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arXiv2013

A one-dimensional tunable magnetic metamaterial

We present experimental data on a one-dimensional superconducting metamaterial that is tunable over a broad frequency band. The basic building block of this magnetic thin-film medium is a single-junction (rf-) superconducting quantum interference device (SQUID). Due to the nonlinear inductance of such an element, its resonance frequency is tunable in situ by applying a dc magnetic field. We demonstrate that this results in tunable effective parameters of our metamaterial consisting of 54 SQUIDs. In order to obtain the effective magnetic permeability from the measured data, we employ a technique that uses only the complex transmission coefficient S21.

cond-mat.supr-con↗
arXiv2018

Resonating valence bonds and spinon pairing in the Dicke model

Resonating valence bond (RVB) states are a class of entangled quantum many body wavefunctions with great significance in condensed matter physics. We propose a scheme to synthesize a family of RVB states using a cavity QED setup with two-level atoms (with states $\vert 0 \rangle$ and $\vert 1 \rangle$) coupled to a common photon mode. In the lossy cavity limit, starting with an initial state of $M$ atoms excited and $N$ atoms in the ground state, we show that this setup can be configured as a Stern Gerlach experiment. A measurement of photon emission collapses the wavefunction of atoms onto an RVB state composed of resonating long-ranged singlets of the form $\frac{1}{\sqrt{2}}[\vert 0 1 \rangle - \vert 1 0 \rangle]$. Each emitted photon reduces the number of singlets by unity, replacing it with a pair of lone spins or `spinons'. As spinons are formed coherently in pairs, they are analogous to Cooper pairs in a superconductor. To simulate pair fluctuations, we propose a protocol in which photons are allowed to escape the cavity undetected. This leads to a mixed quantum state with a fluctuating number of spinon pairs -- an inchoate superconductor. Remarkably, in the limit of large system sizes, this protocol reveals an underlying quantum phase transition. Upon tuning the initial spin polarization ($M-N$), the emission exhibits a continuous transition from a dark state to a bright state. This is reflected in the spinon pair number distribution which can be tuned from sub-poissonian to super-poissonian regimes. This opens an exciting route to simulate RVB states and superconductivity.

quant-ph↗
arXiv2021

The role of orbital nesting in the superconductivity of Iron-based Superconductors

We analyze the magnetic excitations and the spin-mediated superconductivity in iron-based superconductors within a low-energy model that operates in the band basis but fully incorporates the orbital character of the spin excitations. We show how the orbital selectivity, encoded in our low-energy description, simplifies substantially the analysis and allows for analytical treatments, while retaining all the main features of both spin-excitations and gap functions computed using multiorbital models. Importantly, our analysis unveils the orbital matching between the hole and electron pockets as the key parameter to determine the momentum-dependence and the hierarchy of the superconducting gaps, instead of the Fermi surface matching as in the common nesting scenario.

cond-mat.supr-con↗
arXiv2012

Engineering and manipulating topological qubits in 1D quantum wires

We investigate the Josephson effect in TNT and NTN junctions, consisting of topological (T) and normal (N) phases of semiconductor-superconductor 1D heterostructures in the presence of a Zeeman field. A key feature of our setup is that, in addition to the variation of the phase of the superconducting order parameter, we allow the orientation of the magnetic field to change along the junction. We find a novel magnetic contribution to the Majorana Josephson coupling that permits the Josephson current to be tuned by changing the orientation of the magnetic field along the junction. We also predict that a spin current can be generated by a finite superconducting phase difference, rendering these materials potential candidates for spintronic applications. Finally, this new type of coupling not only constitutes a unique fingerprint for the existence of Majorana bound states but also provides an alternative pathway for manipulating and braiding topological qubits in networks of wires.

cond-mat.mes-hall↗
arXiv2012

Robust magnetic order of Ce 4f-electrons coexisting with superconductivity in CeFeAsO$_{1-x}$F$_x$

The electrical resistance of CeFeAsO$_{1-x}$F$_x$ (x = 0.06 and 0.08) has been measured in a magnetic field up to 40T. At zero field, the sample with x = 0.06 shows a structural phase transition around T$_S$~100K, followed by a spin-density-wave (SDW) transition around T$_{SDW}$~30K. For x = 0.08, the structural phase transition is suppressed down to T$_S$~60K without a clear anomaly associated with the Fe-SDW transition, and superconductivity shows up at T$_C$~25K. At lower temperatures, both samples show a clear resistive peak around T$_N$~4K, which is associated with the antiferromagnetic (AFM) transition of Ce-4f electrons. Strikingly, zero resistance is recovered upon further lowering temperature below T$_N$ for x = 0.08. Moreover, we found that the AFM transition of Ce 4f-electrons at 4K hardly changes with applying a magnetic field up to 40T, even in the case of x = 0.08, where superconductivity has been partially suppressed at such a large field.

cond-mat.supr-con↗
arXiv2013

Aspects of Superfluid Cold Atomic Gases in Optical Lattices

We review our studies on Bose and Fermi superfluids of cold atomic gases in optical lattices at zero temperature. Especially, we focus on superfluid Fermi gases along the crossover between the Bardeen-Cooper-Schrieffer (BCS) and the Bose-Einstein condensate (BEC) states, which enable us to study the Bose and the Fermi superfluids in a unified point of view. We discuss basic static and long-wavelength properties (such as the equation of state, incompressibility, and effective mass), energetic stability, and energy band structures of the superfluid Fermi gases in an optical lattice periodic along one spatial direction. The periodic potential causes pairs of atoms to be strongly bound, and this can affect the static and long-wavelength properties and the stability of the superflow. Regarding the band structure, a peculiar loop structure called "swallowtail" can appear in superfluid Fermi gases and in the Bose case, but the mechanism of emergence in the Fermi case is very different from that in bosonic case. Other quantum phases that the cold atomic gases in optical lattices can show are also briefly discussed based on their roles as quantum simulators of Hubbard models.

cond-mat.quant-gas↗
arXiv2013

Observation of Higher-Order Sideband Transitions and First-Order Sideband Rabi Oscillations in a Superconducting Flux Qubit Coupled to a SQUID Plasma Mode

We report results of spectroscopic measurements and time-domain measurements of a superconducting flux qubit. The dc superconducting quantum interference device (SQUID), used for readout of the qubit, and a shunt capacitor formed an LC resonator generating a SQUID plasma mode. Higher-order red and blue sidebands were observed in a simple measurement scheme because the resonant energy of the resonator, 600 MHz, was comparable to the thermal energy. We also observed Rabi oscillations on the carrier transition and the first-order sideband transitions. Because the qubit was coupled to a single arm of the dc SQUID, the qubit-SQUID coupling was significant at zero bias current, where these phenomena were observed. The ratios between the Rabi periods for the carrier transition and the sideband transitions are compared with those estimated from the coupling constant, which was separately determined. The result may be explained by assuming initial excitation of the resonator.

cond-mat.supr-con↗
arXiv2013

Specific Heat and Upper Critical Field of Sc5Ir4Si10 Superconductor

Temperature and magnetic field dependent specific heat of textured Sc5Ir4Si10 superconductor was investigated in detail. Based on the fitting of zero-field electronic specific heat by different gap structures as well as the discussion on field-induced specific heat coefficient, Sc5Ir4Si10 is proved to be an anisotropic s-wave superconductor with a gap anisotropy of 0.53. The anisotropy of upper critical field suggests the weakly one-dimensional Fermi surface, and the value is consistent with the gap anisotropy result obtained from the specific heat data.

cond-mat.supr-con↗
arXiv2013

Robust spin fluctuations and $s\pm$ pairing in the heavily electron doped iron-based superconductors

We theoretically study the spin fluctuation and superconductivity in La1111 and Sm1111 iron-based superconductors for a wide range of electron doping. When we take into account the band structure variation by electron doping, the hole Fermi surface originating from the $d_{X^2-Y^2}$ orbital turns out to be robust against electron doping, and this gives rise to large spin fluctuations and consequently $s\pm$ pairing even in the heavily doped regime. The stable hole Fermi surface is larger for Sm1111 than for La1111, which can be considered as the origin of the apparent difference in the phase diagram.

cond-mat.supr-con↗
arXiv2013

Multi-Orbital Superconductivity in SrTiO3 /LaAlO3 Interface and SrTiO3 Surface

We investigate the superconductivity in two-dimensional electron systems formed in SrTiO3 nanostructures. Our the- oretical analysis is based on the three-orbital model, which takes into account t2g orbitals of Ti ions. Because of the interfacial breaking of mirror symmetry, a Rashba-type antisymmetric spin-orbit coupling arises from the cooperation of intersite and interorbital hybridyzation and atomic LS coupling. This model shows a characteristic spin texture and carrier density dependence of Rashba spin-orbit coupling through the orbital degree of freedom. Superconductivity is mainly caused by heavy quasiparticles consisting of dyz and dzx orbitals at high carrier densities. We find that the Rashba spin-orbit coupling stabilizes a quasi-one-dimensional superconducting phase caused by one of the dyz or dzx orbitals at high magnetic fields along interfaces. This quasi-one-dimensional superconducting phase is protected against para- magnetic depairing effects by the Rashba spin-orbit coupling and realizes a large upper critical field Hc2 beyond the Pauli-Clogston-Chandrasekhar limit. This finding is consistent with an extraordinarily large upper critical field observed in SrTiO3 /LaAlO3 interfaces and its carrier density dependence. The possible coexistence of superconductivity and fer- romagnetism in SrTiO3 /LaAlO3 interfaces may also be attributed to this quasi-one-dimensional superconducting phase.

cond-mat.supr-con↗
arXiv2013

Spin Susceptibility in Non-centrosymmetric Superconductors with Topological Transition of Fermi Surfaces

The non-centrosymmetric superconductors Li2Pd3B and Li2Pt3B show different superconducting properties despite having the same crystal symmetry. Motivated by experimental results, we investigate the spin susceptibility of non-centrosymmetric superconductors accompanied by the topological transition of Fermi surfaces due to antisymmetric spin-orbit coupling, which is indicated by the first-principles band structure calculation for Li2Pt3B. We study three types of topological transition, namely, (A) the disappearance of the Fermi surface, (B) crossing the Dirac point, and (C) crossing the saddle point van-Hove singularity. The spin susceptibility in the superconducting state is increased by the topological transitions (A) and (C), while it is decreased by (B). We discuss the unusual magnetic properties observed in Li2Pt3B on the basis of these results.

cond-mat.supr-con↗
arXiv2013

Suppression of Structural Phase Transition in IrTe2 by Isovalent Rh Doping

We report that the isovalent Rh doping of IrTe2 suppresses the structural/electronic phase transition and induces superconductivity at 2.6 K. The doping level of Rh that is necessary for suppressing the transition is three times higher than those of Pt and Pd. Ir1-xRhxTe2 might provide us with a unique opportunity to study the origin of the structural/electronic phase transition in IrTe2.

cond-mat.supr-con↗
arXiv2013

Electronic structure reconstruction by orbital symmetry breaking in IrTe2

We report an angle-resolved photoemission spectroscopy (ARPES) study on IrTe2 which exhibits an interesting lattice distortion below 270 K and becomes triangular lattice superconductors by suppressing the distortion via chemical substitution or intercalation. ARPES results at 300 K show multi-band Fermi surfaces with six-fold symmetry which are basically consistent with band structure calculations. At 20 K in the distorted phase, whereas the flower shape of the outermost Fermi surface does not change from that at 300 K, topology of the inner Fermi surfaces is strongly modified by the lattice distortion. The Fermi surface reconstruction by the distortion depends on the orbital character of the Fermi surfaces, suggesting importance of Ir 5d and/or Te 5p orbital symmetry breaking.

cond-mat.supr-con↗
arXiv2013

Zigzag chain structure transition and orbital fluctuations in Ni-based superconductors

We investigate the electronic state and structure transition of BaNi2As2, which shows a similar superconducting phase diagram as Fe-based superconductors. We construct the ten-orbital tight-binding model for BaNi2As2 by using the maximally localized Wannier function method. The Coulomb and quadrupole-quadrupole interactions are treated within the random-phase approximation. We obtain the strong developments of charge quadrupole susceptibilities driven by the in-plane and out-of-plane oscillations of Ni ions. The largest susceptibility is either O_{X^2-Y^2}-quadrupole susceptibility at q = (pi, 0, pi) or O_{XZ(YZ)}-quadrupole susceptibility at q = (pi, pi, pi), depending on the level splitting between d_{X^2-Y^2} and d_{XZ(YZ)}. These antiferro-quadrupole fluctuations would then be the origin of the strong coupling superconductivity in Ni-based superconductors. Also, we propose that the antiferro-quadrupole O_{X^2-Y^2} order with q = (pi, 0, pi) is the origin of the zigzag chain structure reported in experiments. We identify similarities and differences between Ni- and Fe-based superconductors.

cond-mat.supr-con↗
arXiv2013

Topological Insulator Materials

Topological insulators represent a new quantum state of matter which is characterized by peculiar edge or surface states that show up due to a topological character of the bulk wave functions. This review presents a pedagogical account on topological insulator materials with an emphasis on basic theory and materials properties. After presenting a historical perspective and basic theories of topological insulators, it discusses all the topological insulator materials discovered as of May 2013, with some illustrative descriptions of the developments in materials discoveries in which the author was involved. A summary is given for possible ways to confirm the topological nature in a candidate material. Various synthesis techniques as well as the defect chemistry that are important for realizing bulk-insulating samples are discussed. Characteristic properties of topological insulators are discussed with an emphasis on transport properties. In particular, the Dirac fermion physics and the resulting peculiar quantum oscillation patterns are discussed in detail. It is emphasized that proper analyses of quantum oscillations make it possible to unambiguously identify surface Dirac fermions through transport measurements. The prospects of topological insulator materials for elucidating novel quantum phenomena that await discovery conclude the review.

cond-mat.mtrl-sci↗
arXiv2013

Pressure-Induced Superconductivity in Mineral Calaverite AuTe2

The pressure dependences of resistivity and ac susceptibility have been measured in the mineral calaverite AuTe$_2$. Resistivity clearly shows a first-order phase transition into a high-pressure phase, consistent with the results of a previous structural analysis. We found zero resistivity and a diamagnetic shielding signal at low temperatures in the high-pressure phase, which clearly indicates the appearance of superconductivity. Our experimental results suggest that bulk superconductivity appears only in the high-pressure phase. For AuTe$_2$, the highest superconducting transition temperature under pressure is $T_{\rm c}$ = 2.3 K at 2.34 GPa; it was $T_{\rm c}$ = 4.0 K for Pt-doped (Au$_{0.65}$Pt$_{0.35}$)Te$_2$. The difference in $T_{\rm c}$ between the two systems is discussed on the basis of the results obtained using the band calculations and McMillan's formula.

cond-mat.supr-con↗
arXiv2013

Bulk Superconductivity in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ Induced by Annealing in Se and S Vapor

We reported that bulk superconductivity in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ can be induced by annealing in Se and S vapor. The well-annealed samples show a large critical current density, emph{J}$_c$ $\sim$ 2 - 4 $\times$ 10$^5$ A/cm$^2$. Combined with our previous reports about O$_2$ and Te annealing effects, we proved that bulk superconductivity can be successfully induced in Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ by annealing in atmosphere of all chalcogens, such as O, S, Se and Te.

cond-mat.supr-con↗