We present measurements of the low temperature specific heat of single crystals of kappa-(ET)_2Cu[N(CN)_2]Br as a function of the cooling rate through the glasslike structure transition at $\sim$ 80K. We find that rapid cooling produces a small (< 4%) decrease in the superconducting transition temperature accompanied by a substantial (up to 50%) decrease in the normal-state electronic specific heat. A natural explanation of our data is that there is a macroscopic phase separation between superconducting and insulating regions in rapidly cooled samples.
Extension of the Brinkman-Rice picture and the Mott transition
In order to explain the metal-Mott-insulator transition, the Brinkman-Rice (BR) picture is extended. In the case of less than one as well as one electron per atom, the on-site Coulomb repulsion is given by U={kappa}{rho}^2U_c by averaging the electron charge per atom over all atomic sites, where {kappa} is the correlation strength of U, {rho} is the band filling factor, and U_c is the critical on-site Coulomb energy. The effective mass of a quasiparticle is found to be m*/m=1/{1-{kappa}^2{rho}^4} for 0<{kappa}{rho}^2<1 and seems to follow the heat capacity data of Sr_{1-x} La_xTiO_3 and YBa_2C…
Mott Transition from a Spin Liquid to a Fermi Liquid in the Spin-Frustrated Organic Conductor kappa-(ET)2Cu2(CN)3
用$^1$ H NMR和电阻率测量研究了三角晶格自旋液体模型体系$κ$ -（ET）$_2$ Cu $_2$（CN）$_3$的压温相图.在压力下，自旋液相在莫特转变为金属或超导相之前是持久的。在Mott转变时，自旋涨落被迅速抑制，自旋-晶格弛豫速率和电阻率随温度的变化具有费米液体特征。相图中Mott边界的特征曲率反映了自旋阻挫对Mott相变的重要影响。
In the most studied family of organic superconductors kappa-(BEDT-TTF)_2X, the BEDT-TTF molecules that make up the conducting planes are coupled as dimers. For some anions X, an antiferromagnetic insulator is found at low temperatures adjacent to superconductivity. With an average of one hole carrier per dimer, the BEDT-TTF band is effectively 1/2-filled. Numerous theories have suggested that fluctuations of the magnetic order can drive superconducting pairing in these models, even as direct calculations of superconducting pairing in monomer 1/2-filled band models find no superconductivity. He…
In this work, we investigate the thermoelectric properties of a hybrid junction realised coupling surface states of a three-dimensional topological insulator with a conventional $s$-wave superconductor. We focus on the ballistic devices and study the quasiparticle flow, carrying both electric and thermal currents, adopting a scattering matrix approach based on conventional Blonder-Tinkham-Klapwijk formalism. We calculate the cooling efficiency of the junction as a function of the microscopic parameters of the normal region (i.e. the chemical potential etc.). The cooling power increases when mo…
Electronic cooling in hybrid normal metal-insulator-superconductor junctions is a promising technology for the manipulation of thermal loads in solid state nanosystems. One of the main bottlenecks for efficient electronic cooling is the electron-phonon coupling, as it represents a thermal leakage channel to the phonon bath. Graphene is a two-dimensional material that exhibits a weaker electron-phonon coupling compared to standard metals. For this reason, we study the electron cooling in graphene-based systems consisting of a graphene sheet contacted by two insulator/superconductor junctions. W…
In a normal-metal/insulator/superconductor (NIS) tunnel junction refrigerator, the normal-metal electrons are cooled and the dissipated power heats the superconducting electrode. This paper presents a review of the mechanisms by which heat leaves the superconductor and introduces overlayer quasiparticle traps for more effective heatsinking. A comprehensive thermal model is presented that accounts for the described physics, including the behavior of athermal phonons generated by both quasiparticle recombination and trapped quasiparticles. We compare the model to measurements of a large area (>4…
The Sb2Te3 crystals are grown using the conventional self flux method via solid state reaction route, by melting constituent elements (Sb and Te) at high temperature (850 C), followed by slow cooling (2 C per hour).The weak anti localization (WAL) related low field (2 Tesla) magneto-conductance at low temperatures (2.5 K and 20 K) has been analysed and discussed using the Hikami- Larkin - Nagaoka (HLN) model. Summarily, the short letter reports an easy and versatile method for crystal growth of bulk Sb2Te3 topological insulator (TI) and its brief physical property characterization.
We have successfully fabricated micron-scale Cu-AlO$_{x} $-Al-NbN normal metal-insulator-superconductor (NIS) tunnel junction devices, using pulsed laser deposition (PLD) for NbN film growth, and electron-beam lithography and shadow evaporation for the final device fabrication. The subgap conductance of these devices exhibit a strong temperature dependence, rendering them suitable for thermometry from $\sim$ 0.1 K all the way up to the superconducting transition temperature of the NbN layer, which was here $\sim 11$ K, but could be extended up to $\sim 16$ K in our PLD chamber. Our data fits w…
We consider a model NISIN system with two junctions in series, where N is a normal metal, S is a superconductor and I is an insulator. We assume that the resistance of the first junction is high, while the resistance of the second one is low. In this case the first junction cools the left normal electrode, while the second junction partially removes excited quasiparticles from the superconductor. We consider cooling properties of this double junction structure. It is shown that the cooling power depends strongly on the ratio of the resistances of the two junctions. In conclusion, we derive a g…
We investigate electron cooling based on a clean normal-metal/spin-filter/superconductor junction. Due to the suppression of the Andreev reflection by the spin-filter effect, the cooling power of the system is found to be extremely higher than that for conventional normal-metal/nonmagnetic-insulator/superconductor coolers. Therefore we can extract large amount of heat from normal metals. Our results strongly indicate the practical usefulness of the spin-filter effect for cooling detectors, sensors, and quantum bits.
1T-TaS$_2$ undergoes successive phase transitions upon cooling and eventually enters an insulating state of mysterious origin. Some consider this state to be a band insulator with interlayer stacking order, yet others attribute it to Mott physics that support a quantum spin liquid state.Here, we determine the electronic and structural properties of 1T-TaS$_2$ using angle-resolved photoemission spectroscopy and X-Ray diffraction. At low temperatures, the 2$π$/2c-periodic band dispersion, along with half-integer-indexed diffraction peaks along the c axis, unambiguously indicates that the ground …
Observation of unconventional band topology in a superconducting doped topological insulator, Cux-Bi2Se3: Topological Superconductor or non-Abelian superconductor?
最近发现掺铜的拓扑绝缘体Bi $_2$ Se $_3$在冷却时经历超导转变，这提高了它是第一个已知的“拓扑超导体”或实现新的非阿贝尔超导状态的可能性。它的真正性质取决于关键的体和表面状态带拓扑结构。我们提出了第一个光电子能谱的结果，通过检查在许多不同的铜掺杂值的能带拓扑，我们发现，拓扑保护的自旋螺旋表面状态保持良好的保护和分离的费米能级，其中铜配对发生在最佳掺杂的拓扑绝缘体的体狄拉克带。铜的加入导致非线性电子掺杂和强重整化的拓扑表面态。这些极不寻常的观测结果强烈地表明，在Cu $_x$ Bi $_2$ Se $_3$的拓扑表面上的超导性不能是任何常规类型的一般拓扑理论。首次报道了三维体狄拉克能带结构的超导掺杂态和拓扑不变性质，这将有助于为这种新型超导体制定一个具体的理论。
3D non-abelian anyons have been theoretically proposed to exist in heterostructures composed of type II superconductors and topological insulators. We use realistic material parameters for a device derived from Bi$_2$Se$_3$ to quantitatively predict the temperature and magnetic field regimes where an experiment might detect the presence of these exotic states by means of a cooling effect. Within the appropriate parameter regime, an adiabatic increase of the magnetic field will result in a decrease of system temperature when anyons are present. If anyons are not present, the same experiment wou…
We report the development of superconducting tantalum nitride (TaN$_{x} $) normal metal-insulator-superconductor (NIS) tunnel junctions. For the insulating barrier, we used both AlO$_{x}$ and TaO$_{x}$ (Cu-AlO$_{x}$-Al-TaN$_{x} $ and Cu-TaO$_{x}$-TaN$_{x} $), with both devices exhibiting temperature dependent current-voltage characteristics which follow the simple one-particle tunneling model. The superconducting gap follows a BCS type temperature dependence, rendering these devices suitable for sensitive thermometry and bolometry from the superconducting transition temperature $T_{\text{C}}$ …
Heat management and refrigeration are key concepts for nanoscale devices operating at cryogenic temperatures. The design of an on-chip mesoscopic refrigerator that works thanks to the input heat is presented, thus realizing a solid state implementation of the concept of cooling by heating. The system consists of a circuit featuring a thermoelectric element based on a ferromagnetic insulator-superconductor tunnel junction (N-FI-S) and a series of two normal metal-superconductor tunnel junctions (SINIS). The N-FI-S element converts the incoming heat in a thermovoltage, which is applied to the SI…
We have investigated the effect of a difference in the tunnelling resistances of the individual normal metal-insulator-superconductor (NIS) tunnel junctions in a double junction SINIS device, with particular emphasis on the impact on the conductance, cooling and thermometric performance. By solving the electrical and thermal equations of the junctions in a self-consistent way, we find that asymmetry gives rise to many new features, such as appearance of an excess sub-gap current, improved cooling performance, exhibition of negative differential resistance, and improved temperature range of the…
Comparison of the phase diagram of the half-filled layered organic superconductors with the phase diagram of the RVB theory of the Hubbard-Heisenberg model
We have successfully fabricated Cu/AlOx-Al/Nb normal-metal/insulator/superconductor tunnel junction devices with a high value of the superconducting gap (up to $\sim 1$ mV), using electron-beam lithography and angle evaporation techniques in the sub-micron scale. The subgap conductance of these junctions shows the expected strong temperature dependence, rendering them suitable for thermometry all the way from 100 mK to 6 K. In addition, some direct electronic cooling of the normal metal was also seen at bias values near the gap edge. The device performance was strongly influenced by the detail…
Efficient electron-refrigeration based on a normal-metal/spin-filter/superconductor junction is proposed and demonstrated theoretically. The spin-filtering effect leads to values of the cooling power much higher than in conventional normal-metal/nonmagnetic-insulator/superconductor coolers and allows for an efficient extraction of heat from the normal metal. We demonstrate that highly efficient cooling can be realized in both ballistic and diffusive multi-channel junctions in which the reduction of the electron temperature from 300 mK to around 50 mK can be achieved. Our results indicate the p…
We report synthesis, structural details and electrical transport properties of topological insulator Bi2Te3. The single crystalline specimens of Bi2Te3 are obtained from high temperature (950C) melt and slow cooling (2C/hour). The resultant crystals were shiny, one piece (few cm) and of bright silver color. The Bi2Te3 crystal is found to be perfect with clear [00l] alignment. The powder XRD pattern being carried out on crushed crystals showed that Bi2Te3 crystallized in R3m symmetry with a = b = 4.3866(2) A, c = 30.4978(13) A and Gamma = 120degree. The Bi position is refined to (0, 0, 0.4038 (…
We investigate heat and charge transport through a diffusive SIF1F2N tunnel junction, where N (S) is a normal (superconducting) electrode, I is an insulator layer and F1,2 are two ferromagnets with arbitrary direction of magnetization. The flow of an electric current in such structures at subgap bias is accompanied by a heat transfer from the normal metal into the superconductor, which enables refrigeration of electrons in the normal metal. We demonstrate that the refrigeration efficiency depends on the strength of the ferromagnetic exchange field h and the angle α between the magnetizations o…
Many complex electronic systems exhibit so-called pseudogaps, which are poorly-understood suppression of low-energy spectral intensity in the absence of an obvious gap-inducing symmetry. Here we investigate the superconductor $Ba_{1-x}K_{x}BiO_{3}$ near optimal doping, where unconventional transport behavior and evidence of pseudogap(s) have been observed above the superconducting transition temperature $T_{c}$, and near an insulating phase with long-range lattice distortions. Angle-resolved photoemission spectroscopy (ARPES) reveals a dispersive band with vanishing quasiparticle weight and "t…
We present inelastic neutron scattering results of phonons in (Pb$_{0.5}$Sn$_{0.5}$)$_{1-x}$In$_x$Te powders, with $x=0$ and 0.3. The $x=0$ sample is a topological crystalline insulator, and the $x=0.3$ sample is a superconductor with a bulk superconducting transition temperature $T_c$ of 4.7 K. In both samples, we observe unexpected van Hove singularities in the phonon density of states at energies of 1--2.5 meV, suggestive of local modes. On cooling the superconducting sample through $T_c$, there is an enhancement of these features for energies below twice the superconducting-gap energy. We …
Demonstration of a scalable all-solid-state refrigerator exploiting diffusion geometries and limiting interfacial conductances at temperatures below 1 kelvin
