北京高压科学研究中心
Center for High Pressure Science &Technology Advanced Research

Superconducting and Magnetic Materials under Extreme Conditions


SMM: research group



Welcome to the homepage of the research group for Superconducting and Magnetic Materials under Extreme Conditions.  Our group is a young research unit of the HPSTAR (research area of Physics of Strongly Correlated Materials). Our group is currently being built up at the HPSTAR in Beijing, China. We investigate awide range of strongly correlated electron systems under extreme conditions.



Recent topics:


May 2017: High-pressure behavior of superconducting boron-doped diamond

Despite great efforts in the study of group-IV covalent semiconductors, many unresolved questions and unexplained results require further investigation. We have investigated the high-pressure structure of freestanding superconducting  (Tc = 4.3 K) boron-dopeddiamond (BDD) and how it affects the electronic and vibrational properties using Raman spectroscopy and x-ray diffraction in the 0–30 GPa range. With this viewpoint, we measured the heat capacity, magnetization, and electrical resistivity to obtain its physical properties. we discussed  the relevance of phonon-mediatedmechanisms in the grains by conducting high-pressure transport studies. In our experiment, Tc reduces with applied pressure by a pressure coefficient of −0.09 K/GPa. The suppression of superconductivity at high pressure via a decrease of the electron-phonon coupling parameter in BDD has been theoretically demonstrated.  This work is published in Physical Review B. 95, 174519 (2017).



Feb. 2017: NbS3: Quasi 1D conductor with three charge density wave transitions

Anotable feature of quasi-1D CDWs is their ability to slide in a sufficiently high electric field, resulting in nonlinear conductivity. In this work, we show the features of the charge density wave (CDW) conductor NbS3 (phase II) and include several additional results from transport, compositional, and structural studies. Particularly, we highlight three central results: (1)  CDW transitions at TP1 = 360K and TP2 = 150 K, and third CDW transition occurs at a much higher temperature TP0 = 620 - 650 K; evidence for the nonlinear conductivity of this CDW is presented. (2) We show that the CDW associated with the TP2 transition arises from S vacancies acting as donors. Such a CDW transition has not been observed before. (3) We demonstrate the exceptional coherence of the TP1 CDWat room temperature. The effects of uniaxial strain on the CDWtransition temperature and transport are reported. This work is published in Physical Review B. 95, 035110 (2017).


October 2016: FeSeS: What is the consensus on its gap structure?




A basic property of any superconductor is its gap structure, which is related to the symmetry of its pairing state.  Amongst iron-based superconductors, the simple material FeSe has attracted much attention because, when made in thin-film form, its superconductivityappears to persist to a critical temperature Tc 100 K. In bulk form, FeSe is unusual in that it undergoes the standard tetragonal to-orthorhombic structural transition without the usual accompanying antiferromagnetic transition. This raises fundamental questions about the role of magnetism in causing superconductivity and nematicity. To answer an open question on the superconducting gap, we report on an angle resolved photoemission study on the superconducting gap structure in the nematic state. Our results show that the superconducting gap shows twofold anisotropy around the Z point (0, 0, π), and it is undetectable around the hole pocket near Γ (0, 0, 0) and the electron pockets at the zone corners (π, π, kz).The experimental results presented in this article will help the theorists currently developing the theory of one of the most enigmatic and intriguing quantum phenomena that finds more and more applications in modern technology. This work is published in Phy. Rev. Lett.. 117, 157003 (2016).


August 2016: TaS2: Phase diagram and enhancement of Tc by one order of magnitude

For more than four decades, one of the major subjects in condensed matter physics has been the coexistence of
the charge density wave (CDW) order and superconductivity in TMDs). In
CDW materials such a coupling between the electrons and the soft-phonon mode describes the phase transition
from the CDW to a normal state.  Amongst many TMD materials, 2H-TaS2  becomes superconducting
at ambient pressure and without doping. So far, this compound is one of the very few materials where a
chiral and polar charge-ordered phase is suggested to exist.

Through transport and thermodynamic measurements, we report on the fieldtemperature
phase diagram in 2H-TaS2 single crystals. We show that the superconducting transition
temperature (Tc) increases by one order of magnitude from temperatures at 0.98 K up to 9.15 K at
8.7 GPa when the Tc becomes very sharp. Additionally, the effects of 8.7 GPa illustrate a suppression
of the CDW ground state, with critically small Fermi surfaces. Below the Tc the lattice of magnetic flux
lines melts from a solid-like state to a broad vortex liquid phase region. Our measurements indicate an
unconventional s-wave-like picture with two energy gaps evidencing its multi-band nature.
This work is published in Sci. Rep. 6, 31824 (2016).

June 2016: Chemical and physical pressure to the Fe- Chacogenides

A comprehensive study of the doping dependence of the phase diagram of FeSe-based superconductors is still
required due to the lack of a clean and systematic means of doping control. Here, we report on the  thermodynamic and transport properties studies of impurity scattering in stoichiometric FeSe single crystals. Co doping at the Fe site is found
to decrease the superconducting transition temperature. The upper critical field and specific heat all indicatea possible multiband superconductivity with strong coupling in the Co-doped system. A remarkable feature in
FeSe is that its temperature dependent resistivity exhibits a wide hump at high temperatures, a signature of
a crossover from a semiconductinglike behavior to metallic behavior. A structural tetragonal-to-orthorhombic
phase transition (Ts) (a consequence of the electronic nematicity) is suppressed by either physical or chemical
pressures. We found that the temperature-induced Lifshitz transition is much higher than the temperature for the
nematic order. This work is published in Physical Review B. 93, 224508 (2016).


April 2016: Superconductivity at 46 K in FeSe

Iron-based superconductor with layered structure is currently the family with the second highest critical temperature, behind the cuprates.The observation of Tcfrom 65 K to 109 Kin FeSe monolayergrown on SrTiO3 (STO)substrates has attracted considerable attention and interest since 2012. Such a highTcis unexpected in the FeSe system, because the bulk FeSe exhibits aTc of only 8 K at ambient pressure. This discovery could be thought as a break through in FeSe system in recent five years. The origin of high-temperature superconductivity in two-dimensionalFeSesystem is crucial for probing the superconductivity mechanism in the Fe-based superconductors, and thus providing insights on how to boost high-temperature superconductivity in general.

In this work we used photoemission spectroscopy to directly probe the K doped thick FeSe films and FeSe0.93S0.07 bulk crystals, establishing the phase diagram of FeSe as a function of electron doping. They found that the correlation strength remarkably increases with increasing doping, while an insulating phase emerges in the heavily overdoped regime. Between the nematic phase and the insulating phase, a dome of enhanced superconductivity is observed, with the maximum superconducting transition temperature of about 46 K.  This work is published in Nat. Comm. 7, 10840 (2016).