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Register a new userTwo-Fold-Symmetric Magnetoresistance in Single Crystals of Tetragonal BiCh2-Based Superconductor LaO0.5F0.5BiSSe
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We have investigated the in-plane anisotropy of the c-axis magnetoresistance for single crystals of a BiCh2-based superconductor LaO0.5F0.5BiSSe under in-plane magnetic fields. We observed two-fold symmetry in the c-axis magnetoresistance in the ab-plane of LaO0.5F0.5BiSSe while the crystal possessed a tetragonal square plane with four-fold symmetry. The observed symmetry lowering in magnetoresistance from the structural symmetry may be related to the nematic states, which have been observed in the superconducting states of several unconventional superconductors.
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We have investigated the in-plane anisotropy of the c-axis magnetoresistance (MR) in both superconducting and normal states of single crystals of NdO0.7F0.3BiS2 under in-plane magnetic fields. In the superconducting states of NdO0.7F0.3BiS2, four-fold-symmetric in-plane anisotropy of the c-axis MR was observed below the superconducting transition temperature. Since the crystal structure of NdO0.7F0.3BiS2 is tetragonal, the rotational symmetry in the superconducting state is preserved in the present compound. This result is clearly different from the previous report observed in LaO0.5F0.5BiSSe single crystals, where the in-plane MR in the superconducting state shows two-fold symmetry. On the other hand, in the normal states of NdO0.7F0.3BiS2, two-fold symmetric MR with a small amplitude was observed. The possible origin of the two-fold-symmetric behavior was discussed with the presence of local structural disorder in the conducting plane of BiCh2-based compounds.
We investigate the superconducting properties and possible nematic superconductivity of self-doped BiCh2-based (Ch: S, Se) superconductor CeOBiS1.7Se0.3 through the measurements of in-plane anisotropy of magnetoresistance. Single crystals of CeOBiS1.7Se0.3 were grown using a flux method. Single-crystal structural analysis revealed that the crystal structure at room temperature is tetragonal (P4/nmm). Bulk superconductivity with a transition temperature of 3.3 K was observed through electrical resistivity and magnetization measurements. Investigation of anisotropy of upper critical field suggested relatively low anisotropy in the crystal as compared to other BiCh2-based superconductors. In the superconducting states of CeOBiS1.7Se0.3, two-fold symmetric in-plane anisotropy of magnetoresistance was observed, which indicates the in-plane rotational symmetry breaking in the tetragonal structure and hence the possibility of nematic superconductivity in CeOBiS1.7Se0.3.
Transition-metal oxides offer an opportunity to explore unconventional superconductors, where the superconductivity (SC) is often interrelated with novel phenomena such as spin/charge order, fluctuations, and Fermi surface instability (1-3). LiTi2O4 (LTO) is a unique compound in that it is the only known spinel oxide superconductor. In addition to electron-phonon coupling, electron-electron and spin fluctuation contributions have been suggested as playing important roles in the microscopic mechanism for its superconductivity (4-8). However, the lack of high quality single crystals has thus far prevented systematic investigation of their transport properties (9). Here, we report a careful study of transport and tunneling spectroscopy in epitaxial LTO thin films. In the superconducting state, the energy gap was found to decrease as a quadratic function of magnetic field. In the normal state, an unusual magnetoresistance (MR) was observed where it changes from anisotropic positive to isotropic negative as the temperature is increased. A constant charge carrier concentration without any abrupt change in lattice parameters as a function of temperature suggests that the isotropic MR stems from the suppression of spin scattering/fluctuations, while the anisotropic term originates from an orbital contribution. These observations point to an important role strong correlations play in this unique superconductor.
A subtle balance between competing interactions in strongly correlated systems can be easily tipped by additional interfacial interactions in a heterostructure. This often induces exotic phases with unprecedented properties, as recently exemplified by high-Tc superconductivity in FeSe monolayer on the nonmagnetic SrTiO3. When the proximity-coupled layer is magnetically active, even richer phase diagrams are expected in iron-based superconductors (FeSCs), which however has not been explored due to the lack of a proper material system. One promising candidate is Sr2VO3FeAs, a naturally-assembled heterostructure of a FeSC and a Mott-insulating vanadium oxide. Here, using high-quality single crystals and high-accuracy 75As and 51V nuclear magnetic resonance (NMR) measurements, we show that a novel electronic phase is emerging in the FeAs layer below T0 ~ 155 K without either static magnetism or a crystal symmetry change, which has never been observed in other FeSCs. We find that frustration of the otherwise dominant Fe stripe and V Neel fluctuations via interfacial coupling induces a charge/orbital order with C4-symmetry in the FeAs layers, while suppressing the Neel antiferromagnetism in the SrVO3 layers. These findings demonstrate that the magnetic proximity coupling is effective to stabilize a hidden order in FeSCs and, more generally, in strongly correlated heterostructures.
Two-dimensional transition metal dichalcogenides (TMDs) have been attracting significant interest due to a range of properties, such as layer-dependent inversion symmetry, valley-contrasted Berry curvatures, and strong spin-orbit coupling (SOC). Of particular interest is niobium diselenide (NbSe2), whose superconducting state in few-layer samples is profoundly affected by an unusual type of SOC called Ising SOC. Combined with the reduced dimensionality, the latter stabilizes the superconducting state against magnetic fields up to ~35 T and could lead to other exotic properties such as nodal and crystalline topological superconductivity. Here, we report transport measurements of few-layer NbSe$_2$ under in-plane external magnetic fields, revealing an unexpected two-fold rotational symmetry of the superconducting state. In contrast to the three-fold symmetry of the lattice, we observe that the magnetoresistance and critical field exhibit a two-fold oscillation with respect to an applied in-plane magnetic field. We find similar two-fold oscillations deep inside the superconducting state in differential conductance measurements on NbSe$_2$/CrBr$_3$ superconductor-magnet junctions. In both cases, the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute the behavior to the mixing between two closely competing pairing instabilities, namely, the conventional s-wave instability typical of bulk NbSe$_2$ and an unconventional d- or p-wave channel that emerges in few-layer NbSe2. Our results thus demonstrate the unconventional character of the pairing interaction in a few-layer TMD, opening a new avenue to search for exotic superconductivity in this family of 2D materials.
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