arXiv subjects

C. Q. Jin

Publications and source records attributed to C. Q. Jin.

At least 19 recordsLinked to original sources

arXiv2024

Superconductivity Discovered in Niobium Polyhydride at High Pressures

Niobium polyhydride was synthesized at high pressure and high temperature conditions by using diamond anvil cell combined with in situ high pressure laser heating techniques. High pressure electric transport experiments demonstrate that superconducting transition occurs with critical temperature(Tc) 42 K at 187 GPa. The shift of Tc as function of external applied magnetic field is in consistent to the nature of superconductivity while the upper critical field at zero temperature Hc2(0) is estimated to~16.8 Tesla while the GL coherent length ~57 angstrom is estimated. The structure investigation using synchrotron radiation implies that the observed superconductivity may come from Fm-3m phase of NbH3.

cond-mat.supr-con↗

arXiv2024

Superconductivity with Tc 116K discovered in antimony polyhydrides

Superconductivity (SC) was experimentally observed for the first time in antimony polyhydride. The diamond anvil cell combined with laser heating system was used to synthesize the antimony polyhydride sample at high pressure and high temperature conditions. In-situ high pressure transport measurements as function of temperature with applied magnet are performed to study the SC properties. It was found that the antimony polyhydride samples show superconducting transition with critical temperature Tc 116 K at 184 GPa. The investigation of SC at magnetic field revealed that the superconducting coherent length ~40 angstroms based on Ginzburg Landau (GL) equation. Antimony polyhydride superconductor has the second highest Tc in addition to sulfur hydride among the polyhydrides of elements from main group IIIA to VIIA in periodic table.

cond-mat.supr-con↗

arXiv2023

Superconductivity Observed in Tantalum Polyhydride at High Pressure

We report experimental discovery of tantalum polyhydride superconductor. It was synthesized at high pressure and high temperature conditions using diamond anvil cell combined with in-situ high pressure laser heating techniques. The superconductivity was investigated via resistance measurements at pressures. The highest superconducting transition temperature Tc was found to be ~30 K at 197 GPa in the sample that was synthesized at the same pressure with ~2000 K heating. The transitions are shifted to low temperature upon applying magnetic fields that supports the superconductivity nature. The upper critical field at zero temperature μ0Hc2(0) of the superconducting phase is estimated to be ~20 T that corresponds to GL coherent length ~40 angstroms. Our results suggest that the superconductivity may arise from I-43d phase of TaH3. It is, for the first time to our best knowledge, experimental realization of superconducting hydrides for the VB group of transitional metals.

cond-mat.supr-con↗

arXiv2022

Record High Tc Element Superconductivity Achieved In Titanium

It is challenging to search for high Tc superconductivity (SC) in transition metal elements wherein d electrons are usually not favored by conventional BCS theory. Here we report discovery of surprising SC up to 310 GPa with Tc above 20 K in wide pressure range from 108 GPa to 240 GPa in titanium. The maximum Tc^onset above 26 K and zero resistance Tc^zero of 21 K are record high values hitherto achieved among element superconductors. The Hc2(0) is estimated to be about 32 Tesla with coherence length 32 angstrom. The results show strong s-d transfer and d-band dominance, indicating correlation driven contributions to high Tc SC in dense titanium. This finding is in sharp contrast to the theoretical predications based on pristine electron-phonon coupling scenario. The study opens a fresh promising avenue for rational design and discovery of high Tc superconductors among simple materials via pressure tuned unconventional mechanism.

cond-mat.supr-con↗

arXiv2022

Superconductivity above 80 K in polyhydrides of hafnium

Studies on polyhydrides are attracting growing attentions recently due to their potential high temperature superconductivity (SC). We here report the discovery of SC in hafnium polyhydrides at high pressures. The hafnium superhydrides are synthesized at high pressure and high temperature conditions using diamond anvil cell in combination with in-situ high pressure laser heating technique. The SC was investigated by in-situ high pressure resistance measurements in applied magnetic fields. A superconducting transition with onset Tc ~83 K was observed at 243 GPa. The upper critical field Hc2(0) was estimated to be 24 Tesla by GL theory and the consequent superconducting coherent length to be ~37 angstrom. Our results suggest that the superconducting phase is from C2/m-HfH14. This is the first 5d transition metal polyhydride superconductor with Tc above the liquid nitrogen temperature.

cond-mat.supr-con↗

arXiv2022

Superconductivity above 200 K Observed in Superhydrides of Calcium

Searching for superconductivity with Tc near room temperature is of great interest both for fundamental science & many potential applications. Here we report the experimental discovery of superconductivity with maximum critical temperature (Tc) above 210 K in calcium superhydrides, the new alkali earth hydrides experimentally showing superconductivity above 200 K in addition to sulfur hydride & rare-earth hydride system. The materials are synthesized at the synergetic conditions of 160~190 GPa and ~2000 K using diamond anvil cell combined with in-situ laser heating technique. The superconductivity was studied through in-situ high pressure electric conductance measurements in an applied magnetic field for the sample quenched from high temperature while maintained at high pressures. The upper critical field Hc(0) was estimated to be ~268 T while the GL coherent length is ~11 angstrom. The in-situ synchrotron X-ray diffraction measurements suggest that the synthesized calcium hydrides are primarily composed of CaH6 while there may also exist other calcium hydrides with different hydrogen contents.

cond-mat.supr-con↗

arXiv2022

Superconductivity in Zirconium Polyhydrides with Tc above 70 K

We report the experimental discovery of superconductivity with critical temperature Tc above 70 K in zirconium polyhydrides, the record high so far for 3d transition metal hydrides. The superconducting zirconium polyhydrides are synthesized at the extreme conditions above 200 GPa & 2000 K by using diamond anvil cell combined with in-situ laser heating. Superconductivity was observed in the samples maintained at the same pressure upon quenched from heating.

cond-mat.supr-con↗

arXiv2022

Magnetic Properties and Electronic Configurations of Mn Ions in the Diluted Magnetic Semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ Studied by X-ray Magnetic Circular Dichroism and Resonant Inelastic X-ray Scattering

The magnetic properties and the electronic excitations of the new diluted magnetic semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ have been studied by x-ray magnetic circular dichroism (XMCD) and resonant inelastic x-ray scattering (RIXS) at the Mn $L_{2,3}$ edge. The sum rule analysis of the XMCD spectra yields the net spin moment of $0.45μ_{\text{B}}$/Mn and the small orbital moment of $0.05μ_{\text{B}}$/Mn. This indicates that the Mn atoms are in the high-spin configurations of $d^{5}$, whereas the presence of competing ferromagnetic and antiferromagnetic interactions between the Mn ions reduces the net spin moment. RIXS spectra show broad peaks from 1 to 6 eV energy loss, which originate from the $d$-$d$ crystal field excitations of the Mn ions. Based on a comparison of the RIXS line shapes with those of Ga$_{1-x}$Mn$_{x}$As, we conclude that the ground state of Mn in Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ consists not only of the charge-transferred $3d^{5}\underline{L}$ electron configuration ($\underline{L}$: ligand hole) with weakly bound holes as in Ga$_{1-x}$Mn$_{x}$As, but also of the pure $3d^{5}$ configuration with free holes.

cond-mat.str-el↗

arXiv2021

Nature of the ferromagnetic-antiferromagnetic transition in Y$_{1-x}$La$_{x}$TiO$_{3}$

We explore the magnetically-ordered ground state of the isovalently-substituted Mott-insulator Y$_{1-x}$La$_{x}$TiO$_{3}$ for $x$ $\leq$ 0.3 via single crystal growth, magnetometry, neutron diffraction, x-ray magnetic circular dichroism (XMCD), muon spin rotation ($μ$SR) and small-angle neutron scattering (SANS). We find that the decrease in the magnetic transition temperature on approaching the ferromagnetic (FM) - antiferromagnetic (AFM) phase boundary at the La concentration $x_c$ $\approx$ 0.3 is accompanied by a strong suppression of both bulk and local ordered magnetic moments, along with a volume-wise separation into magnetically-ordered and paramagnetic regions. The thermal phase transition does not show conventional second-order behavior, since neither a clear signature of dynamic critical behavior nor a power-law divergence of the magnetic correlation length is found for the studied substitution range; this finding becomes increasingly obvious with substitution. Finally, from SANS and magnetometry measurements, we discern a crossover from easy-axis to easy-plane magneto-crystalline anisotropy with increasing La substitution. These results indicate complex changes in magnetic structure upon approaching the phase boundary.

cond-mat.str-el↗

arXiv2021

Nodal superconducting gap in LiFeP revealed by NMR: contrast with LiFeAs

Identifying the uniqueness of FeP-based superconductors may shed new lights on the mechanism of superconductivity in iron-pnictides. Here, we report nuclear magnetic resonance(NMR) studies on LiFeP and LiFeAs which have the same crystal structure but different pnictogen atoms. The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state and can provide the information on the superconducting pairing symmetry through the temperature dependence of London penetration depth $λ_L$. We find that $λ_L$ saturates below $T \sim 0.2$ $T_c$ in LiFeAs, where $T_c$ is the superconducting transition temperature, indicating nodeless superconducting gaps. Furthermore, by using a two-gaps model, we simulate the temperature dependence of $λ_L$ and obtain the superconducting gaps of LiFeAs, as $Δ_1 = 1.2$ $k_B T_c$ and $Δ_2 = 2.8$ $k_B T_c$, in agreement with previous result from spin-lattice relaxation. For LiFeP, in contrast, the London penetration depth $λ_L$ does not show any saturation down to $T \sim 0.03 $ $T_c$, indicating nodes in the superconducting energy gap function. Finally, we demonstrate that the strong spin fluctuations with diffusive characteristics exist in LiFeP, as in some cuprate high temperature superconductors.

cond-mat.supr-con↗

arXiv2020

The Hund's Superconductor Li(Fe,Co)As

We combine transport, angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy to investigate several low energy manifestations of the Hund coupling in a canonical FeSC family Li(Fe,Co)As. We determine the doping dependence of the coherent-incoherent crossover temperature and the quasi-particle effective mass enhancement in the normal state. Our tunneling spectroscopy result in the superconducting state supports the idea that superconductivity emerging from Hund's metal state displays a universal maximal superconducting gap vs transition temperature (2$Δ_{max}/k_{B}T_{c}$) value, which is independent of doping level and $T_{c}$.

cond-mat.supr-con↗

arXiv2020

Orbital selectivity of layer resolved tunneling on iron superconductor Ba0.6K0.4Fe2As2

We use scanning tunneling microscopy/spectroscopy (STM/S) to elucidate the Cooper pairing of the iron pnictide superconductor Ba0.6K0.4Fe2As2. By a cold-cleaving technique, we obtain atomically resolved termination surfaces with different layer identities. Remarkably, we observe that the low-energy tunneling spectrum related to superconductivity has an unprecedented dependence on the layer-identity. By cross-referencing with the angle-revolved photoemission results and the tunneling data of LiFeAs, we find that tunneling on each termination surface probes superconductivity through selecting distinct Fe-3d orbitals. These findings imply the real-space orbital features of the Cooper pairing in the iron pnictide superconductors, and propose a new and general concept that, for complex multi-orbital material, tunneling on different terminating layers can feature orbital selectivity.

cond-mat.supr-con↗

arXiv2018

Electron Mass Enhancement near a Nematic Quantum Critical Point in NaFe$_{1-x}$Co$_{x}$As

A magnetic order can be completely suppressed at zero temperature($T$), by doping carriers or applying pressure, at a quantum critical point(QCP), around which physical properties change drastically. However, the situation is unclear for an electronic nematic order that breaks rotation symmetry. Here we report nuclear magnetic resonance(NMR) studies on NaFe$_{1-x}$Co$_{x}$As where magnetic and nematic transitions are well separated. The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state, which is related to London penetration depth $λ_{\rm L}$ that measures the electron mass $m^*$. We discovered two peaks in the doping dependence of $λ_{\rm L}^2$($T\sim$0); one at $x_{\rm M}$=0.027 where the spin-lattice relaxation rate shows quantum critical behavior, and another at $x_{\rm c}$=0.032 around which the nematic transition temperature extrapolates to zero and the electrical resistivity shows a $T$-linear variation. Our results indicate that a nematic QCP lies beneath the superconducting dome at $x_{\rm c}$ where $m^*$ is enhanced. The impact of the nematic fluctuations on superconductivity is discussed.

cond-mat.supr-con↗

arXiv2018

Single Crystal Growth and Spin Polarization Measurements of Diluted Magnetic Semiconductor (BaK)(ZnMn)2As2

Recently a new type diluted magnetic semiconductor (BaK)(ZnMn)2As2 (BZA) with high Cure temperature (Tc) was discovered showing independent spin and charge doping mechanism. This makes BZA a promising material for spintronics devices. Here we report for the first time the successful growth of BZA single crystal. An Andreev reflection junction that can be used to evaluate spin polarization was fabricated based on the BZA single crystal, a 66% spin polarization of the BZA single crystal was hence obtained by Andreev reflection spectroscopy analysis.

cond-mat.mtrl-sci↗

arXiv2018

Decoupled Pairing Amplitude and Electronic Coherence in Iron-Based Superconductors

Here we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi liquid phase (LiFeAs) and an incoherent bad-metal phase (FeTe0.55Se0.45). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum superconducting pairing amplitude, which shows a universal scaling that is valid for all FeSCs. Our observation excludes pairing scenarios in the BCS and the BEC limit for FeSCs and calls for a universal strong coupling pairing mechanism for the FeSCs.

cond-mat.supr-con↗

arXiv2018

Weak doping dependence of the antiferromagnetic coupling between nearest-neighbor Mn$^{2+}$ spins in (Ba$_{1-x}$K$_x$)(Zn$_{1-y}$Mn$_y$)$_2$As$_2$

Dilute magnetic semiconductors (DMS) are nonmagnetic semiconductors doped with magnetic transition metals. The recently discovered DMS material (Ba$_{1-x}$K$_{x}$)(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ offers a unique and versatile control of the Curie temperature, $T_{\mathrm{C}}$, by decoupling the spin (Mn$^{2+}$, $S=5/2$) and charge (K$^{+}$) doping in different crystallographic layers. In an attempt to describe from first-principles calculations the role of hole doping in stabilizing ferromagnetic order, it was recently suggested that the antiferromagnetic exchange coupling $J$ between the nearest-neighbor Mn ions would experience a nearly twofold suppression upon doping 20\% of holes by potassium substitution. At the same time, further-neighbor interactions become increasingly ferromagnetic upon doping, leading to a rapid increase of $T_{\mathrm{C}}$. Using inelastic neutron scattering, we have observed a localized magnetic excitation at about 13 meV, associated with the destruction of the nearest-neighbor Mn-Mn singlet ground state. Hole doping results in a notable broadening of this peak, evidencing significant particle-hole damping, but with only a minor change in the peak position. We argue that this unexpected result can be explained by a combined effect of superexchange and double-exchange interactions.

cond-mat.mtrl-sci↗

arXiv2017

Ba(Zn,Co)2As2: a II-II-V Diluted Ferromagnetic Semiconductor with N-type Carriers

Diluted ferromagnetic semiconductors (DMSs) that combine the properties of semiconductors with ferromagnetism have potential application in spin-sensitive electronics (spintronics) devices. The search for DMS materials exploded after the observation of ferromagnetic ordering in III-V (Ga,Mn)As films. Recently, a series of DMS compounds isostructural to iron-based superconductors have been reported. Among them, the highest Curie temperature $T_C$ of 230 K has been achieved in (Ba,K)(Zn,Mn)$_2$As$_2$. However, most DMSs, including (Ga,Mn)As, are p-type, i.e., the carriers that mediate ferromagnetism are holes. For practical applications, DMS with n-type carriers are also advantageous. Here we report the successful synthesis of a II-II-V diluted ferromagnetic semiconductor with n-type carriers, Ba(Zn,Co)$_2$As$_2$. Magnetization measurements show that the ferromagnetic transition occurs up to $T_{C} \sim$ 45 K. Hall effect and Seebeck effect measurements jointly confirm that the dominant carriers are electrons. Through muon spin relaxation ($μ$SR), a volume sensitive magnetic probe, we have also confirmed that the ferromagnetism in Ba(Zn,Co)$_2$As$_2$ is intrinsic and the internal field is static.

cond-mat.mtrl-sci↗

arXiv2017

Magneto-elastic coupling in Fe-based superconductors

We used polarization-resolved Raman scattering to study the magneto-elastic coupling in the parent compounds of several families of Fe-based superconductors (BaFe2As2, EuFe2As2, NaFeAs, LiFeAs, FeSe and LaFeAsO). We observe an emergent Ag-symmetry As phonon mode in the XY scattering geometry whose intensity is significantly enhanced below the magneto-structural transition only for compounds showing magnetic ordering. We conclude that the small lattice anisotropy is insufficient to induce the in-plane electronic polarizability anisotropy necessary for the observed phonon intensity enhancement, and interpret this enhancement below the Neel temperature in terms of the anisotropy of the magnetic moment and magneto-elastic coupling. We evidence a Fano line- shape in the XY scattering geometry resulting from a strong coupling between the Ag (As) phonon mode and the B2g symmetry-like electronic continuum. Strong electron-phonon coupling may be relevant to superconductivity.

cond-mat.supr-con↗