Welcome! Our laboratory is hosted by the Department of Physics, The Chinese University of Hong Kong. We investigate a wide range of strongly correlated electron systems under extreme conditions. The group is led by Dr. Swee K. Goh (Associate Professor).

Recent topics:

February 2020 As a leading candidate of type-II Weyl semimetal, Td-MoTe2 has attracted a lot of attention. Thus, the understanding of its Fermiology is important. The bulk electronic structure of Td-MoTe2 features large hole pockets at the Brillouin zone center, which have never been conclusively detected in quantum oscillations. The existence of these hole pockets is important for understanding the Weyl physics. Here, we successfully solve this puzzle by detecting these elusive hole pockets in our quantum oscillation measurements. Combining with the high-pressure quantum oscillation study and the DFT + U calculations, we reveal that the hole pockets have a large Fermi surface curvature at ambient pressure, making the associated quantum oscillation signals weak. This probably explains their absence in previous quantum oscillation experiments. This work is published in Physical Review Letters.

December 2019 To measure the diamagnetism associated with the superconductivity under high pressure is not an easy task. In collaboration with Sen Yang's group, we accomplish this mission by utilising the nitrogen vacancies in diamond particles as a local sensor of magnetic field. We successfully detected the Tc, Hc1 and Hc2 in BaFe2(As0.59P0.41)2 under pressure with spatial resolution. Depending on the positions of diamond particles relative to the sample, the effects "felt" by the NV centres can be different, enabling the magnetic field profile around the superconductor to be extracted. The crystals used were prepared and characterised by the teams at Kyoto University and The University of Tokyo. This work is published in Science. See press coverage at CUHK's CPR office, Physics World and HK China News Agency (in Chinese).

March 2019 Superconductivity in the type-II Weyl semimetal candidate MoTe2 has attracted much attention due to the possible realization of topological superconductivity. We have constructed its temperature-pressure phase diagram. The magnetoresistance (MR) and Hall coefficient of MoTe2 are found to decrease with an increasing pressure. The Kohler's scalings for the MR data above ~11 kbar show a change of exponent whereas the data at lower pressure can be well scaled with a single exponent. These results are suggestive of a Fermi surface reconstruction when the structure changes from the Td to 1T' phase. We have performed a detailed study of the upper critical field Hc2 of MoTe2 under pressure. The angular dependence of Hc2 at 15 kbar can be well described by the Tinkham model, suggesting the two-dimensional nature of superconductivity in the high-pressure 1T' phase. This is in sharp contrary to our previous work on WTe2, in which the angular dependence of of Hc2 follows the 3D anisotropic mass model. This work is published in Physical Review Materials.

October 2018 We have performed inelastic x-ray scattering at SPring-8 to probe the phonon softening accompanied by the second-order structural transition in (CaxSr1-x)3Rh4Sn13 for various calcium contents (x = 0, 0.5, 0.75 and 0.85). We detected a complete phonon softening at the M point when approaching the structural transition temperature from above. The temperature dependence of the soft mode energy follows Landau theory with the mean-field approximation. Intriguingly, at x = 0.85, the energy squared of the soft mode at the M point extrapolates to zero at (-5.7 ± 7.7) K, providing the first compelling microscopic evidence of a structural quantum critical point in (CaxSr1-x)3Rh4Sn13. The observed phonon softening around the M point enhances the phonon density of states at low energy, providing the essential ingredient for realizing strong-coupling superconductivity near the structural quantum critical point. This work is published in Physical Review B (Rapid Communication).

July 2018 Rare-earth monopnictides RX (where R = rare-earth, X = Sb, Bi) have drawn attention recently due to intriguing magneto-transport properties. In collaboration with National Cheng Kung University, we have carried out the transport study on single crystalline ScSb. Unlike most monopnictides, ScSb has an additional hole pocket at the Gamma point. Our measurements reveal a large magnetoresistance and field-induced resistivity plateau at low temperatures. Kohler scaling is obeyed over a wide temperature range. Furthermore, the Shubnikov-de Haas oscillations allow the full construction of Fermi surface and estimation of electron concentration (n) and the hole concentration (p), which indicate that ScSb is a nearly compensated semimetal. The calculated band structure (provided by J. Y. Zhu's group) agrees extremely well with SdH data, and the calculation indicates the absence of a band inversion. Our results show that the large magnetoresistance in ScSb can be adequately attributed to the nearly perfect compensation of electrons and holes, possibly accompanied by mobility mismatch. This work is published in Physical Review B.

June 2018 At ambient pressure, LiOsO3 undergoes a `ferroelectric-like' structural change from the centrosymmetric crystal structure R-3c to the noncentrosymmetic structure R3c below 140 K. In collaboration with NYU Shanghai and NIMS Japan, we have studied the pressure dependence of the polar to non-polar transition temperature of LiOsO3 up to 6.48 GPa. We found that the transition temperature can be substantially enhanced with pressure. The origin of the increased stability of the R3c phase at high pressure is discussed with the help of density functional theory (DFT) calculations. This work is published in Applied Physics Letters.

March 2018 In collaboration with Tokyo Metropolitan University and Kyoto University, we have studied ambient-pressure-grown LaO0.5F0.5BiS2 single crystals and constructed the H-T phase diagrams displaying the temperature dependence of the out-of-plane upper critical field and the in-plane upper critical field. Furthermore, the angular dependence of Hc2 at fixed temperatures has also been measured. Our results are well described by the two-gap model. Moreover, the in-plane upper critical field at low temperatures are found to exceed the Pauli paramagnetic limit, which can be attributed to hidden spin polarization due to the breaking of the local inversion symmetry at the electronically active BiS2 bilayers. The anisotropy of the superconductivity reaches around 16 on approaching 0 K. We conclude that LaO0.5F0.5BiS2 is a highly anisotropic two-gap spin-orbit coupled superconductor. This work is published in Physical Review B.

December 2017 In collaboration with University of Cambridge, we have investigated the dimensionality of superconductivity in WTe2 at 98.5 kbar, where the superconducting transition temperature (Tc) is maximal, through electrical resistivity measurements. Despite the layered crystal structure, the angular dependence of the upper critical field (Hc2) from 0.01 Tc to 0.67 Tc can be described by the anisotropic mass Ginzburg-Landau model. Analysis of the temperature dependence of Hc2 along the in-plane and out-of-plane directions gives a temperature-independent and extremely low anisotropy factor of ~1.7 from 0.01 Tc to Tc. Our results conclude that WTe2 is a nearly isotropic superconductor with one of the lowest anisotropy factor among layered superconductors. This work is published in Physical Review B (Rapid Communication).

August 2017 In collaboration with University of Tokyo and Kyoto University, we have followed the pressure dependence of the superconducting transition temperature (Tc) in FeSe1-xSx using ac susceptibility. The pressure dependence of Tc at the low-pressure region agrees with literature data obtained resistively. However, a substantial weakening of the diamagnetic shielding was detected beyond the pressure around which the maximum Tc of 30 K was reached near the verge of the pressure-induced magnetic phase. This observation suggests a strong competition between magnetism and superconductivity at high pressure in FeSe1-xSx. This work is published in Physical Review B (Rapid Communication). Note added in October 2017: see a related pressure work on FeSe1-xSx by Matsuura et al. in Nature Communications.

June 2017 Pressure-induced superconductivity has recently been discovered in CrAs. In collaboration with Kobe University and Kyoto University, we have studied the magnetoresistance of CrAs under pressure. Near the magnetic instability, we observe a large, non-saturating quasilinear magnetoresistance at low temperatures. Our analysis of the bandstructure reveals a nontrivial band crossing at Y, with an energy-momentum dispersion which is linear along the Y-Γ direction, and parabolic along the Y-S direction. Furthermore, this band crossing is protected by nonsymmorphic crystal symmetry. We show that the quasilinear magnetoresistance arises from an intricate interplay between this nontrivial band crossing and strong magnetic fluctuations. This work is published in Nature Communications. See also press coverage at EurekAlert! and Phys.org.

April 2017 We have analyzed the lattice constants of several isovalent A3T4Sn13 compounds (A=Ca,Sr, T=Co,Rh,Ir), and constructed the temperature-lattice constant phase diagram. The analysis shows that Ca3Co4Sn13 is far away from structural instability, and thus represents an ideal reference compound to other compositions we studied before (see here and here) that are in the vicinity of the structural quantum critical point. With this viewpoint, we measured the heat capacity and electrical resistivity to obtain the approximate phonon density of states and the electron-phonon transport coupling function of Ca3Co4Sn13 and selected compounds close to the quantum phase transition. Our results support the scenario of phonon softening close to the structural quantum critical point, and explain the enhancement of the coupling strength on approaching structural instability. This work is published in Physical Review B.

June 2016 Using heat capacity, electrical resistivity, X-ray diffraction as well as density functional theory, we probe the nature of the phase transition at T*=152 K in La3Co4Sn13. The data collected on our high quality single crystals allow us to conclude that the transition is a second-order structural transition. It has been shown that the structural transition in La3Co4Sn13 can be suppressed using pressure. Therefore, the confirmation of the second-order nature of the phase transition raises an interesting possibility of reaching a structural quantum critical point when T* is suppressed to 0 K. This scenario has recently been proposed in related systems Sr3Ir4Sn13 and Sr3Rh4Sn13. La3Co4Sn13 also superconducts at low temperatures. Therefore, La3Co4Sn13 can be regarded as another promising system for exploring the interplay between structural transition and superconductivity. This work is published in Physical Review B (Rapid Communication).

May 2016 In collaboration with Hong Kong University of Science and Technology, we investigated the vortex dynamics of the superconducting Bi2Te3/FeTe heterostructures which had been exposed to air for two years. We found that both the superconductivity and the two-dimensional vortex dynamics remain robust in these aged heterostructures, even when the thickness of the Bi2Te3 epilayer is down to 3 nm. This work is published in Scientific Reports.

November 2015 We have carried out specific heat studies on (CaxSr1-x)3Rh4Sn13 for various doping concentration x straddling across the structural quantum critical point. We found that the effective Debye temperature and the Einstein temperature are significantly reduced around the critical concentration xc~0.9. The superconducting transition temperature is also found to peak around x=0.9, and this is accompanied by a remarkable enhancement in 2Δ/kBTc and ΔC/γTc far beyond the BCS values. The stannide (CaxSr1-x)3Rh4Sn13 thus provides a model system to study the interplay between structural quantum criticality and phonon-mediated superconductivity. This work is published in Physical Review Letters.

March 2015 As part of an international collaboration with scientists from Cambridge, Oxford, Imperial College and Kyoto, we investigated the entire substitution series of (CaxSr1-x)3Rh4Sn13 under pressure. We found that a second-order phase transition of structural origin can be completely suppressed by a suitable combination of pressure and calcium concentration. In particular, the structural transition temperature T* reaches zero when xc~0.9, without applying pressure. The accessibility of the structural quantum critical point will enable a wide range of techniques to be applied to extract the physics of structural quantum criticality. This work is published in Physical Review Letters.

November 2014 We have investigated the pressure dependence of the superconducting critical temperature, Tc, in TlNi2Se2-xSx. We found that the superconducting state collapses suddenly at moderate pressure, and for TlNi2SeS, a dome-shaped pressure dependence of Tc is detected. In order to understand our experimental observation, we also conducted band structure calculations to track the evolution of the Fermi surfaces in TlNi2Se2-xSx. This study is published in Physical Review B (Rapid Communication).