arXiv subjects

Hai-Hu Wen

Publications and source records attributed to Hai-Hu Wen.

At least 109 records · Page 6Linked to original sources

arXiv2018

Vortex lattice and vortex bound states in CsFe$_2$As$_2$ investigated by scanning tunneling microscopy/spectroscopy

We investigate the vortex lattice and vortex bound states in CsFe$_2$As$_2$ single crystals by scanning tunneling microscopy/spectroscopy (STM/STS) under various magnetic fields. A possible structural transition or crossover of vortex lattice is observed with the increase of magnetic field, i.e., the vortex lattice changes from a distorted hexagonal lattice to a distorted tetragonal one at the magnetic field near 0.5 T. It is found that a mixture of stripelike hexagonal and square vortex lattices emerges in the crossover region. The vortex bound state is also observed in the vortex center. The tunneling spectra crossing a vortex show that the bound-state peak position holds near zero bias with STM tip moving away from the vortex core center. The Fermi energy estimated from the vortex bound state energy is very small. Our investigations provide experimental information to both the vortex lattice and the vortex bound states in this iron-based superconductor.

cond-mat.supr-con↗

arXiv2018

Discovery of a new nontoxic cuprate superconducting system Ga-Ba-Ca-Cu-O

Superconductivity is observed in a new nontoxic cuprate system Ga-Ba-Ca-Cu-O, with Tc = 82K for GaBa2Ca5Cu6O14+δ (Ga-1256) and Tc = 116K probably for GaBa2Ca3Cu4O10+δ (Ga-1234) or GaBa2Ca2Cu3O8+δ (Ga-1223), respectively. All compounds are fabricated by solid state reaction method under high pressure and high temperature. Samples are characterized by resistivity, magnetization and X-ray diffraction (XRD) measurements. The temperature dependence of magnetization measured in both zero-field-cooled and field-cooled processes on one sample (S1) shows two superconducting transitions at about 82K and 113K. The estimated shielding fraction for the phase with Tc of 82K is about 67%, while the fraction for another phase with Tc of 113K is quite small. The XRD Rietveld refinement for S1 indicates two main phases existing in the sample, Ga-1256 with fraction of about 58% and non-superconducting Ca0.85CuO2 with fraction of about 42%, respectively. Thus, we can conclude the superconducting phase with transition temperature of 82K is due to Ga-1256. The resistivity measurement also confirms the superconductivity for S1, and the resistivity reaches zero at about 82K. The temperature dependence of magnetization for another sample (S2) shows much higher superconducting shielding fraction for the phase with Tc of 116K, which may be a promising prospective for the synthesis of Ga-1234 or Ga-1223 phase.

cond-mat.supr-con↗

arXiv2018

Unprecedented High Irreversibility Line in Nontoxic Cuprate Superconductor (Cu,C)Ba2Ca3Cu4O11+delta

One of the key factors limiting the high power applications for a type-II superconductor is the irreversibility line Hirr(T) which reflects the very boundary of resistive dissipation in the phase diagram of magnetic field versus temperature. In cuprate family, the Y-, Bi-, Hg- and Tl-based systems have superconducting transition temperatures exceeding the liquid nitrogen boiling temperature (~77K). However, the toxic elements Hg and Tl in the latter two systems strongly constrain the possible applications. The Bi-based (2223) system is nontoxic, but the irreversibility magnetic field is strongly suppressed in the liquid nitrogen temperature region. For this purpose, the best perspective so far is relying on the YBa2Cu3O7 (Tc~90 K) system which is nontoxic and has a relatively high irreversibility magnetic field. Here we report the study of a nontoxic superconductor (Cu,C)Ba2Ca3Cu4O11+δ with Tc = 116K. It is found that the irreversibility magnetic field is unprecedentedly high among all superconductors and it thus provides a great potential of applications in the liquid nitrogen temperature region.

cond-mat.supr-con↗

arXiv2018

Direct Evidence of Superconductivity with Twofold Symmetry in Bi2Te3 Thin Film Deposited on FeTe0.55Se0.45

Topological superconductor is a timely and frontier topic in condensed matter physics. In superconducting state, an order parameter will be established with the basic or subsidiary symmetry of the crystalline lattice. In doped Bi2Se3 with a basic threefold symmetry, it was predicted however that superconductivity may have a twofold symmetry of odd parity. Here we report the proximity effect induced superconductivity in Bi2Te3 thin film on top of an iron-based superconductor FeTe0.55Se0.45. By using the quasiparticle interference technique, we demonstrate clear evidence of twofold symmetry of the superconducting gap. The gap minimum is along one of the main crystalline axis following the so-called Delta_4y notation. This is also accompanied by the elongated vortex shape mapped out by the density of states within the superconducting gap, with probably the Majorana mode within the vortex core. Our results reveal the direct evidence of superconductivity with odd parity in Bi2Te3 thin film.

cond-mat.supr-con↗

arXiv2018

Sign reversal superconducting gaps revealed by phase referenced quasi-particle interference of impurity induced bound states in (Li$_{1-x}$Fe$_x$)OHFe$_{1-y}$Zn$_y$Se

By measuring the spatial distribution of differential conductance near impurities on Fe sites, we have obtained the quasi-particle interference (QPI) patterns in the (Li$_{1-x}$Fe$_x$)OHFe$_{1-y}$Zn$_y$Se superconductor with only electron Fermi surfaces. By taking the Fourier transform on these patterns, we investigate the scattering features between the two circles of electron pockets formed by folding or hybridization. We treat the data by using the recent theoretical approach [arXiv:1710.09089] which is specially designed for the impurity induced bound states. It is found that the superconducting gap sign is reversed on the two electron pockets, which can be directly visualized by the phase-referenced QPI technique, indicating that the Cooper pairing is induced by the repulsive interaction. Our results further show that this method is also applicable for data measured for multiple impurities, which provides an easy and feasible way for detecting the gap function of unconventional superconductors.

cond-mat.supr-con↗

arXiv2018

Structures and Physical Properties of CsV$_2$Se$_{2-x}$O and V$_2$Se$_2$O

By using solid-state reactions, we successfully synthesize new oxyselenides CsV$_2$Se$_{2-x}$O (x = 0, 0.5). These compounds containing V$_2$O planar layers with a square lattice crystallize in the CeCr$_2$Si$_2$C structure with the space group of $P4/mmm$. Another new compound V$_2$Se$_2$O which crystallizes in space group $I4/mmm$ is fabricated by topochemical deintercalation of cesium from CsV$_2$Se$_2$O powder with iodine in tetrahydrofuran(THF). Resistivity measurements show a semiconducting behavior for CsV$_2$Se$_2$O, while a metallic behavior for CsV$_2$Se$_{1.5}$O, and an insulating feature for V$_2$Se$_2$O. A charge- or spin-density wave-like anomaly has been observed at 168 K for CsV$_2$Se$_2$O and 150 K for CsV$_2$Se$_{1.5}$O, respectively. And these anomalies are also confirmed by the magnetic susceptibility measurements. The resistivity in V$_2$Se$_2$O exhibits an anomalous log(1/$T$) temperature dependence, which is similar to the case in parent phase or very underdoped cuprates indicating the involvement of strong correlation. Magnetic susceptibility measurements show that the magnetic moment per V-site in V$_2$Se$_2$O is much larger than that of CsV$_2$Se$_{2-x}$O, which again suggests the correlation induced localization effect in the former.

cond-mat.mtrl-sci↗

arXiv2018

Plastic pinning replaces collective pinning as the second magnetization peak disappears in the pnictide superconductor Ba-KFe$_2$As$_2$

We report a detailed study of isofield magnetic relaxation and isothermal magnetization measurements with $H$$\parallel$c on an underdoped Ba$_{0.75}$K$_{0.25}$Fe$_2$As$_2$ pnictide single crystal, with superconducting transition temperature $T_c$ = 28 K. The second magnetization peak (SMP) has been observed at temperatures below $T_c$/2 and vanished at higher temperatures. The observed behaviour of the SMP has been studied by measuring the magnetic field dependence of relaxation rate, $R(H)$ and by performing the Maley's analysis. The results suggest that the crossover from collective to plastic pinning observed in the SMP disappears above 12 K with plastic pinning replacing collective pinning. An interesting $H$-$T$ phase diagram is obtained. The critical current density ($J_c$) was estimated using Bean's model and found to be $\sim$ $3.4 \times 10^9$ A/m$^2$ at 10 K in the SMP region, which is comparable to an optimally doped Ba-KFe$_2$As$_2$ superconductor and may be exploited for potential technological applications. The pinning mechanism is found to be unconventional and does not follow the usual $δl$ and $δT_c$ pinning models, which suggest the intrinsic nature of pinning in the compound.

cond-mat.supr-con↗

arXiv2017

Microscopic pairing fingerprint of the iron-based superconductor ${\rm Ba_{1-x}K_xFe_2As_2}$

Resolving the microscopic pairing mechanism and its experimental identification in unconventional superconductors is among the most vexing problems of contemporary condensed matter physics. We show that Raman spectroscopy provides an avenue for this quest by probing the structure of the pairing interaction at play in an unconventional superconductor. As we study the spectra of the prototypical Fe-based superconductor ${\rm Ba_{1-x}K_xFe_2As_2}$ for $0.22\le x \le 0.70$ in all symmetry channels, Raman spectroscopy allows us to distill the leading $s$-wave state. In addition, the spectra collected in the $B_{1g}$ symmetry channel reveal the existence of two collective modes which are indicative of the presence of two competing, yet sub-dominant, pairing tendencies of $d_{x^2-y^2}$ symmetry type. A comprehensive functional Renormalization Group (fRG) and random-phase approximation (RPA) study on this compound confirms the presence of the two sub-leading channels, and consistently matches the experimental doping dependence of the related modes. The synopsis of experimental evidence and theoretical modelling supports a spin-fluctuation mediated superconducting pairing mechanism.

cond-mat.supr-con↗

arXiv2017

Pressure Induced Superconductivity in the New Compound ScZrCo1-$δ$

It is widely perceived that the correlation effect may play an important role in several unconventional superconducting families, such as cuprate, iron-based and heavy-fermion superconductors. The application of high pressure can tune the ground state properties and balance the localization and itineracy of electrons in correlated systems, which may trigger unconventional superconductivity. Moreover, non-centrosymmetric structure may induce the spin triplet pairing which is very rare in nature. Here, we report a new compound ScZrCo1-$δ$ crystallizing in the Ti2Ni structure with the space group of FD3-MS without a spatial inversion center. The resistivity of the material at ambient pressure shows a bad metal and weak semiconducting behavior. Furthermore, specific heat and magnetic susceptibility measurements yield a rather large value of Wilson ratio ~4.47. Both suggest a ground state with correlation effect. By applying pressure, the up-going behavior of resistivity in lowering temperature at ambient pressure is suppressed and gradually it becomes metallic. At a pressure of about 19.5 GPa superconductivity emerges. Up to 36.05 GPa, a superconducting transition at about 3.6 K with a quite high upper critical field is observed. Our discovery here provides a new platform for investigating the relationship between correlation effect and superconductivity.

cond-mat.supr-con↗

arXiv2017

Discrete energy levels of Caroli-de Gennes-Martricon states in quantum limit due to small Fermi energy in FeTe$_{0.55}$Se$_{0.45}$

Caroli-de Gennes-Martricon (CdGM) states were predicted in 1964 as low energy excitations within vortex cores of type-II superconductors. In the quantum limit, namely $T/T_\mathrm{c} \ll Δ/E_\mathrm{F}$, the energy levels of these states were predicted to be discrete with the basic levels at $E_μ= \pm μΔ^2/E_\mathrm{F}$ ($μ= 1/2$, $3/2$, $5/2$, ...). However, due to the small ratio of $Δ/E_\mathrm{F}$ in most type-II superconductors, it is very difficult to observe the discrete CdGM states, but rather a symmetric peak appears at zero-bias at the vortex center. Here we report the clear observation of these discrete energy levels of CdGM states in FeTe$_{0.55}$Se$_{0.45}$. The rather stable energies of these states versus space clearly validates our conclusion. Analysis based on the energies of these CdGM states indicates that the Fermi energy in the present system is very small.

cond-mat.supr-con↗

arXiv2017

BCS-like critical fluctuations with limited overlap of Cooper pairs in FeSe

In conventional superconductors, very narrow superconducting fluctuation regions are observed above $T_c$ because of the strong overlapping of Cooper pairs in a coherence volume. In the bulk form of iron chalcogenide superconductor FeSe, it is argued that the system may locate in the crossover region of BCS to BEC, indicating a strong superconducting fluctuation. In this respect, we have carried out measurements of magnetization, specific heat and Nernst effect on FeSe single crystals in order to investigate the superconducting fluctuation effect near $T_c$. The region of diamagnetization induced by superconducting fluctuation seems very narrow above $T_c$. The crossing point of temperature dependent magnetization curves measured at different magnetic fields, which appears in many systems of cuprate superconductors and is regarded as indication of strong critical fluctuation, is however absent. The magnetization data can be scaled based on the Ginzburg-Landau fluctuation theory for a quasi-two-dimensional system, but the scaling result cannot be described by the theoretical function of the fluctuation theory because of the limited fluctuation regions. The specific heat jump near $T_c$ is rather sharp without the trace of strong superconducting fluctuation. This is also supported by the Nernst effect measurements which indicate a very limited region for vortex motion above $T_c$. Associated with very small value of Ginzburg number and further analyses, we conclude that the superconducting fluctuation is very weak above $T_c$ in this material. Our results are strongly against the picture of significant phase fluctuation in FeSe single crystals, although the system has a very limited overlapping of Cooper pairs in the coherence volume. This dichotomy provides new insights into the superconducting mechanism when the system is with a dilute superfluid density.

cond-mat.supr-con↗

arXiv2017

Magnetization of potassium doped p-terphenyl and p-quaterphenyl by high pressure synthesis

By using high pressure synthesis method, we have fabricated the potassium doped para-terphenyl. The temperature dependence of magnetization measured in both zero-field-cooled and field-cooled processes shows step like transitions at about 125 K. This confirms earlier report about the possible superconductivity like transition in the same system. However, the magnetization hysteresis loop exhibits a weak ferromagnetic background. After removing this ferromagnetic background, a Meissner effect like magnetic shielding can be found. A simple estimate on the diamagnetization of this step tells that the diamagnetic volume is only about 0.0427% at low temperatures, if we assume the penetration depth is much smaller than the size of possible superconducting grains. This magnetization transition does not shift with magnetic field but is suppressed and becomes almost invisible above 1.0 T. The resistivity measurements are failed because of an extremely large resistance. By using the same method, we also fabricated the potassium doped para-quaterphenyl. A similar step like transition at about 125 K was also observed by magnetization measurement. Since there is an unknown positive background and the diamagnetic volume is too small, it is insufficient to conclude that this step is derived from superconductivity although it looks like.

cond-mat.supr-con↗

arXiv2017

Relationship between critical current and flux-flow resistivity in the mixed state of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

We studied the temperature and magnetic field dependence of vortex dissipation and critical current in the mixed-state of unconventional superconducting alloys Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0.044 \leq x \leq 0.100$) through current-voltage measurements. Our results reveal that all the electric field $E$ vs current density $j$ curves in the Ohmic regime merge to one point ($j_0,E_0$) and that there is a simple relationship between the critical current density $j_c$ and flux-flow resistivity $ρ_{\rm ff}$: $ρ_{\rm ff}/ρ_{\rm n} = (1- j_{c}/j_{0})^{-1}$, where $ρ_{\rm n}=E_0/j_0$ is the normal-state resistivity just above the superconducting transition. In addition, $E_0$ is positive for all five dopings, reflecting the abnormal behavior of the flux-flow resistivity $ρ_{\rm ff}$: it increases with decreasing magnetic field. In contrast, $E_0$ is negative for the conventional superconductor Nb since, as expected, $ρ_{\rm ff}$ decreases with decreasing magnetic field. Furthermore, in the under-doped and over-doped single crystals of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, the parameter $E_0$ remains temperature independent, while it decreases with increasing temperature for the single crystals around optimal doping ($ 0.060\leq x\leq 0.072 $). This result points to the co-existence of superconductivity with some other phase around optimal doping.

cond-mat.supr-con↗

arXiv2017

Experimental Demonstration of the Sign Reversal of the Order Parameter in (Li1-xFex)OHFe1-yZnySe

Iron pnictides are the only known family of unconventional high-temperature superconductors besides cuprates. Until recently, it was widely accepted that superconductivity is spin-fluctuation driven and intimately related to their fermiology, specifically, hole and electron pockets separated by the same wave vector that characterizes the dominant spin fluctuations, and supporting order parameters (OP) of opposite signs. This picture was questioned after the discovery of a new family, based on the FeSe layers, either intercalated or in the monolayer form. The critical temperatures there reach ~40 K, the same as in optimally doped bulk FeSe - despite the fact that intercalation removes the hole pockets from the Fermi level and, seemingly, undermines the basis for the spin-fluctuation theory and the idea of a sign-changing OP. In this paper, using the recently proposed phase-sensitive quasiparticle interference technique, we show that in LiOH intercalated FeSe compound the OP does change sign, albeit within the electronic pockets, and not between the hole and electron ones. This result unifies the pairing mechanism of iron based superconductors with or without the hole Fermi pockets and supports the conclusion that spin fluctuations play the key role in electron pairing.

cond-mat.supr-con↗

arXiv2017

Highly anisotropic superconducting gaps and possible evidence of antiferromagnetic order in FeSe single crystals

Specific heat has been measured in FeSe single crystals down to 0.414 K under magnetic fields up to 16 T. A sharp specific heat anomaly at about 8.2 K is observed and is related to the superconducting transition. Another jump of specific heat is observed at about 1.08 K which may either reflect an antiferromagnetic transition of the system or a superconducting transition arising from Al impurity. We would argue that this anomaly in low temperature region may be the long sought antiferromagnetic transition in FeSe. Global fitting in wide temperature region shows that the models with a single contribution with isotropic s-wave, anisotropic s-wave, and d-wave gap all do not work well, nor the two isotropic s-wave gaps. We then fit the data by a model with two components in which one has the gap function of $Δ_0(1+αcos2θ)$. To have a good global fitting and the entropy conservation for the low temperature transition, we reach a conclusion that the gap minimum should be smaller than 0.15 meV ($α$ = 0.9 to 1), indicating that the superconducting gap(s) are highly anisotropic. Our results are very consistent with the gap structure derived recently from the scanning tunneling spectroscopy measurements and yield specific heat contributions of about 32\% weight from the hole pocket and 68\% from the electron pockets.

cond-mat.supr-con↗

arXiv2017

Collective vortex pinning and crossover between second order to first order transition in optimally doped Ba1-xKxBiO3 single crystals

Measurements on magnetization and relaxation have been carried out on an optimally doped Ba$_{0.59}$K$_{0.47}$BiO$_{3+δ}$ single crystal with $T_c$ = 31.3 K. Detailed analysis is undertaken on the data. Both the dynamical relaxation and conventional relaxation have been measured leading to the self-consistent determination of the magnetization relaxation rate. It is found that the data are well described by the collective pinning model leading to the glassy exponent of about $μ\approx$ 1.64 to 1.68 with the magnetic fields of 1 and 3 T. The analysis based on Maley's method combining with the conventional relaxation data allows us to determine the current dependent activation energy $U$ which yields a $μ$ value of about 1.23 to 1.29 for the magnetic fields of 1 and 3 T. The second magnetization peaks appear in wide temperature region from 2 K to 24 K. The separation between the second peak field and the irreversibility field becomes narrow when temperature is increased. When the two fields are close to each other, we find that the second peak evolves into a step like transition of magnetization, suggesting a crossover from the second order to first order transition. Finally, we present a vortex phase diagram and demonstrate that the vortex dynamics in Ba$_{1-x}$K$_x$BiO$_3$ can be used as a model system for studying the collective vortex pining.

cond-mat.supr-con↗

arXiv2016

Superconductivity in LiOHFeS single crystals with a shrunk c-axis lattice constant

By using a hydrothermal ion-exchange method, we have successfully grown superconducting crystals of LiOHFeS with ${T_c}$ of about 2.8 K. Being different from the sister sample (Li${_{1-x}}$Fe${_x}$)OHFeSe, the energy dispersion spectrum analysis on LiOHFeS shows that the Fe/S ratio is very close to 1:1, suggesting an almost charge neutrality and less electron doping in the FeS planes of the system. Comparing with the non superconducting LiOHFeS crystal, each peak of the X-ray diffraction pattern of the superconducting crystal splits into two, and the diffraction peaks locating at lower reflection angles are consistent with that of non-superconducting ones. The rest set of diffraction peaks with higher reflection angles is corresponding to the superconducting phase, suggesting that the superconducting phase may has a shrunk c-axis lattice constant. Magnetization measurements indicate that the magnetic shielding due to superconductivity can be quite high under a weak magnetic field. The resistivity measurements under various magnetic fields show that the upper critical field is quite low, which is similar to the tetragonal FeS superconductor.

cond-mat.supr-con↗

arXiv2016

Universality and unconventional enhancement of flux-flow resistivity in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

Measurements of the current-voltage characteristics were performed on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with doping level $0.044 \leq x \leq 0.1$. An unconventional increase in the flux-flow resistivity $ρ_{\rm ff}$ with decreasing magnetic field was observed across this doping range. Such an abnormal field dependence of flux-flow resistivity is in contrast with the linear field dependence of $ρ_{\rm ff}$ in conventional type-II superconductors, but is similar to the behavior recently observed in the heavy-fermion superconductor CeCoIn$_5$. A significantly enhanced $ρ_{\rm ff}$ was found for the x=0.06 single crystals, implying a strong single-particle energy dissipation around the vortex cores. At different temperatures and fields and for a given doping concentration, the normalized $ρ_{\rm ff}$ scales with normalized field and temperature. The doping level dependence of these parameters strongly suggests that the abnormal upturn flux-flow resisitivity is likely related to the enhancement of spin fluctuations around the vortex cores of the optimally doped samples.

cond-mat.supr-con↗