arXiv subjects

Huan Yang

Publications and source records attributed to Huan Yang.

At least 361 records · Page 20Linked to original sources

arXiv2010

Observation of Lattice and Andreev Bound States of Vortices in Ba0.6K0.4Fe2As2 Single Crystals with Scanning Tunneling Microscopy/Spectroscopy

For a type-II superconductor, when the applied magnetic field is higher than the lower critical value Hc1, the magnetic flux will penetrate into the superconductor and form quantized vortices, which usually are arranged in an Abrikosov lattice. For the newly discovered iron pnictide superconductors, previous measurements have shown that, in electron-doped BaFe2As2, the vortices form a highly disordered structure. In addition, the density of states (DOS) within the vortex cores1 do not exhibit the Andreev bound states in conventional superconductors. In this Letter, we report the observation of a triangular vortex lattice and the Andreev bound states in hole-doped BaFe2As2 by using a low temperature scanning tunneling microscope (STM). Detailed study of the vortex cores reveals that the spectrum of the Andreev bound states inside the vortex core exhibits a distinct spatial evolution: at the center of the vortex core, it appears as a single peak at 0.5 mV below the Fermi-energy; away from the core center, it gradually evolves into two sub-peaks and they eventually fade out. The drastic differences between the vortex cores of the electron-doped and hole-doped counterparts are illusive to the pairing mechanism of the iron pnictide superconductors.

cond-mat.supr-con↗

arXiv2010

Preparing a mechanical oscillator in non-Gaussian quantum states

We propose a protocol for coherently transferring non-Gaussian quantum states from optical field to a mechanical oscillator. The open quantum dynamics and continuous-measurement process, which can not be treated by the stochastic-master-equation formalism, are studied by a new path-integral-based approach. We obtain an elegant relation between the quantum state of the mechanical oscillator and that of the optical field, which is valid for general linear quantum dynamics. We demonstrate the experimental feasibility of such protocol by considering the cases of both large-scale gravitational-wave detectors and small-scale cavity-assisted optomechanical devices.

quant-ph↗

arXiv2009

Nernst effect and dimensionality in the quantum limit

Nernst effect, the transverse voltage generated by a longitudinal thermal gradient in presence of magnetic field has recently emerged as a very sensitive, yet poorly understood, probe of electron organization in solids. Here we report on an experiment on graphite, a macroscopic stack of graphene layers, which establishes a fundamental link between dimensionality of an electronic system and its Nernst response. In sharp contrast with single-layer graphene, the Nernst signal sharply peaks whenever a Landau level meets the Fermi level. This points to the degrees of freedom provided by finite interlayer coupling as a source of enhanced thermoelectric response in the vicinity of the quantum limit. Since Landau quantization slices a three-dimensional Fermi surface, each intersection of a Landau level with the Fermi level modifies the Fermi surface topology. According to our results, the most prominent signature of such a topological phase transition emerges in the transverse thermoelectric response.

cond-mat.str-el↗

arXiv2009

Specific-Heat Measurement of Residual Superconductivity in the Normal State of Underdoped Cuprate Superconductors

We have measured the magnetic field and temperature dependence of specific heat on $Bi_2Sr_{2-x}La_xCuO_{6+δ}$ single crystals in wide doping and temperature regions. The superconductivity related specific heat coefficient $γ_{sc}$ and entropy $S_{sc}$ are determined. It is found that $γ_{sc}$ has a hump-like anomaly at $T_c$ and behaves as a long tail which persists far into the normal state for the underdoped samples, but for the heavily overdoped samples the anomaly ends sharply just near $T_c$. Interestingly, we found that the entropy associated with superconductivity is roughly conserved when and only the long tail part in the normal state is taken into account for the underdoped samples, indicating the residual superconductivity above T$_c$.

cond-mat.supr-con↗

arXiv2009

Hall plateaus at magic angles in bismuth beyond the quantum limit

We present a study of the angular dependence of the resistivity tensor up to 35 T in elemental bismuth complemented by torque magnetometry measurements in a similar configuration. For at least two particular field orientations a few degrees off the trigonal axis, the Hall resistivity was found to become field-independent within experimental resolution in a finite field window corresponding to a field which is roughly three times the frequency of quantum oscillations. The Hall plateaus rapidly vanish as the field is tilted off theses magic angles. We identify two distinct particularities of these specific orientations, which may play a role in the emergence of the Hall plateaus.

cond-mat.str-el↗

arXiv2008

Physical properties of noncentrosymmetric superconductor Ru$_7$B$_3$

Transition metal boride Ru$_7$B$_3$ was found to be a noncentrosymmetric superconductor with $T_{C}$ equal to 3.3 K. Superconducting and normal state properties of Ru$_7$B$_3$ were determined by a self-consistent analysis through resistivity($ρ_{xx}$ and $ρ_{xy}$), specific heat, lower critical field measurement and electronic band structure calculation. It is found that Ru$_7$B$_3$ belongs to an s-wave dominated single band superconductor with energy gap 0.5 meV and could be categorized into type II superconductor with weak electron-phonon coupling. Unusual 'kink' feature is clearly observed in field-broadening resistivity curves, suggesting the possible mixture of spin triplet induced by the lattice without inversion symmetry.

cond-mat.supr-con↗

arXiv2008

Growth and characterization of A_{1-x}K_xFe_2As_2 (A = Ba, Sr) single crystals with x=0 - 0.4

Single crystals of A$_{1-x}$K$_x$Fe$_2$As$_2$ (A=Ba, Sr) with high quality have been grown successfully by FeAs self-flux method. The samples have sizes up to 4 mm with flat and shiny surfaces. The X-ray diffraction patterns suggest that they have high crystalline quality and c-axis orientation. The non-superconducting crystals show a spin-density-wave (SDW) instability at about 173 K and 135 K for Sr-based and Ba-based compound, respectively. After doping K as the hole dopant into the BaFe$_2$As$_2$ system, the SDW transition is smeared, and superconducting samples with the compound of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (0 $< x \leqslant$ 0.4) are obtained. The superconductors characterized by AC susceptibility and resistivity measurements exhibit very sharp superconducting transition at about 36 K, 32 K, 27 K and 23 K for x= 0.40,0.28,0.25 and 0.23, respectively.

cond-mat.supr-con↗

arXiv2008

Fishtail effect and the vortex phase diagram of single crystal Ba0.6K0.4Fe2As2

By measuring the magnetization hysteresis loops of superconducting Ba0.6K0.4Fe2As2 single crystals, we obtained the high upper critical field and large current carrying ability, which point to optimistic applications. The fishtail (or second peak) effect is also found in the material, and the position of the vortex pinning force shows a maximum at 1/3 of the reduced field, being consistent with the picture of vortex pinning by small size normal cores in the sample. Together with the resistive measurements, for the first time the vortex phase diagram is obtained for superconductor Ba0.6K0.4Fe2As2.

cond-mat.supr-con↗

arXiv2008

Magnetization Relaxation and Collective Vortex Pinning in the Fe-Based Superconductor SmFeAsO_0.9F_0.1

By measuring the dynamic and traditional magnetization relaxations we investigate the vortex dynamics of the newly discovered superconductor SmFeAsO_0.9F_0.1 with Tc = 55K. It is found that the relaxation rate is rather large reflecting a small characteristic pinning energy. Moreover it shows a weak temperature dependence in wide temperature region, which resembles the behavior of the cuprate superconductors. Combining with the resistive data under different magnetic fields, a vortex phase diagram is obtained. Our results strongly suggest that the model of collective vortex pinning applies to this new superconductor very well.

cond-mat.supr-con↗

arXiv2008

Hall effect and magnetoresistance in single crystals of NdFeAsO$_{1-x}$F$_{x}$

Hall effect and magnetoresistance have been measured on single crystals of $NdFeAsO_{1-x}F_{x}$ with x = 0 ($T_c$ = 0 $ $K) and x = 0.18 ($T_c$ = 50 $ $K). For the undoped samples, strong Hall effect and magnetoresistance with strong temperature dependence were found below about 150 K. The magnetoresistance was found to be as large as 30% at 15 K at a magnetic field of 9 T. From the transport data we found that the transition near 155 K was accomplished in two steps: first one occurs at 155 K which may be associated with the structural transition, the second one takes place at about 140 K which may correspond to the spin-density wave like transition. In the superconducting sample with $T_c$ = 50 $ $K, it is found that the Hall coefficient also reveals a strong temperature dependence with a negative sign. But the magnetoresistance becomes very weak and does not satisfy the Kohler's scaling law. These dilemmatic results (strong Hall effect and very weak magnetoresistance) prevent to understand the normal state electric conduction by a simple multi-band model by taking account the electron and hole pockets. Detailed analysis further indicates that the strong temperature dependence of $R_H$ cannot be easily understood with the simple multi-band model either. A picture concerning a suppression to the density of states at the Fermi energy in lowering temperature is more reasonable. A comparison between the Hall coefficient of the undoped sample and the superconducting sample suggests that the doping may remove the nesting condition for the formation of the SDW order, since both samples have very similar temperature dependence above 175 K.

cond-mat.supr-con↗

arXiv2008

Fully Band Resolved Scattering Rate in MgB2 Revealed by Nonlinear Hall Effect and Magnetoresistance Measurements

We have measured the normal state temperature dependence of the Hall effect and magnetoresistance in epitaxial MgB2 thin films with variable disorders characterized by the residual resistance ratio RRR ranging from 4.0 to 33.3. A strong nonlinearity of the Hall effect and magnetoresistance have been found in clean samples, and they decrease gradually with the increase of disorders or temperature. By fitting the data to the theoretical model based on the Boltzmann equation and ab initio calculations for a four-band system, for the first time, we derived the scattering rates of these four bands at different temperatures and magnitude of disorders. Our method provides a unique way to derive these important parameters in multiband systems.

cond-mat.supr-con↗

arXiv2008

Evidence for Two Gaps and Breakdown of the Uemura Plot in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Single Crystals

We report a detailed investigation on the lower critical field $H_{c1}$ of the superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ (FeAs-122) single crystals. A pronounced kink is observed on the $H_{c1}(T)$ curve, which is attributed to the existence of two superconducting gaps. By fitting the data $H_{c1}(T)$ to the two-gap BCS model in full temperature region, a small gap of $Δ_a(0)=2.0\pm 0.3$ meV and a large gap of $Δ_b(0)=8.9\pm 0.4$ meV are obtained. The in-plane penetration depth $λ_{ab}(0)$ is estimated to be 105 nm corresponding to a rather large superfluid density, which points to the breakdown of the Uemura plot in FeAs-122 superconductors.

cond-mat.supr-con↗

arXiv2008

Angular dependence of resistivity in the superconducting state of NdFeAsO$_{0.82}$F$_{0.18}$ single crystals

We report the results of angle dependent resistivity of NdFeAsO$_{0.82}$F$_{0.18}$ single crystals in the superconducting state. By doing the scaling of resistivity within the frame of the anisotropic Ginzburg-Landau theory, it is found that the angle dependent resistivity measured under different magnetic fields at a certain temperature can be collapsed onto one curve. As a scaling parameter, the anisotropy $Γ$ can be determined for different temperatures. It is found that $Γ(T)$ increases slowly with decreasing temperature, varying from $Γ\simeq$ 5.48 at T=50 K to $Γ\simeq$ 6.24 at T=44 K. This temperature dependence can be understood within the picture of multi-band superconductivity.

cond-mat.supr-con↗

arXiv2008

Superconducting properties of SmO1-xFxFeAs wires with Tc = 52 K prepared by the powder-in-tube method

We demonstrate that Ta sheathed SmO1-xFxFeAs wires were successfully fabricated by the powder-in-tube (PIT) method for the first time. Structural analysis by mean of x-ray diffraction shows that the main phase of SmO1-xFxFeAs was obtained by this synthesis method. The transition temperature of the SmO0.65F0.35FeAs wires was confirmed to be as high as 52 K. Based on magnetization measurements, it is found that a globe current can flow on macroscopic sample dimensions with Jc of ~3.9x10^3 A/cm^2 at 5 K and self field, while a high Jc about 2x10^5 A/cm^2 is observed within the grains, suggesting that a significant improvement in the globle Jc is possible. It should be noted that the Jc exhibits a very weak field dependence behavior. Furthermore, the upper critical fields (Hc2) determined according to the Werthamer-Helfand-Hohenberg formula are (T= 0 K) = 120 T, indicating a very encouraging application of the new superconductors.

cond-mat.supr-con↗

arXiv2008

Doping Dependence of Superconductivity and Lattice Constants in Hole Doped $La_{1-x}Sr_xFeAsO$

By using solid state reaction method we have fabricated the hole doped $La_{1-x}Sr_xFeAsO$ superconductors with Sr content up to 0.13. It is found that the sharp anomaly at about 150 K and the low temperature upturn of resistivity are suppressed by doping holes into the parent phase. Interestingly both the superconducting transition temperature $T_c$ and the lattice constants (a-axis and c-axis) increase monotonously with hole concentration, in sharp contrast with the electron doped side where the $T_c$ increases with a continuing shrinkage of the lattice constants either by dope more fluorine or oxygen vacancies into the system. Our data clearly illustrate that the superconductivity can be induced by doping holes via substituting the trivalent La with divalent Sr in the LaFeAsO system with single FeAs layer, and the $T_c$ in the present system exhibits a symmetric behavior at the electron and hole doped sides, as we reported previously.

cond-mat.supr-con↗

arXiv2008

Critical Fields and Anisotropy of NdO0.82F0.18FeAs Single Crystals

The newly discovered iron-based superconductors have stimulated enormous interests in the field of superconductivity. Since the new superconductor is a layered system, the anisotropy is a parameter with the first priority to know. Meanwhile any relevant message about the critical fields (upper critical field and irreversibility line) are essentially important. By using flux method, we have successfully grown the single crystals NdO0.82F0.18FeAs at ambient pressure. Resistive measurements reveal a surprising discovery that the anisotropy Γ= (mc/mab)^{1/2} is below 5, which is much smaller than the theoretically calculated results. The data measured up to 400 K show a continuing curved feature which prevents a conjectured linear behavior for an unconventional metal. The upper critical fields determined based on the Werthamer-Helfand-Hohenberg formula are H_{c2}^{H||ab}(T=0 K) = 304 T and H_{c2}^{H||c}(T=0 K)=62-70 T, indicating a very encouraging application of the new superconductors.

cond-mat.supr-con↗

arXiv2008

Linear Temperature Dependence of the Lower Critical Field $H_{c1}$ in F-Doped LaOFeAs Superconductors

We present the first experimental results of the lower critical field $H_{c1}$ of the newly discovered F-doped superconductor LaO$_{0.9}$F$_{0.1}$FeAs (F-LaOFeAs) by global and local magnetization measurements. It is found that $H_{c1}$ showed an clear linear-$T$ dependence down to a temperature of 2 K, indicative of an unconventional pairing symmetry with a nodal gap function. Based on the d-wave model, we estimated a maximum gap value $Δ_0=4.0 \pm 0.6$ meV, in consistent with the recent specific heat and point-contact tunneling measurements. Taking the demagnetization factor into account, the absolute value of $H_{c1}(0)$ is determined to be about 54 Oe, manifesting a low superfluid density for LaO$_{0.9}$F$_{0.1}$FeAs.

cond-mat.supr-con↗

arXiv2008

Superconductivity at 36 K in Gadolinium-arsenide Oxides GdO$_{1-x}$F$_{x}$FeAs

In this paper we report the fabrication and superconducting properties of GdO$_{1-x}$F$_{x}$FeAs. It is found that when x is equal to 0.17, GdO$_{0.83}$F$_{0.17}$FeAs is a superconductor with the onset transition temperature T$_{c}^{on}\approx$ 36.6K. Resistivity anomaly near 130K was observed for all samples up to x = 0.17, such a phenomenon is similar to that of LaO$_{1-x}$F$_{x}$FeAs. Hall coefficient indicates that GdO$_{0.83}$F$_{0.17}$FeAs is conducted by electron-like charge carriers.

cond-mat.supr-con↗