W. Z. Hu

Boston University

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Gang Li

Hong Kong Polytechnic University

P. Zheng

Chinese Academy of Sciences

Jing Dong

National Engineering Research Center for Information Technology in Agriculture

Z. Li

Chinese Academy of Sciences

Genfu Chen

Czech Academy of Sciences, Institute of Physics

A. Cavalleri

University of Oxford

D. Nicoletti

Max Planck Institute for the Structure and Dynamics of Matter

Gui Chen

South China Normal University

B. Keimer

Max Planck Institute for Solid State Research

Zhiguo Chen

Institute of Physics

Cooperative Institutions

Chinese Academy of Sciences

Institute of Physics

University of Chinese Academy of Sciences

National Laboratory for Superconductivity

Universität Hamburg

Max Planck Institute for the Structure and Dynamics of Matter

Czech Academy of Sciences, Institute of Physics

Max Planck Society

Center for Free-Electron Laser Science

Max Planck Institute for Solid State Research

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Transient gap generation in BaFe$_2$As$_2$ driven by coherent lattice vibrations

arXiv (Cornell University) (2022)

Jacob A. Warshauer Daniel Alejandro Bustamante Lopez Qingxin Dong Genfu Chen W. Z. Hu

The electronic structure and the magnetic properties of iron-based superconductors are highly sensitive to the pnictogen height. Coherent excitation of the $A_{1g}$ phonon by femtosecond laser directly modulates the pnictogen height, which has been used to control the physical properties of iron-based superconductors. Previous studies show that the driven $A_{1g}$ phonon resulted in a transient increase of the pnictogen height in BaFe$_2$As$_2$, favoring an enhanced Fe magnetic moment. Here, we use time-resolved broadband terahertz spectroscopy to investigate the dynamics of BaFe$_2$As$_2$ in the $A_{1g}$ phonon driven state. Below the spin-density wave (SDW) transition temperature, we observe a transient gap generation at early time delays. A similar transient feature is observed in the normal state up to room temperature.

Pnictogen

Transient (computer programming)

10.48550/arxiv.2212.03337

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Element substitution effect in transition metal oxypnictide Re(O$_{1-x}$F$_x$)TAs (Re=rare earth, T=transition metal)

arXiv (Cornell University) (2008)

G. F. Chen Z. Li Di Wu J. K. Dong G. Li

Different element substitution effects in transition metal oxypnictide Re(O$_{1-x}$F$_x$)TAs with Re=La, Ce, Nd, Eu, Gd, Tm, T=Fe, Ni, Ru, were studied. Similar to the La- or Ce-based systems, we found that the pure NdOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. Electron doping by F increases T$_c$ to about 50 K. While in the case of Gd, the T$_c$ is reduced below 10 K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity was found when La was replaced by Ce in both cases, instead a ferromagnetic ordering transition was likely to form at low temperature in F-doped sample. We also synthesized LaO$_{1-x}$F$_x$RuAs and CeO$_{1-x}$F$_x$RuAs compounds. Metallic behavior was observed down to 4 K.

Tetragonal crystal system

10.48550/arxiv.0803.4384

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Coherent Control of High Harmonic Generation via Dual-Gas Multijet Arrays

Physical Review Letters (2011)

A. Willner F. Tavella M. Yeung T. Dzelzainis Christos Kamperidis

High harmonic generation (HHG) is a central driver of the rapidly growing field of ultrafast science. We present a novel quasiphase-matching (QPM) concept with a dual-gas multijet target leading, for the first time, to remarkable phase control between multiple HHG sources (>2) within the Rayleigh range. The alternating jet structure with driving and matching zones shows perfect coherent buildup for up to six QPM periods. Although not in the focus of the proof-of-principle studies presented here, we achieved competitive conversion efficiencies already in this early stage of development.

Harmonic

10.1103/physrevlett.107.175002

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Citations (87)

Evidence for a band broadening across the ferromagnetic transition ofCr1/3NbSe2

Physical Review B (2008)

W. Z. Hu Guangtao Wang Rongwei Hu C. Petrović E. Morosan

The electronic structure of ${\text{Cr}}_{1/3}{\text{NbSe}}_{2}$ is studied via optical spectroscopy. We observe two low-energy interband transitions in the paramagnetic phase, which split into four peaks as the compound enters the ferromagnetic state. The band structure calculation indicates the four peaks are interband transitions to the spin up Cr ${e}_{g}$ states. We show that the peak splitting below the Curie temperature is not due to the exchange splitting of spin-up and spin-down bands, but directly reflects a band broadening effect in Cr-derived states upon the spontaneous ferromagnetic ordering.

10.1103/physrevb.78.085120

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High energy pseudogap and its evolution with doping in Fe-based superconductors as revealed by optical spectroscopy

Journal of Physics Condensed Matter (2012)

N L Wang W. Z. Hu Zhiguo Chen R. H. Yuan Gang Li

We report optical spectroscopic measurements on electron- and hole-doped BaFe2As2. We show that the compounds in the normal state are not simple metals. The optical conductivity spectra contain, in addition to the free carrier response at low frequency, a temperature-dependent gap-like suppression at rather high energy scale near 0.6 eV. This suppression evolves with the As-Fe-As bond angle induced by electron- or hole-doping. Furthermore, the feature becomes much weaker in the Fe-chalcogenide compounds. We elaborate that the feature is caused by the strong Hund's rule coupling effect between the itinerant electrons and localized electron moment arising from the multiple Fe 3d orbitals. Our experiments demonstrate the coexistence of itinerant and localized electrons in iron-based compounds, which would then lead to a more comprehensive picture about the metallic magnetism in the materials.

Pseudogap

Optical conductivity

Magnetism

10.1088/0953-8984/24/29/294202

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Electronic structure: Wide-band, narrow-band, and strongly correlated systems - Optical study of the charge-density-wave mechanism in 2H-TaS2 and NaxTaS2 (5 pages)

Gang Li Guorong Wu N L Wang Jiaxi Yan Haoliang Wen

Charge Density Wave

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Origin of the structural phase transition at 130 K in BaNi$_2$As$_2$: a combined study of optical spectroscopy and band structure calculations

arXiv (Cornell University) (2009)

Z. G. Chen Gang Xu W. Z. Hu X. D. Zhang P. Zheng

BaNi$_2$As$_2$ exhibits a first order structural transition at 130 K. Understanding this structural transition is a crucial step towards understanding the electronic properties of the material. We present a combined optical spectroscopy and band structure calculation study on the compound across the transition. The study reveals that BaNi$_2$As$_2$ is a good metal with a rather high plasma frequency. The phase transition leads to a small reduction of conducting carriers. We identify that this reduction is caused by the removal of several small Fermi surface sheets contributed dominantly from the As-As bonding and Ni-As antibonding states.

Antibonding molecular orbital

10.48550/arxiv.0905.0841

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Chiral Spin-Liquid-Like State in Pyrochlore Iridate Thin Films

arXiv (Cornell University) (2024)

Xiaoran Liu Jong‐Woo Kim Yao Wang Michael Terilli Xun Jia

The pyrochlore iridates have become ideal platforms to unravel fascinating correlated and topolog?ical phenomena that stem from the intricate interplay among strong spin-orbit coupling, electronic correlations, lattice with geometric frustration, and itinerancy of the 5d electrons. The all-in-all?out antiferromagnetic state, commonly considered as the magnetic ground state, can be dramatically altered in reduced dimensionality, leading to exotic or hidden quantum states inaccessible in bulk. Here, by means of magnetotransport, resonant elastic and inelastic x-ray scattering experiments, we discover an emergent quantum disordered state in (111) Y2Ir2O7 thin films (thickness less than 30 nm) per?sisting down to 5 K, characterized by dispersionless magnetic excitations. The anomalous Hall effect observed below an onset temperature near 135 K corroborates the presence of chiral short-range spin configurations expressed in non-zero scalar spin chirality, breaking the macroscopic time-reversal symmetry. The origin of this chiral state is ascribed to the restoration of magnetic frustration on the pyrochlore lattice in lower dimensionality, where the competing exchange interactions together with enhanced quantum fluctuations suppress any long-range order and trigger spin-liquid-like behavior with degenerate ground-state manifold.

10.48550/arxiv.2403.06170

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Optical properties of FeAs-based parent compound: A comparative study for polycrystalline EuFe2As2 and LaFeAsO

Frontiers of Physics in China (2009)

W. Z. Hu Jing Dong Gang Li Z. Li P. Zheng

Optical conductivity

10.1007/s11467-009-0044-4

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Citations (2)

Anomalous Metallic State ofCu0.07TiSe2: An Optical Spectroscopy Study

Physical Review Letters (2007)

Gang Li W. Z. Hu J. K. Dong Dong Qian David Hsieh

We report an optical spectroscopy study on the newly discovered superconductor Cu0.07TiSe2. Consistent with the development from a semimetal or semiconductor with a very small indirect energy gap upon doping TiSe2, it is found that the compound has a low carrier density. Most remarkably, the study reveals a substantial shift of the screened plasma edge in reflectance towards high energy with decreasing temperature. This phenomenon, rarely seen in metals, indicates either a sizable increase of the conducting carrier concentration or/and a decrease of the effective mass of carriers with reducing temperature. We attribute the shift primarily to the latter effect.

10.1103/physrevlett.99.167002

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Citations (14)