboard patterns. These characteristics can serve as signatures of the FFLO states.

Structural, electrical, and magnetic properties of a series of molecular conductors based on BDT-TTP and lanthanoid nitrate complex anions (BDT-TTP = 2,5-Bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene)

The platelike crystals of a series of novel molecular conductors, which are based on the pi-donor molecules BDT-TTP (2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene) with a tetrathiapentalene skeleton and lanthanide nitrate complex anions [Ln(NO3)x](3-x)(Ln = La, Ce, (Pr), Tb, Dy, Ho, Er, Tm, Yb, Lu) with localized 4f magnetic moments, were synthesized. Except for the Ce complex, the salts were composed of (BDT-TTP)(5)[Ln(NO(3))(5)] and were isostructural. Even though the Ce crystal had a different composition, (BDT-TTP)(6)[Ce(NO(3))(6)](C(2)H(5)OH)(x)() (x approximately 3), the crystals all had the space group P(-)1. Although the X-ray examination of the Pr salts was insufficient, the existence of two modifications was suggested in these systems by preliminary X-ray examination. Previously, we reported the crystal structures and unique magnetic properties of (BDT-TTP)(5)[Ln(NO(3))(5)] (Ln = Sm, Eu, Nd, Gd). Thus, by combining the results of this work with previous one, we for the first time succeeded in obtaining a complete set of organic conductors composed of the identical pi-donors (BDT-TTP in this case) and all the lanthanide nitrate complex anions (except the complex with Pm(3+)). The crystals were all metallic down to 2 K. Electronic band structure calculations resulted in two-dimensional Fermi surfaces, which was consistent with their stable metallic states. Except for the Lu complex, which lacked paramagnetic moments, the magnetic susceptibilities were measured on the six heavy lanthanide ion complex salts by a SQUID magnetometer (Ln = Tb, Dy, Ho, Er, Tm, Yb). The large paramagnetic susceptibilities, which were caused by the paramagnetic moments of the rare-earth ions, were obtained. The Curie-Weiss law fairly accurately reproduced the temperature dependence of the magnetic susceptibilities of (BDT-TTP)(5)[Ho(NO(3))(5)] in the experimental temperature range (2-300 K) and a comparatively large Weiss temperature (|THETAV;|) was obtained (THETAV;(Ho) = -15 K). A Weiss temperature (THETAV;(Tm) = -8 K) was also obtained for Tm. The |THETAV;| values of other (BDT-TTP)(5)[Ln(NO(3))(5)] salts and (BDT-TTP)(6)[Ce(NO(3))(6)](C(2)H(5)OH)x(x approximately 3) were as follows: |THETAV;|/K = 4 (Er), < or =2 (Ce, Tb, Dy, Yb). The comparatively strong intermolecular magnetic interaction between Ho(3+) ions, which was suggested by the |THETAV;| value, is inconsistent with the traditional image of strongly localized 4f orbitals shielded by the electrons in the outer 5s and 5p orbitals. The dipole interactions between Ln(3+) ions causing the Curie-Weiss behavior and the comparatively large THETAV; value of (BDT-TTP)(5)[Ho(NO(3))(5)] is inconsistent with the data, since the complexes exhibit isostructural properties and there is not a clear relationship between the magnitudes of THETAV; values and those of magnetic moments. Therefore, it is possible that the 4f orbitals of Ho atom are sensitive to the ligand field, which will have an effect on the orbital moment of the Ho(3+) ion and/or produce a small amount of mixing between 4f and ligand orbitals to give rise to "real" intermolecular antiferromagnetic interaction through intermolecular overlapping between pi (BDT-TTP) and ligand orbitals of lanthanide nitrate complex anions.

A general method for the synthesis of alkylenedithio- and bis(alkylenedithio)tetraselenafulvalenes

A general synthetic method toward a series of alkylenedithio- and bis(alkylenedithio)tetraselenafulvalenes, i.e., methylenedithio- (MDT-TSF, 1a), ethylenedithio- (EDT-TSF, 1b), propylenedithio- (PDT-TSF, 1c), bis(methylenedithio)- (BMDT-TSF, 2a), bis(ethylenedithio)- (BETS, 2b), and bis(propylenedithio)tetraselenafulvalene (BPDT-TSF, 2c), as superior electron donors for organic conductors has been developed. This method is advantageous to ready access to a series of compounds from common synthetic intermediates, 2-methylthio-3-(2-methoxycarbonylethylthio)-tetraselenafulvalene (6) and 2,6(7')-bis(methylthio)-3,7(6')-bis(2-methoxycarbonylethylthio)tetraselenafulvalene (7), for the asymmetrical alkylenedithio- and symmetrical bis(alkylenedithio)-TSFs, respectively. These key intermediates are readily prepared by phosphite-promoted coupling reactions of 4-methylthio-5-(2-methoxycarbonylethylthio)-1,3-selenole-2-selone (5) or by a reaction of TSF with LDA and methyl 3-thiocyanatopropionate. The latter method provides not only the successful conversion of TSF to these heterocycle derivatives but also a generally acceptable route to them, since TSF is accessible without the toxic and less easily available CSe(2).

Nanoengineered magnetic-field-induced superconductivity

The perpendicular critical fields of a superconducting film have been strongly enhanced by using a nanoengineered lattice of magnetic dots (dipoles) on top of the film. Magnetic-field-induced superconductivity is observed in these hybrid superconductor/ferromagnet systems due to the compensation of the applied field between the dots by the stray field of the dipole array. By switching between different magnetic states of the nanoengineered field compensator, the critical parameters of the superconductor can be effectively controlled.

Intrinsic pinning of vortices as a direct probe of the nonuniform Larkin-Ovchinnikov-Fulde-Ferrell state in layered superconductors

Previously the search for the modulated superconducting Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state was performed by means of measurements which do not give direct information on spatial modulation of the superconducting state. We propose to measure interlayer conductivity in Josephson-coupled layered superconductors as a function of the strength and the orientation of the parallel magnetic field. We show that interlayer critical current and the conductivity have peaks when the magnetic field is perpendicular to the in-plane wave vector of the LOFF state and when the period of the Josephson vortex lattice induced by the magnetic field is commensurate with the LOFF period.

Dual-action molecular superconductors with magnetic anions

Dual-action organic superconductors, whose conducting properties can be sharply controlled by an external magnetic field, have been discovered in systems consisting of organic conduction layers based on bis(ethylenedithio)tetraselenafulvalene (BETS) molecules and magnetic anions. Owing to the metamagnetic nature of the anion layers, the superconducting state of kappa-BETS2FeBr4 can be switched on or off by applying the external field. In lambda-BETS2Fe0.4Ga0.6Cl4, exhibiting a field-induced superconducting transition for the field parallel to the conduction plane, the insulating, metallic, and superconducting states can be realized in a stepwise manner by slightly tuning the external magnetic field.

An indication of magnetic-field-induced superconductivity in a bifunctional layered organic conductor, kappa-(BETS)(2)FeBr(4)

Hybrid systems consisting of the conducting layers of organic donor molecules and the magnetic layers of inorganic anions have been focused on as possible bifunctional materials, whose conducting properties can be tuned by controlling the magnetic state of the anion layers on an application of magnetic field. Here we report the magnetoresistance of the antiferromagnetic organic superconductor, kappa-(BETS)2FeBr4 [BETS = bis(ethylenedithio)tetraselenafulvalene], consisting of the two-dimensional superconducting layers of the BETS semications and the insulating layers of the FeBr4- anions. Due to the metamagnetic nature of the Fe3+ spin system, characteristic resistivity decrease was observed just below the antiferromagnetic superconductor-to-ferromagnetic metal transition at 1.6 T. Furthermore, an indication of the onsets of the magnetic-field-induced superconductivity was discovered around 12.5 T.

Magnetic molecular conductors based on BETS molecules and divalent magnetic anions [BETS = bis(ethylenedithio)tetraselenafulvalene]

Several conducting salts based on BETS [where BETS = bis(ethylenedithio)tetraselenafulvalene] molecules and divalent magnetic anions such as the (CoCl(4))(2-), (CoBr(4))(2-), and (MnBr(4))(2-) were prepared. Electrocrystallization by using the (CoCl(4))(2-) anion gave two kinds of crystals. Block-shaped crystals were cleared to be (BETS)(2)CoCl(4), which is an insulator with the high-spin state of cobalt 3d spin. On the other hand, the X-ray crystal structure analysis of a plate-shaped crystal of the (CoCl(4))(2-) salt revealed the system to be kappa-(BETS)(4)CoCl(4)(EtOH), which is metallic down to 0.7 K. The electronic band structure calculation gave a typical two-dimensional cylindrical Fermi surface. However, there is only very weak antiferromagnetic interaction between the S = 3/2 cobalt 3d spins because of its anion-solvent-intermingled layer structure. On the other hand, the electrocrystallization by using the (MnBr(4))(2-) anion yielded the plate-shaped black crystals of the (MnBr(4))(2-) salt. Crystal structure analysis of the (MnBr(4))(2-) salt showed that the salt is theta;-(BETS)(4)MnBr(4)(EtOH)(2) with alternating donor and anion-solvent mixed layers. The stacking direction in one donor layer is perpendicular to those of the neighboring layers. The electrical and magnetic properties of the theta;-(BETS)(4)MnBr(4)(EtOH)(2) salt showed the metallic behavior down to approximately 30 K and the paramagnetism of the high-spin manganese 3d spins. Band structure calculation of this salt gave an elliptical cylindrical Fermi surface. Because the Fermi surfaces of the adjacent donor layers are rotated to each other by 90 degrees, the theta-(BETS)(4)MnBr(4)(EtOH)(2) salt becomes a two-dimensionally isotropic metal.

New superconducting phases in field-induced organic superconductor lambda-(BETS)2FeCl4

We derive the parallel upper critical field, Hc2, as a function of the temperature T in quasi-2D organic compound lambda-(BETS)2FeCl4, accounting for the formation of the nonuniform Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) state. To further check the 2D LOFF model, we propose to study the Hc2(T) curve at low T in tilted fields, where the vortex state is described by the high Landau level functions characterized by the index n. We predict a cascade of first-order transitions between vortex phases with different n, between phases with different types of the symmetry at given n and the change of the superconducting transition from the second order to the first order as FeCl4 ions are replaced partly by GaCl4 ions.