arXiv subjects

S. J. Zhang

Publications and source records attributed to S. J. Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Revealing Hidden Inversion Symmetry Breaking in ZrTe$_5$ via Phonon-Assisted Heterodyne Amplification

ZrTe$_5$ is a sensitive topological material where small perturbations can alter its electronic structure. Its equilibrium crystal structure has been widely regarded as centrosymmetric, while recent experiments have raised the possibility of inversion-symmetry breaking. Here we probe this hidden symmetry lowering using nonlinear optical spectroscopy. Although conventional second-harmonic generation does not resolve an equilibrium symmetry-breaking signal, terahertz-field-induced second-harmonic generation (TFISH) reveals it through phonon-assisted heterodyne amplification. A coherently driven infrared-active phonon acts as a local oscillator for the vanishingly weak second-order susceptibility $χ^{(2)}$, converting an otherwise undetectable symmetry-breaking response into a phonon-frequency modulation of the TFISH signal. The field-linear scaling of this modulation demonstrates $χ^{(2)}$ is an equilibrium susceptibility rather than a response induced by the THz field. Polarization- and temperature-dependent measurements identify a bulk polar distortion along the crystallographic $a$ axis that persists to room temperature, while the $c$ axis remains nonpolar. These results provide direct optical evidence for equilibrium inversion-symmetry breaking in bulk ZrTe$_5$ and establish a structural constraint for understanding its electronic and topological properties.

cond-mat.mtrl-sci↗

arXiv2026

Identifying and tracking magnetically induced polarization in Fe$_2$Mo$_3$O$_8$ by static and time-resolved second harmonic generation

Multiferroic materials offer a promising platform for ultrafast optical control of coupled magnetic and polar orders. However, a prerequisite for such control is to precisely identify how the magnetically induced polarization manifests itself on the ultrafast timescale, and then to probe its dynamics upon external perturbations. Here, we address this issue in the polar magnet Fe$_2$Mo$_3$O$_8$ by combining static and time-resolved second harmonic generation (SHG). Temperature-dependent static SHG reveals that, among the symmetry-allowed tensor elements, only $χ^{(2)}_{ccc}$ exhibits a pronounced anomaly at the antiferromagnetic transition ($T_{\rm N} \approx 60$ K), identifying the $c$-axis polar response as the primary degree of freedom coupled to the magnetic order. Guided by this result, time-resolved SHG selectively tracks the dynamics of this tensor element following ultrafast photoexcitation. We observe a rapid enhancement of the $χ^{(2)}_{ccc}$-related SHG signal, followed by biexponential recovery. The response is independent of the pump polarization, consistent with an ultrafast thermal origin, but is achieved at a fluence significantly below that required for conventional lattice heating. These results establish SHG as tensor-selective probe of ultrafast magnetoelectric dynamics and demonstrate the high sensitivity of the magnetically induced polarization in Fe$_2$Mo$_3$O$_8$ to optical excitation.

cond-mat.str-el↗

arXiv2025

Transition of blue-core helicon discharge

This study explores the transitional characteristics of blue-core helicon discharge, which to our knowledge was not particularly focused on before. Parameters are measured on recently built advanced linear plasma device, i.e. Multiple Plasma Simulation Linear Device (MPS-LD) by various diagnostics including Langmuir probe, optical emission spectrometer, and standard high-speed camera. It is found that the jump direction of electron density (from low level to high level) is opposite to that of electron temperature (from high level to low level). Electron density increases significantly and the radial profile becomes localized near the axis when the blue-core transition occurs. With increased field strength, electron density increases whereas electron temperature drops. The radial profile of electron temperature looks like a ``W" shape, i.e. minimizing around the edge of blue-core column. Electron density increases with background pressure, while electron temperature peaks around certain pressure value. High-speed videos show that the plasma column oscillates radially and experiences azimuthal instabilities with high rate once entered blue-core mode. An electromagnetic solver (EMS) based on Maxwell's equations and a cold-plasma dielectric tensor is also employed to compute the wave field and power absorption during blue-core transition, to provide more details that are valuable for understanding the transitional physics but not yet available in experiment. The results show that wave field in both radial and axial directions changes significantly during the transition, its structure differs from antenna to downstream, and the power dependence of wave magnetic field is overall opposite to that of wave electric field. This work presents comprehensive characteristics of the transitional blue-core discharge and is important to both physics understanding and practical applications.

physics.plasm-ph↗

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↗

arXiv2024

Robust spin order and fragile charge order in Na0.5CoO2 as revealed by time-resolved terahertz spectroscopy

Near-infrared (NIR) pump-terahertz (THz) probe spectroscopy is used to investigate the charge and spin exciations in a strongly correlated electron compound Na0.5CoO2. This compound exhibits a coexistence of various charge and spin orders arising from intricate interactions among charge, spin, and orbital degrees of freedom. NIR pulses create significantly diverse effects on the charge and spin orders; while the charge order is easily melted,coherent magnon excitations are present in all fluences examined. Furthermore, a novel π phase shift of the coherent magnon oscillations is observed in the pump-induced change of the terahertz electric field between regions of increasing and decreasing field change. These results unequivocally illustrate that ultrashort laser pulses enable the disentanglement of different interactions within complex systems characterized by multiple orders, providing a fresh perspective on the interplay between itinerant and localized electrons within the Co 3d t2g multiplets.

cond-mat.str-el↗

arXiv2024

Light-Induced Melting of Competing Stripe Orders without Introducing Superconductivity in La$_\mathbf{2-x}$Ba$_\mathbf{x}$CuO$_\mathbf{4}$

The ultrafast manipulation of quantum material has led to many novel and significant discoveries. Among them, the light-induced transient superconductivity in cuprates achieved by melting competing stripe orders represents a highly appealing accomplishment. However, recent investigations have shown that the notion of photoinduced superconductivity remains a topic of controversy, and its elucidation solely through c-axis time-resolved terahertz spectroscopy remains an arduous task. Here, we measure the in-plane and out-of-plane transient terahertz responses simultaneously in the stripe-ordered non-superconducting La$_{2-x}$Ba$_{x}$CuO$_4$ after near-infrared excitations. We find that although a pump-induced reflectivity edge appears in the c-axis reflectance spectrum, the reflectivity along the CuO$_2$ planes decreases simultaneously, indicating an enhancement in the scattering rate of quasiparticles. This in-plane transient response is clearly distinct from the features associated with superconducting condensation. Therefore, we conclude the out-of-plane transient responses cannot be explained by an equivalent of Josephson tunneling. Notably, those pump-induced terahertz responses remain consistent even when we vary the near-infrared optical pump wavelengths and hole concentrations. Our results provide a critical evidence that transient three-dimensional superconductivity cannot be induced by melting the competing stripe orders with pump pulses whose photon energy is much higher than the superconducting gap of cuprates.

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↗

arXiv2023

Flat optical conductivity in the topological kagome magnet TbMn$_6$Sn$_6$

Kagome magnet TbMn$_6$Sn$_6$ is a new type of topological material that is known to support exotic quantum magnetic states. Experimental work has identified that TbMn$_6$Sn$_6$ hosts Dirac electronic states that could lead to topological and Chern quantum phases, but the optical response of the Dirac fermions of TbMn$_6$Sn$_6$ and its properties remain to be explored. Here, we perform optical spectroscopy measurement combined with first-principles calculations on single-crystal sample of TbMn$_6$Sn$_6$ to investigate the associated exotic phenomena. TbMn$_6$Sn$_6$ exhibits frequency-independent optical conductivity spectra in a broad range from 1800 to 3000 cm$^{-1}$ (220-370 meV) in experiments. The theoretical band structures and optical conductivity spectra are calculated with several shifted Fermi energy to compare with the experiment. The theoretical spectra with 0.56 eV shift for Fermi energy are well consistent with our experimental results. Besides, the massive quasi-two-dimensional (quasi-2D) Dirac bands, which have linear band dispersion in $k_x$-$k_y$ plane and no band dispersion along the $k_z$ direction, exist close to the shifted Fermi energy. According to tight-binding model analysis, the quasi-2D Dirac bands give rise to a flat optical conductivity, while its value is smaller than, about one tenth of, that from the calculations and experiments. It indicates that the other trivial bands also contribute to the flat optical conductivity.

cond-mat.str-el↗

arXiv2022

Revealing strong coupling of collective modes between superconductivity and pseudogap in cuprate superconductor by terahertz third harmonic generation

The study of interaction between different degrees of freedom in solids is of fundamental importance to understand the functionalities of materials. One striking example of such interaction is the intertwined coupling or competition between superconductivity (SC), charge density wave (CDW), pseudogap state (PG), and other exotic phases in cuprate superconductors. Recent emergence of nonlinear Terahertz (THz) third harmonic generation (THG) spectroscopy provides a powerful tool for exploring the collective (Higgs) modes of superconductivity order parameters, and its interaction with intertwined/competing phases. In this study, we report on nonlinear THz THG spectroscopy of the YBa$_2$Cu$_3$O$_{6+x}$ (YBCO) thin films with different doping. We identify a characteristic temperature $T_{THG}$, below which third order suscepetility $χ^{(3)}$ emerges. Notably, the $T_{THG}$ is coincident with the crossover temperature $T^*$ of pseudogap in a wide range doping of phase diagram. Upon entering the superconducting state, THG increases sharply but exhibits an abnormal dip feature near $T_c$ which is more clearly seen in optimally doped sample. Strikingly, we observe a beating structure directly in the measured real time waveform of THG signal. Fourier transformation of the time domain waveform gives two separate modes below and above original THG frequency. The observation strongly indicates that an additional mode, presumably Higgs mode, appears at $T_c$ and couples to the mode already developed below $T^*$. The strong coupling effect offers new insight into the interplay between superconductivity and pseudogap. The result unambiguously suggests that the pseudogap phase is not a precursor of superconductivity but represents a distinct order.

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

Optical spectroscopy and ultrafast pump-probe study of a quasi-one-dimensional charge density wave in CuTe

CuTe is a two-dimensional (2D) layered material, yet forming a quasi-one-dimensional (quasi-1D) charge-density-wave (CDW) along the a-axis in the ab-plane at high transition temperature $T_{CDW}=335$ K. However, the anisotropic properties of CuTe remain to be explored. Here we performed combined transport, polarized infrared reflectivity, and ultrafast pump-probe spectroscopy to investigate the underlying CDW physics of CuTe. Polarized optical measurement clearly revealed that an energy gap gradually forms along the a-axis upon cooling, while optical evidence of gap signature is absent along the b-axis, suggesting pronounced electronic anisotropy in this quasi-2D material. Time-resolved optical reflectivity measurement revealed that the amplitude and relaxation time of photo-excited quasiparticles change dramatically across the CDW phase transition. Taking fast Fourier transformation of the oscillation signals arising from collective excitations, we identify the 1.65-THz mode as the CDW amplitude mode, whose energy softens gradually at elevated temperatures. Consequently, we provide further evidence for the formation of completely anisotropic CDW order in CuTe, which is quite rare in quasi-2D materials.

cond-mat.str-el↗

arXiv2021

Optical spectroscopy and ultrafast pump-probe study of structural phase transition in 1T'-TaTe2

1T'-TaTe2 exhibits an intriguing first-order structural phase transition at around 170 K. Understanding the electronic structural properties is a crucial way to comprehend the origin of the structural phase transition. We performed a combined optical and ultrafast pump-probe study on the compound across the transition temperature. The phase transition leads to abrupt changes of both optical spectra and ultrafast electronic relaxation dynamics. The measurements revealed a sudden reconstruction of band structure. We elaborate that the phase transition is of the first order and can not be attributed to the conventional density-wave type instability. Our work is illuminating for understanding the origin of the structural phase transition.

cond-mat.str-el↗

arXiv2021

Magnetic excitations of the field induced states in BaCo2(AsO4)2 probed by time-domain terahertz spectroscopy

Searching for Kitaev quantum spin liquid (QSL) is a fascinating and challenging problem. Much effort has been devoted to honeycomb lattice candidates with strong spin-orbit coupling in 5d-electron iridates and 4delectron RuCl3. Recently, theoretical studies suggested that the 3d7 Co-based honeycomb materials with high spin state S=3/2 and effective orbital angular momentum L=1 could also be promising candidates of Kitaev QSL. One of the candidates, BaCo2(AsO4)2, was revisited recently. The long range magnetic order in BaCo2(AsO4)2 can be suppressed by very weak in-plane magnetic field, suggesting its proximity to Kitaev QSL. Here we perform time domain terahertz spectroscopy measurement to study the magnetic excitations on BaCo2(AsO4)2. We observe different magnon excitations upon increasing external magnetic field. In particular, the system is easily driven to a field-polarized paramagnetic phase, after the long range magnetic order is suppressed by a weak field Hc 2. The spectra beyond Hc2 are dominated by single magnon and two magnon excitations without showing signature of QSL. We discuss the similarity and difference of the excitation spectra between BaCo2(AsO4)2 and the widely studied Kitaev QSL candidate RuCl3.

cond-mat.str-el↗

arXiv2021

Unconventional charge density wave and photoinduced lattice symmetry change in Kagome Metal CsV$_3$Sb$_5$ probed by time-resolved spectroscopy

Recently, kagome lattice metal AV$_3$Sb$_5$ (A = K, Rb, Cs) family has received wide attention due to its presence of superconductivity, charge density wave (CDW) and peculiar properties from topological nontrivial electronic structure. With time-resolved pump-probe spectroscopy, we show that the excited quasiparticle relaxation dynamics can be explained by formation of energy gap below the phase transition being similar to a usual second-order CDW condensate, by contrast, the structure change is predominantly first order phase transition. Furthermore, no CDW amplitude mode is identified in the ordered phase. The results suggest that the CDW order is very different from the traditional CDW condensate. We also find that weak pump pulse can non-thermally melt the CDW order and drive the sample into its high temperature phase, revealing the fact that the difference in lattice potential between those phases is small.

cond-mat.supr-con↗