The application of a sufficiently strong magnetic field to a superconductor will, in general, destroy the superconducting state. Two mechanisms are responsible for this. The first is the Zeeman effect, which breaks apart the paired electrons if they are in a spin-singlet (but not a spin-triplet) state. The second is the so-called 'orbital' effect, whereby the vortices penetrate into the superconductors and the energy gain due to the formation of the paired electrons is lost. For the case of layered, two-dimensional superconductors, such as the high-Tc copper oxides, the orbital effect is reduced when the applied magnetic field is parallel to the conducting layers. Here we report resistance and magnetic-torque experiments on single crystals of the quasi-two-dimensional organic conductor lambda-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene. We find that for magnetic fields applied exactly parallel to the conducting layers of the crystals, superconductivity is induced for fields above 17 T at a temperature of 0.1 K. The resulting phase diagram indicates that the transition temperature increases with magnetic field, that is, the superconducting state is further stabilized with magnetic field.

We report the ac magnetic susceptibility ac and resistivity measurements of EuFe 2 As 2 under high pressure P. By observing nearly 100% superconducting shielding and zero resistivity at P ¼ 28 kbar, we establish that P-induced superconductivity occurs at T c $ 30 K in EuFe 2 As 2 . shows an anomalous nearly linear temperature dependence from room temperature down to T c at the same P. ac indicates that an antiferromagnetic order of Eu 2þ moments with T N $ 20 K persists in the superconducting phase. The temperature dependence of the upper critical field is also determined.KEYWORDS: iron pnictides, pressure-induced superconductivity, susceptibility, upper critical field DOI: 10.1143/JPSJ.78.083701The discovery of superconductivity (SC) at a transition temperature T c ¼ 26 K in LaFeAsO 1Àx F x by Kamihara et al.1) has triggered extensive studies of SC in layered iron pnictides and related compounds. Rotter et al. found that BaFe 2 As 2 with a simpler structure can be made superconducting by doping:2) Perhaps more importantly, it is reported that 122 compounds of the form AFe 2 As 2 (A ¼ Ca, Sr, Ba, and Eu) can be tuned to SC by the application of high pressure P. 3-10)P tuning can provide opportunities to determine the nature of the iron-pnictide high-temperature SC without being adversely affected by disorder due to doping. However, most of these reports are based only on resistivity measurements and hence cannot establish the bulk nature of P-induced SC.11) Even when magnetic measurements are reported, results are not conclusive: In ref. 5, magnetic measurements were performed on SrFe 2 As 2 and BaFe 2 As 2 , but the observed volume fraction was expressed in arbitrary units. In ref. 9, the volume fraction of the P-induced superconducting phase of CaFe 2 As 2 was estimated to be at least 50%, while in ref. 12 CaFe 2 As 2 was reported not to exhibit SC under hydrostatic P produced by the use of helium as a pressure-transmitting medium.EuFe 2 As 2 exhibits two phase transitions, at T o $ 190 K and T N $ 19 K, at ambient P.13-16) The transition at T o is a combined structural and magnetic transition, similar to those in the other 122 compounds: the crystal structure changes from tetragonal to orthorhombic and the Fe 2þ moments order antiferromagnetically. The transition at T N is due to the antiferromagnetic (AFM) ordering of the Eu 2þ moments. The AFM coupling of the Eu 2þ moments is rather weak: the field-induced paramagnetic state with a saturated moment of $7 B /Eu is easily reached by the application of $1 or 2 T in the ab-plane or along the c-axis, respectively. 17) A temperature (T)-P phase diagram has been determined from measurements: 10) while T o decreases with P and is not detected above P ¼ 23 kbar, T N is nearly P-independent up to 26 kbar (the highest P in ref. 10). The authors of ref. 10 state that P-induced SC at T c $ 30 K occurs above 20 kbar. However, their data (at P ¼ 21:6 kbar) shows only a partial drop and approximately half of the normal-state appears to remain as T ! 0. Obviously, further e...

Recently, VIB-group layered transition metal dichalcogenides (TMDs), MX 2 (M = Mo, W; X = S, Se, Te), attracted extensive attention due to rich physiochemical properties, ranging from catalysis, [1][2][3] topological states, [4][5][6][7][8][9][10][11][12][13][14][15][16] valley polarization, [17][18][19][20][21][22] even to superconductivity. [23][24][25][26][27][28] These multiple electronic properties essentially originate from varied crystal structures of TMD materials. The typical crystal structure in TMD materials is the 2H-type structure with [X-M-X] atoms in ABA stacking in each monolayer (Figure S1a, Supporting Information). Usually, 2H MX 2 materials are semiconducting, such as, 2H MoS 2 , where the valley polarization was widely studied. [17][18][19] Also, Ising superconductivity was observed when the TMD materials are reduced down to a few or even oneRecently the metastable 1T′-type VIB-group transition metal dichalcogenides (TMDs) have attracted extensive attention due to their rich and intriguing physical properties, including superconductivity, valleytronics physics, and topological physics. Here, a new layered WS 2 dubbed "2M" WS 2 , is constructed from 1T′ WS 2 monolayers, is synthesized. Its phase is defined as 2M based on the number of layers in each unit cell and the subordinate crystallographic system. Intrinsic superconductivity is observed in 2M WS 2 with a transition temperature T c of 8.8 K, which is the highest among TMDs not subject to any fine-tuning process. Furthermore, the electronic structure of 2M WS 2 is found by Shubnikov-de Haas oscillations and first-principles calculations to have a strong anisotropy. In addition, topological surface states with a single Dirac cone, protected by topological invariant Z 2 , are predicted through first-principles calculations. These findings reveal that the new 2M WS 2 might be an interesting topological superconductor candidate from the VIB-group transition metal dichalcogenides.

We report resistivity measurements performed on KFe 2 As 2 single crystals down to T = 0.3 K and in magnetic fields up to 17.5 T. The in-plane resistivity vs. T curve has a convex shape down to ∼50 K and shows a T 2 dependence below ∼45 K. The ratio of the c-axis to in-plane resistivities is ∼10 at room temperature and ∼40 at 4.2 K. The superconducting

We investigate by electrical transport the field-induced superconducting state (FISC) in the organic conductor λ-(BETS)2FeCl4. Below 4 K, antiferromagnetic-insulator, metallic, and eventually superconducting (FISC) ground states are observed with increasing in-plane magnetic field. The FISC state survives between 18 and 41 T, and can be interpreted in terms of the Jaccarino-Peter effect, where the external magnetic field compensates the exchange field of aligned Fe 3+ ions. We further argue that the Fe 3+ moments are essential to stabilize the resulting singlet, two-dimensional superconducting state Superconductivity is usually destroyed by diamagnetic currents induced in the presence of strong magnetic fields. This effect has orbital character and prevails in most conventional "s-wave" superconductors that involve singlet state of the Cooper pairs. In addition, superconductivity can also be suppressed by the Pauli pair breaking mechanism: here the external field destroys the spinsinglet state of the Cooper pair, imposing the so-called Clogston-Chandrasekhar paramagnetic limit [1,2]. Nevertheless, and despite these well known physical limitations, S. Uji et al. [3] have recently reported the observation of a magnetic-field induced superconducting phase (FISC) in the quasi-two-dimensional organic conductor λ-(BETS) 2 FeCl 4 for fields exceeding 18 tesla, applied parallel to the conducting layers. This is particularly remarkable since this compound, at zero field, is an antiferromagnetic insulator (AI) below T p ∼ = 8.5K [4]. The AI state is suppressed by the application of magnetic fields above 10 tesla at low temperatures [5].The present work was motivated by the apparent increase in the critical temperature of the FISC above 18 T with increasing magnetic field (Ref. [3]). Here, for instance, in the case of spin-triplet superconductivity, there would be in principle, no limit on the upper critical field. The presence of Fe 3+ magnetic moments, which coexist with the FISC state, adds further appeal to the triplet state model. To clarify the nature of the FISC, we have studied the λ-(BETS) 2 FeCl 4 compound at low temperatures in steady, tilted magnetic fields up to 42 tesla. Our main result is the observation of reentrance towards the metallic state at a temperature-dependent critical field. We obtain a temperature-magnetic field phase diagram for the FISC state, which we interpret in terms of the Jacarino-Peter (JP) field compensation effect [6]. This implies that the Cooper pairs condense into a spin-singlet state. We argue further that the Fe 3+ magnetic state is indeed necessary to stabilize the singlet superconducting state by suppression of diamagnetic currents in the associated in-plane high magnetic fields.λ-(BETS) 2 FeCl 4 (where BETS stands for Bis(ethylenedithio)tetraselenafulvalene) crystallizes in a triclinic unit cell. The BETS planar molecules are stacked along the crystallographic a-axis, and constitute conducting planes parallel to the a-c plane. These conducting layers alternate along the b-axis...