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H. C. Lei

Publications and source records attributed to H. C. Lei.

31 records · Page 2Linked to original sources
arXiv2020

Observation of a Chiral Wave Function in Twofold Degenerate Quadruple Weyl System BaPtGe

Topological states in quantum materials are defined by non-trivial topological invariants, such as the Chern number, which are properties of their bulk wave functions. A remarkable consequence of topological wave functions is the emergence of edge modes, a phenomenon known as bulk-edge correspondence, that gives rise to quantized or chiral physical properties. While edge modes are widely presented as signatures of non-trivial topology, how bulk wave functions can manifest explicitly topological properties remains unresolved. Here, using high-resolution inelastic x-ray spectroscopy (IXS) combined with first principles calculations, we report experimental signatures of chiral wave functions in the bulk phonon spectrum of BaPtGe, which we show to host a previously undiscovered twofold degenerate quadruple Weyl node. The chirality of the degenerate phononic wave function yields a non-trivial phonon dynamical structure factor, S(Q,$ω$), along high-symmetry directions, that is in excellent agreement with numerical and model calculations. Our results establish IXS as a powerful tool to uncover topological wave functions, providing a key missing ingredient in the study of topological quantum matter.

cond-mat.mtrl-sci↗
arXiv2020

Spin-reorientation-induced band gap in Fe$_3$Sn$_2$: Optical signatures of Weyl nodes

Temperature- and frequency-dependent infrared spectroscopy identifies two contributions to the electronic properties of the magnetic kagome metal Fe$_3$Sn$_2$: two-dimensional Dirac fermions and strongly correlated flat bands. The interband transitions within the linearly dispersing Dirac bands appear as a two-step feature along with a very narrow Drude component due to intraband contribution. Low-lying absorption features indicate flat bands with multiple van Hove singularities. Localized charge carriers are seen as a Drude-peak shifted to finite frequencies. The spectral weight is redistributed when the spins are reoriented at low temperatures; a sharp mode appears suggesting the opening of a gap due to the spin reorientation as the sign of additional Weyl nodes in the system.

cond-mat.str-el↗
arXiv2019

Phononic Helical Nodal Lines with $\mathcal{PT}$ Protection in MoB$_{2}$

While condensed matter systems host both Fermionic and Bosonic quasi-particles, reliably predicting and empirically verifying topological states is only mature for Fermionic electronic structures, leaving topological Bosonic excitations sporadically explored. This is unfortunate, as Bosonic systems such a phonons offer the opportunity to assess spinless band structures where nodal lines can be realized without invoking special additional symetries to protect against spin-orbit coupling. Here we combine first-principles calculations and meV-resolution inelastic x-ray scattering to demonstrate the first realization of parity-time reversal ($\mathcal{PT}$) symmetry protected helical nodal lines in the phonon spectrum of MoB$_{2}$. This structure is unique to phononic systems as the spin-orbit coupling present in electronic systems tends to lift the degeneracy away from high-symmetry locations. Our study establishes a protocol to accurately identify topological Bosonic excitations, opening a new route to explore exotic topological states in crystalline materials.

cond-mat.mtrl-sci↗
arXiv2019

Tunable Berry Curvature Through Magnetic Phase Competition in a Topological Kagome Magnet

Magnetic topological phases of quantum matter are an emerging frontier in physics and material science. Along these lines, several kagome magnets have appeared as the most promising platforms. However, the magnetic nature of these materials in the presence of topological state remains an unsolved issue. Here, we explore magnetic correlations in the kagome magnet Co_3Sn_2S_2. Using muon spin-rotation, we present evidence for competing magnetic orders in the kagome lattice of this compound. Our results show that while the sample exhibits an out-of-plane ferromagnetic ground state, an in-plane antiferromagnetic state appears at temperatures above 90 K, eventually attaining a volume fraction of 80% around 170 K, before reaching a non-magnetic state. Strikingly, the reduction of the anomalous Hall conductivity above 90 K linearly follows the disappearance of the volume fraction of the ferromagnetic state. We further show that the competition of these magnetic phases is tunable through applying either an external magnetic field or hydrostatic pressure. Our results taken together suggest the thermal and quantum tuning of Berry curvature field via external tuning of magnetic order. Our study shows that Co_3Sn_2S_2 is a rare example where the magnetic competition drives the thermodynamic evolution of the Berry curvature field, thus tuning its topological state.

cond-mat.mtrl-sci↗
arXiv2019

Local orbital degeneracy lifting as a precursor to an orbital-selective Peierls transition

Fundamental electronic principles underlying all transition metal compounds are the symmetry and filling of the $d$-electron orbitals and the influence of this filling on structural configurations and responses. Here we use a sensitive local structural technique, x-ray atomic pair distribution function analysis, to reveal the presence of fluctuating local-structural distortions at high temperature of one such compound, \cis . We show that this hitherto overlooked fluctuating symmetry lowering is electronic in origin and will significantly modify the energy-level spectrum and electronic and magnetic properties. The explanation is a local, fluctuating, orbital-degeneracy-lifted state. The natural extension of our result would be that this phenomenon is likely to be widespread amongst diverse classes of partially filled nominally degenerate d-electron systems, with potentially broad implications for our understanding of their properties.

cond-mat.mtrl-sci↗
arXiv2017

High-Tc superconductivity up to 55 K under high pressure in the heavily electron doped Lix(NH3)yFe2Se2 single crystal

We report a high-pressure study on the heavily electron doped Lix(NH3)yFe2Se2 single crystal by using the cubic anvil cell apparatus. The superconducting transition temperature Tc = 44 K at ambient pressure is first suppressed to below 20 K upon increasing pressure to Pc = 2 GPa, above which the pressure dependence of Tc(P) reverses and Tc increases steadily to ca. 55 K at 11 GPa. These results thus evidenced a pressure-induced second high-Tc superconducting (SC-II) phase in Lix(NH3)yFe2Se2 with the highest Tcmax = 55K among the FeSe-based bulk materials. Hall data confirm that in the emergent SC-II phase the dominant electron-type carrier density undergoes a fourfold enhancement and tracks the same trend as Tc(P). Interesting, we find a nearly parallel scaling behavior between Tc and the inverse Hall coefficient for the SC-II phases of both Lix(NH3)yFe2Se2 and (Li,Fe)OHFeSe. The present work demonstrates that high pressure offers a distinctive means to further raising the maximum Tc of heavily electron doped FeSe-based materials by increasing the effective charge carrier concentration via a plausible Fermi surface reconstruction at Pc.

cond-mat.supr-con↗
arXiv2017

Two-dimensional Superconductivity from Dimerization of Atomically Ordered AuTe2Se4/3 Cubes

The emergent phenomena such as superconductivity and topological phase transitions can be observed in strict two-dimensional crystalline matters. Artificial interfaces and one atomic thickness layers are typical 2D materials of this kind. Although having 2D characters, most bulky layered compounds, however, do not possess these striking properties. Here, we report the 2D superconductivity in bulky AuTe2Se4/3,where the reduction in dimensionality is achieved through inducing the elongated covalent Te-Te bonds. The atomic-resolution images reveal that the Au, Te and Se are atomically ordered in a cube, among which are Te-Te bonds of 3.18 A and 3.28 A. The superconductivity at 2.85 K is discovered, which is unraveled to be the quasi-2D nature owing to the BKT topological transition. The nesting of nearly parallel Fermi sheets could give rise to strong electron-phonon coupling. It is proposed to further depleting the thickness could result in more topologically-related phenomena.

cond-mat.supr-con↗
arXiv2017

Evidence of topological insulator state in the semimetal LaBi

By employing angle-resolved photoemission spectroscopy combined with first-principles calculations, we performed a systematic investigation on the electronic structure of LaBi, which exhibits extremely large magnetoresistance (XMR), and is theoretically predicted to possess band anticrossing with nontrivial topological properties. Here, the observations of the Fermi-surface topology and band dispersions are similar to previous studies on LaSb [Phys. Rev. Lett. 117, 127204 (2016)], a topologically trivial XMR semimetal, except the existence of a band inversion along the $Γ$-$X$ direction, with one massless and one gapped Dirac-like surface state at the $X$ and $Γ$ points, respectively. The odd number of massless Dirac cones suggests that LaBi is analogous to the time-reversal $Z_2$ nontrivial topological insulator. These findings open up a new series for exploring novel topological states and investigating their evolution from the perspective of topological phase transition within the family of rare-earth monopnictides.

cond-mat.mtrl-sci↗
arXiv2013

Structural phase transition induced by van Hove singularity in 5d transition metal compound IrTe2

Comprehensive studies of the electronic states of Ir 5d and Te 5p have been performed to elucidate the origin of the structural phase transition in IrTe2 by combining angle-resolved photoemission spectroscopy and resonant inelastic X-ray scattering. While no considerable changes are observed in the configuration of the Ir 5d electronic states across the transition, indicating that the Ir 5d orbitals are not involved in the transition, we reveal a van Hove singularity at the Fermi level (EF) related to the Te px+py orbitals, which is removed from EF at low temperatures. The wavevector connecting the adjacent saddle points is consistent with the in-plane projection of the superstructure modulation wavevector. These results can be qualitatively understood with the Rice-Scott "saddle-point" mechanism, while effects of the lattice distortions need to be additionally involved.

cond-mat.supr-con↗
arXiv2011

77Se NMR Investigation of the K(x)Fe(2-y)Se(2) High Tc Superconductor (Tc=33K)

We report a comprehensive 77Se NMR study of the structural, magnetic, and superconducting properties of a single crystalline sample of the newly discovered FeSe-based high temperature superconductor K(x)Fe(2-y)Se(2) (Tc=33K) in a broad temperature range up to 290 K. We will compare our results with those reported for FeSe (Tc=9K) and FeAs-based high Tc systems.

cond-mat.supr-con↗
arXiv2009

Superconductivity and single crystal growth of Ni0:05TaS2

Superconductivity was discovered in a Ni0:05TaS2 single crystal. A Ni0:05TaS2 single crystal was successfully grown via the NaCl/KCl flux method. The obtained lattice constant c of Ni0:05TaS2 is 1.1999 nm, which is significantly smaller than that of 2H-TaS2 (1.208 nm). Electrical resistivity and magnetization measurements reveal that the superconductivity transition temperature of Ni0:05TaS2 is enhanced from 0.8 K (2H-TaS2) to 3.9 K. The charge-density-wave transition of the matrix compound 2H-TaS2 is suppressed in Ni0:05TaS2. The success of Ni0:05TaS2 single crystal growth via a NaCl/KCl flux demonstrates that NaCl/KCl flux method will be a feasible method for single crystal growth of the layered transition metal dichalcogenides.

cond-mat.supr-con↗
arXiv2008

Crystal growth and superconductivity of FeSe_x

Single crystals FeSe_x have been grown in evacuated sealed quartz tube using a NaCl/KCl flux. The products include two crystal structures of tetragon and hexagon. The electronic transport and magnetic properties measurements of FeSe_x single crystal exhibits a superconducting transition at about 10K.

cond-mat.supr-con↗