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Vortex nucleations in spinor Bose condensates under localized synthetic magnetic fields
Authors:
L. -R. Liu,
S. -C. Wu,
T. -W. Liu,
H. -Y. Hsu,
T. -K. Shen,
S. -K. Yip,
Y. Kawaguchi,
Y. -J. Lin
Abstract:
Gauge fields are ubiquitous in modern quantum physics. In superfluids, quantized vortices can be induced by gauge fields. Here we demonstrate the first experimental observation of vortex nucleations in light-dressed spinor Bose-Einstein condensates under radially-localized synthetic magnetic fields. The light-induced spin-orbital-angular-momentum coupling creates azimuthal gauge potentials…
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Gauge fields are ubiquitous in modern quantum physics. In superfluids, quantized vortices can be induced by gauge fields. Here we demonstrate the first experimental observation of vortex nucleations in light-dressed spinor Bose-Einstein condensates under radially-localized synthetic magnetic fields. The light-induced spin-orbital-angular-momentum coupling creates azimuthal gauge potentials $\vec{A}$ for the lowest-energy spinor branch dressed eigenstate. The observation of the atomic wave function in the lowest-energy dressed eigenstate reveals that vortices nucleate from the cloud center of a vortex-free state with canonical momentum $\vec{p} = 0$. This is because a large circulating azimuthal velocity field $\propto \vec{p}-\vec{A}$ at the condensate center results in a dynamically unstable localized excitation that initiates vortex nucleations. Furthermore, the long-time dynamics to reach the ground state stops in a metastable state when $|\vec{A}|$ is not sufficiently large. Our observation has reasonable agreement with the time-dependent Gross-Pitaevskii simulations.
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Submitted 1 September, 2025; v1 submitted 26 March, 2024;
originally announced March 2024.
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Weyl nodes close to the Fermi energy in NbAs
Authors:
M. Naumann,
F. Arnold,
Z. Medvecka,
S. -C. Wu,
V. Suess,
M. Schmidt,
B. Yan,
N. Huber,
L. Worch,
M. A. Wilde,
C. Felser,
Y. Sun,
E. Hassinger
Abstract:
The noncentrosymmetric transition metal monopnictides NbP, TaP, NbAs and TaAs are a family of Weyl semimetals in which pairs of protected linear crossings of spin-resolved bands occur. These so-called Weyl nodes are characterized by integer topological charges of opposite sign associated with singular points of Berry curvature in momentum space. In such a system anomalous magnetoelectric responses…
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The noncentrosymmetric transition metal monopnictides NbP, TaP, NbAs and TaAs are a family of Weyl semimetals in which pairs of protected linear crossings of spin-resolved bands occur. These so-called Weyl nodes are characterized by integer topological charges of opposite sign associated with singular points of Berry curvature in momentum space. In such a system anomalous magnetoelectric responses are predicted, which should only occur if the crossing points are close to the Fermi level and enclosed by Fermi surface pockets penetrated by an integer flux of Berry curvature, dubbed Weyl pockets. TaAs was shown to possess Weyl pockets whereas TaP and NbP have trivial pockets enclosing zero net flux of Berry curvature. Here, via measurements of the magnetic torque, resistivity and magnetisation, we present a comprehensive quantum oscillation study of NbAs, the last member of this family where the precise shape and nature of the Fermi surface pockets is still unknown. We detect six distinct frequency branches, two of which have not been observed before. A comparison to density functional theory calculations suggests that the two largest pockets are topologically trivial, whereas the low frequencies might stem from tiny Weyl pockets. The enclosed Weyl nodes are within a few meV of the Fermi energy.
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Submitted 25 May, 2021;
originally announced May 2021.
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High magnetic field ultrasound study of spin freezing in La$_{1.88}$Sr$_{0.12}$CuO$_4$
Authors:
M. Frachet,
S. Benhabib,
I. Vinograd,
S. -F. Wu,
B. Vignolle,
H. Mayaffre,
S. Krämer,
T. Kurosawa,
N. Momono,
M. Oda,
J. Chang,
C. Proust,
M. -H. Julien,
D. LeBoeuf
Abstract:
High-$T_{\rm{c}}$ cuprate superconductors host spin, charge and lattice instabilities. In particular, in the antiferromagnetic glass phase, over a large doping range, lanthanum based cuprates display a glass-like spin freezing with antiferromagnetic correlations. Previously, sound velocity anomalies in La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO) for hole doping $p\geq 0.145$ were reported and interpreted as…
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High-$T_{\rm{c}}$ cuprate superconductors host spin, charge and lattice instabilities. In particular, in the antiferromagnetic glass phase, over a large doping range, lanthanum based cuprates display a glass-like spin freezing with antiferromagnetic correlations. Previously, sound velocity anomalies in La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO) for hole doping $p\geq 0.145$ were reported and interpreted as arising from a coupling of the lattice to the magnetic glass [Frachet, Vinograd et al., Nat. Phys. 16, 1064-1068 (2020)]. Here we report both sound velocity and attenuation in LSCO $p=0.12$, i.e. at a doping level for which the spin freezing temperature is the highest. Using high magnetic fields and comparing with nuclear magnetic resonance (NMR) measurements, we confirm that the anomalies in the low temperature ultrasound properties of LSCO are produced by a coupling between the lattice and the spin glass. Moreover, we show that both sound velocity and attenuation can be simultaneously accounted for by a simple phenomenological model originally developed for canonical spin glasses. Our results point towards a strong competition between superconductivity and spin freezing, tuned by the magnetic field. A comparison of different acoustic modes suggests that the slow spin fluctuations have a nematic character.
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Submitted 1 November, 2020;
originally announced November 2020.
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Superconductivity and phonon self-energy effects in Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$
Authors:
S. -F. Wu,
A. Almoalem,
I. Feldman,
A. Lee,
A. Kanigel,
G. Blumberg
Abstract:
We study Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ multi-band superconductor with $T_c=14$K by polarization-resolved Raman spectroscopy. Deep in the superconducting state, we detect pair-breaking excitation at 45cm$^{-1}$ ($2Δ=5.6$meV) in the $XY$($B_{2g}$) scattering geometry, consistent with twice of the superconducting gap energy (3 meV) revealed by ARPES on the hole-like Fermi pocket with…
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We study Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ multi-band superconductor with $T_c=14$K by polarization-resolved Raman spectroscopy. Deep in the superconducting state, we detect pair-breaking excitation at 45cm$^{-1}$ ($2Δ=5.6$meV) in the $XY$($B_{2g}$) scattering geometry, consistent with twice of the superconducting gap energy (3 meV) revealed by ARPES on the hole-like Fermi pocket with $d_{xz}/d_{yz}$ character. We analyze the superconductivity induced phonon self-energy effects for the $B_{1g}$(Fe) phonon and estimate the electron-phonon coupling constant $λ^Γ\approx 0.026$, which is insufficient to explain superconductivity with $T_c=14$K.
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Submitted 17 November, 2019;
originally announced November 2019.
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Observation of Landau quantization and standing waves in HfSiS
Authors:
L. Jiao,
Q. N. Xu,
Y. P. Qi,
S. -C. Wu,
Y. Sun,
C. Felser,
S. Wirth
Abstract:
Recently, HfSiS was found to be a new type of Dirac semimetal with a line of Dirac nodes in the band structure. Meanwhile, Rashba-split surface states are also pronounced in this compound. Here we report a systematic study of HfSiS by scanning tunneling microscopy/spectroscopy at low temperature and high magnetic field. The Rashba-split surface states are characterized by measuring Landau quantiza…
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Recently, HfSiS was found to be a new type of Dirac semimetal with a line of Dirac nodes in the band structure. Meanwhile, Rashba-split surface states are also pronounced in this compound. Here we report a systematic study of HfSiS by scanning tunneling microscopy/spectroscopy at low temperature and high magnetic field. The Rashba-split surface states are characterized by measuring Landau quantization and standing waves, which reveal a quasi-linear dispersive band structure. First-principles calculations based on density-functional theory are conducted and compared with the experimental results. Based on these investigations, the properties of the Rashba-split surface states and their interplay with defects and collective modes are discussed.
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Submitted 24 May, 2018;
originally announced May 2018.
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Decoupled Pairing Amplitude and Electronic Coherence in Iron-Based Superconductors
Authors:
H. Miao,
W. H. Brito,
Z. P. Yin,
R. D. Zhong,
G. D. Gu,
P. D. Johnson,
M. P. M. Dean,
S. Choi,
G. Kotliar,
W. Ku,
X. C. Wang,
C. Q. Jin,
S. -F. Wu,
T. Qian,
H. Ding
Abstract:
Here we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi liquid phase (LiFeAs) and an incoherent bad-metal phase (FeTe0.55Se0.45). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum superconducting pairing ampl…
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Here we use angle-resolved photoemission spectroscopy to study superconductivity that emerges in two extreme cases, from a Fermi liquid phase (LiFeAs) and an incoherent bad-metal phase (FeTe0.55Se0.45). We find that although the electronic coherence can strongly reshape the single-particle spectral function in the superconducting state, it is decoupled from the maximum superconducting pairing amplitude, which shows a universal scaling that is valid for all FeSCs. Our observation excludes pairing scenarios in the BCS and the BEC limit for FeSCs and calls for a universal strong coupling pairing mechanism for the FeSCs.
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Submitted 20 April, 2018;
originally announced April 2018.
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On the origin of critical nematic fluctuations in pnictide superconductors
Authors:
S. -F. Wu,
W. -L. Zhang,
L. Li,
H. B. Cao,
H. -H. Kung,
A. S. Sefat,
H. Ding,
P. Richard,
G. Blumberg
Abstract:
We employ polarization-resolved Raman spectroscopy to study critical nematic fluctuations in Ba(Fe$_{1-x}$Au$_x$)$_2$As$_2$ superconductors above and across well separated tetragonal to orthorhombic phase transition at temperature $T_S(x)$ and the Néel transition at $T_N(x)$. The static Raman susceptibility in $XY$ symmetry channel increases upon cooling from room temperature following the Curie-W…
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We employ polarization-resolved Raman spectroscopy to study critical nematic fluctuations in Ba(Fe$_{1-x}$Au$_x$)$_2$As$_2$ superconductors above and across well separated tetragonal to orthorhombic phase transition at temperature $T_S(x)$ and the Néel transition at $T_N(x)$. The static Raman susceptibility in $XY$ symmetry channel increases upon cooling from room temperature following the Curie-Weiss law, with Weiss temperature $T_θ(x)$ several tens of degrees lower than $T_S(x)$. Data reveals a hidden nematic quantum critical point at $x_{c} = 0.031$ when the system becomes superconducting, indicating a direct connection between quantum critical nematic fluctuations and unconventional superconductivity. We attribute the origin of the nematicity to charge quadrupole fluctuations due to electron transfer between the nearly degenerate $d_{xz}/d_{yz}$ orbitals.
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Submitted 17 December, 2017;
originally announced December 2017.
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Anomalous magneto-elastic coupling in Au-doped BaFe2As2
Authors:
S. -F. Wu,
W. -L. Zhang,
L. Li,
H. -B. Cao,
H. -H. Kung,
A. S. Sefat,
H. Ding,
P. Richard,
G. Blumberg
Abstract:
We used polarization-resolved Raman scattering to study magneto-elastic coupling in Ba(Fe$_{1-x}$Au$_{x}$)$_2$As$_2$ crystals as a function of light Au-doping, materials for which temperatures of the structural transition ($T_S$) and of the magnetic ordering transition ($T_N$) split. We study the appearance of the $A_g$(As)phonon intensity in the $XY$ scattering geometry that is very weak just bel…
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We used polarization-resolved Raman scattering to study magneto-elastic coupling in Ba(Fe$_{1-x}$Au$_{x}$)$_2$As$_2$ crystals as a function of light Au-doping, materials for which temperatures of the structural transition ($T_S$) and of the magnetic ordering transition ($T_N$) split. We study the appearance of the $A_g$(As)phonon intensity in the $XY$ scattering geometry that is very weak just below $T_S$, but for which the intensity is significantly enhanced below $T_N$. In addition, the $A_g$(As) phonon shows an asymmetric line shape below $T_N$ and an anomalous linewidth broadening upon Au-doping in the magnetic phase. We demonstrate that the anomalous behavior of the $A_g$(As) phonon mode in the $XY$ scattering geometry can be consistently described by a Fano model involving the $A_g$(As) phonon mode interacting with the $B_{2g}$ symmetry-like magnetic continuum in which the magneto-elastic coupling constant is proportional to the magnetic order parameter.
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Submitted 5 December, 2017;
originally announced December 2017.
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Magneto-elastic coupling in Fe-based superconductors
Authors:
S. -F. Wu,
W. -L. Zhang,
V. K. Thorsmølle,
G. F. Chen,
G. T. Tan,
P. C. Dai,
Y. G. Shi,
C. Q. Jin,
T. Shibauchi,
S. Kasahara,
Y. Matsuda,
A. S. Sefat,
H. Ding,
P. Richard,
G. Blumberg
Abstract:
We used polarization-resolved Raman scattering to study the magneto-elastic coupling in the parent compounds of several families of Fe-based superconductors (BaFe2As2, EuFe2As2, NaFeAs, LiFeAs, FeSe and LaFeAsO). We observe an emergent Ag-symmetry As phonon mode in the XY scattering geometry whose intensity is significantly enhanced below the magneto-structural transition only for compounds showin…
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We used polarization-resolved Raman scattering to study the magneto-elastic coupling in the parent compounds of several families of Fe-based superconductors (BaFe2As2, EuFe2As2, NaFeAs, LiFeAs, FeSe and LaFeAsO). We observe an emergent Ag-symmetry As phonon mode in the XY scattering geometry whose intensity is significantly enhanced below the magneto-structural transition only for compounds showing magnetic ordering. We conclude that the small lattice anisotropy is insufficient to induce the in-plane electronic polarizability anisotropy necessary for the observed phonon intensity enhancement, and interpret this enhancement below the Neel temperature in terms of the anisotropy of the magnetic moment and magneto-elastic coupling. We evidence a Fano line- shape in the XY scattering geometry resulting from a strong coupling between the Ag (As) phonon mode and the B2g symmetry-like electronic continuum. Strong electron-phonon coupling may be relevant to superconductivity.
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Submitted 5 December, 2017;
originally announced December 2017.
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Quadrupolar charge dynamics in the nonmagnetic FeSe$_{1-x}$S$_x$ superconductors
Authors:
W. -L. Zhang,
S. -F. Wu,
S. Kasahara,
T. Shibauchi,
Y. Matsuda,
G. Blumberg
Abstract:
We use polarization-resolved electronic Raman spectroscopy to study charge dynamics in non-magnetic FeSe$_{1-x}$S$_x$ superconductor. We observe two features of the $XY$ quadrupole symmetry: a low-energy quasi-elastic peak (QEP) and an electronic continuum. The QEP exhibits critical enhancement upon cooling towards the structural transition at $T_S(x)$. Below $T_S(x)$, the QEP diminishes gradually…
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We use polarization-resolved electronic Raman spectroscopy to study charge dynamics in non-magnetic FeSe$_{1-x}$S$_x$ superconductor. We observe two features of the $XY$ quadrupole symmetry: a low-energy quasi-elastic peak (QEP) and an electronic continuum. The QEP exhibits critical enhancement upon cooling towards the structural transition at $T_S(x)$. Below $T_S(x)$, the QEP diminishes gradually, and a gap with temperature evolution reminiscent of a mean-field order parameter opens in the continuum. The intensity of the QEP develops with increasing sulfur doping $x$ and maximizes at $x\approx$ 0.15, while the gap magnitude decreases with the suppression of $T_S(x)$. We interpret the development of the gap in the quadrupole scattering channel as the formation of a stripe quadrupole order: a wave of quadrupole moment without charge or spin modulation.
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Submitted 21 May, 2021; v1 submitted 26 October, 2017;
originally announced October 2017.
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Stability of Weyl points in magnetic half-metallic Heusler compounds
Authors:
S. Chadov,
S. -C. Wu,
C. Felser,
I. Galanakis
Abstract:
We employ {\it ab-initio} fully-relativistic electronic structure calculations to study the stability of the Weyl points in the momentum space within the class of the half-metallic ferromagnetic full Heusler materials, by focusing on Co$_2$TiAl as a well-established prototype compound. Here we show that both the number of the Weyl points together with their $k$-space coordinates can be controlled…
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We employ {\it ab-initio} fully-relativistic electronic structure calculations to study the stability of the Weyl points in the momentum space within the class of the half-metallic ferromagnetic full Heusler materials, by focusing on Co$_2$TiAl as a well-established prototype compound. Here we show that both the number of the Weyl points together with their $k$-space coordinates can be controlled by the orientation of the magnetization. This alternative degree of freedom, which is absent in other topological materials (e.g. in Weyl semimetals), introduces novel functionalities, specific for the class of half-metallic ferromagnets. Of special interest are Weyl points which are preserved irrespectively of any arbitrary rotation of the magnetization axis.
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Submitted 15 February, 2017;
originally announced February 2017.
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Dirac Line-nodes and Effect of Spin-orbit Coupling in Non-symmorphic Critical Semimetal MSiS (M=Hf, Zr)
Authors:
C. Chen,
X. Xu,
J. Jiang,
S. -C. Wu,
Y. P. Qi,
L. X. Yang,
M. X. Wang,
Y. Sun,
N. B. M. Schröter,
H. F. Yang,
L. M. Schoop,
Y. Y. Lv,
J. Zhou,
Y. B. Chen,
S. H. Yao,
M. H. Lu,
Y. F. Chen,
C. Felser,
B. H. Yan,
Z. K. Liu,
Y. L. Chen
Abstract:
Topological Dirac semimetals (TDSs) represent a new state of quantum matter recently discovered that offers a platform for realizing many exotic physical phenomena. A TDS is characterized by the linear touching of bulk (conduction and valance) bands at discrete points in the momentum space (i.e. 3D Dirac points), such as in Na3Bi and Cd3As2. More recently, new types of Dirac semimetals with robust…
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Topological Dirac semimetals (TDSs) represent a new state of quantum matter recently discovered that offers a platform for realizing many exotic physical phenomena. A TDS is characterized by the linear touching of bulk (conduction and valance) bands at discrete points in the momentum space (i.e. 3D Dirac points), such as in Na3Bi and Cd3As2. More recently, new types of Dirac semimetals with robust Dirac line-nodes (with non-trivial topology or near the critical point between topological phase transitions) have been proposed that extends the bulk linear touching from discrete points to 1D lines. In this work, using angle-resolved photoemission spectroscopy (ARPES), we explored the electronic structure of the non-symmorphic crystals MSiS (M=Hf, Zr). Remarkably, by mapping out the band structure in the full 3D Brillouin Zone (BZ), we observed two sets of Dirac line-nodes in parallel with the kz-axis and their dispersions. Interestingly, along directions other than the line-nodes in the 3D BZ, the bulk degeneracy is lifted by spin-orbit coupling (SOC) in both compounds with larger magnitude in HfSiS. Our work not only experimentally confirms a new Dirac line-node semimetal family protected by non-symmorphic symmetry, but also helps understanding and further exploring the exotic properties as well as practical applications of the MSiS family of compounds.
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Submitted 26 January, 2017; v1 submitted 24 January, 2017;
originally announced January 2017.
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ARPES observation of Mn-pnictide hybridization and negligible band structure renormalization in BaMn$_2$As$_2$ and BaMn$_2$Sb$_2$
Authors:
W. -L. Zhang,
P. Richard,
A. van Roekeghem,
S. -M. Nie,
N. Xu,
P. Zhang,
H. Miao,
S. -F. Wu,
J. -X. Yin,
B. B. Fu,
L. -Y. Kong,
T. Qian,
Z. -J. Wang,
Z. Fang,
A. S. Sefat,
S. Biermann,
H. Ding
Abstract:
We performed an angle-resolved photoemission spectroscopy study of BaMn$_2$As$_2$ and BaMn$_2$Sb$_2$, which are isostructural to the parent compound BaFe$_2$As$_2$ of the 122 family of ferropnictide superconductors. We show the existence of a strongly $k_z$-dependent band gap with a minimum at the Brillouin zone center, in agreement with their semiconducting properties. Despite the half-filling of…
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We performed an angle-resolved photoemission spectroscopy study of BaMn$_2$As$_2$ and BaMn$_2$Sb$_2$, which are isostructural to the parent compound BaFe$_2$As$_2$ of the 122 family of ferropnictide superconductors. We show the existence of a strongly $k_z$-dependent band gap with a minimum at the Brillouin zone center, in agreement with their semiconducting properties. Despite the half-filling of the electronic 3$d$ shell, we show that the band structure in these materials is almost not renormalized from the Kohn-Sham bands of density functional theory. Our photon energy dependent study provides evidence for Mn-pnictide hybridization, which may play a role in tuning the electronic correlations in these compounds.
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Submitted 2 November, 2016; v1 submitted 22 August, 2016;
originally announced August 2016.
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Superconductivity and electronic fluctuations in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ studied by Raman scattering
Authors:
S. -F. Wu,
P. Richard,
H. Ding,
H. -H. Wen,
Guotai Tan,
Meng Wang,
Chenglin Zhang,
Pengcheng Dai,
G. Blumberg
Abstract:
Using polarization-resolved electronic Raman scattering we study under-doped, optimally-doped and over-doped Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ samples in the normal and superconducting states. We show that low-energy nematic fluctuations are universal for all studied doping range. In the superconducting state, we observe two distinct superconducting pair breaking peaks corresponding to one large and o…
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Using polarization-resolved electronic Raman scattering we study under-doped, optimally-doped and over-doped Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ samples in the normal and superconducting states. We show that low-energy nematic fluctuations are universal for all studied doping range. In the superconducting state, we observe two distinct superconducting pair breaking peaks corresponding to one large and one small superconducting gaps. In addition, we detect a collective mode below the superconducting transition in the B$_{2g}$ channel and determine the evolution of its binding energy with doping. Possible scenarios are proposed to explain the origin of the in-gap collective mode. In the superconducting state of the under-doped regime, we detect a re-entrance transition below which the spectral background changes and the collective mode vanishes.
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Submitted 13 December, 2016; v1 submitted 22 August, 2016;
originally announced August 2016.
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Collective excitations of dynamic Fermi surface deformations in BaFe$_2$(As$_{0.5}$P$_{0.5}$)$_2$
Authors:
S. -F. Wu,
W. -L. Zhang,
D. Hu,
H. -H. Kung,
A. Lee,
H. -C. Mao,
P. -C. Dai,
H. Ding,
P. Richard,
G. Blumberg
Abstract:
We use electronic Raman scattering to study the low-energy excitations in BaFe$_2$(As$_{0.5}$P$_{0.5}$)$_2$ ($T_c \approx 16$ K) samples. In addition to a superconducting pair breaking peak (2$Δ=6.7$ meV) in the A$_{1g}$ channel with a linear tail towards zero energy, suggesting a nodal gap structure, we detect spectral features associated to Pomeranchuk oscillations in the A$_{1g}$, B$_{1g}$ and…
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We use electronic Raman scattering to study the low-energy excitations in BaFe$_2$(As$_{0.5}$P$_{0.5}$)$_2$ ($T_c \approx 16$ K) samples. In addition to a superconducting pair breaking peak (2$Δ=6.7$ meV) in the A$_{1g}$ channel with a linear tail towards zero energy, suggesting a nodal gap structure, we detect spectral features associated to Pomeranchuk oscillations in the A$_{1g}$, B$_{1g}$ and B$_{2g}$ channels. We argue that the small Fermi energy of the system is an essential condition for these Pomeranchuk oscillations to be underdamped. The Pomeranchuk oscillations have the same frequencies in the B$_{1g}$ and B$_{2g}$ channels, which we explain by the mixing of these symmetries resulting from the removal of the $σ_v$ and $σ_v$ symmetry planes due to a large As/P disorder. Interestingly, we show that the temperature at which the peaks corresponding to the Pomeranchuk oscillations get underdamped is consistent with the non-Fermi liquid to Femi liquid crossover determined by transport, suggesting that the Pomeranchuk instability plays an important role in the low-energy physics of the Fe-based superconductors.
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Submitted 22 July, 2016;
originally announced July 2016.
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Angle-resolved spectroscopy study of Ni-based superconductor SrNi$_2$As$_2$
Authors:
L. -K. Zeng,
P. Richard,
A. van Roekeghem,
J. -X. Yin,
S. -F. Wu,
Z. G. Chen,
N. L. Wang,
S. Biermann,
T. Qian,
H. Ding
Abstract:
We performed an angle-resolved photoemission spectroscopy study of the Ni-based superconductor SrNi$_2$As$_2$. Electron and hole Fermi surface pockets are observed, but their different shapes and sizes lead to very poor nesting conditions. The experimental electronic band structure of SrNi$_2$As$_2$ is in good agreement with first-principles calculations after a slight renormalization (by a factor…
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We performed an angle-resolved photoemission spectroscopy study of the Ni-based superconductor SrNi$_2$As$_2$. Electron and hole Fermi surface pockets are observed, but their different shapes and sizes lead to very poor nesting conditions. The experimental electronic band structure of SrNi$_2$As$_2$ is in good agreement with first-principles calculations after a slight renormalization (by a factor 1.1), confirming the picture of Hund's exchange-dominated electronic correlations decreasing with increasing filling of the $3d$ shell in the Fe-, Co- and Ni-based compounds. These findings emphasize the importance of Hund's coupling and $3d$-orbital filling as key tuning parameters of electronic correlations in transition metal pnictides.
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Submitted 3 August, 2016; v1 submitted 15 April, 2016;
originally announced April 2016.
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Raman scattering investigation of quasi-one-dimensional superconductor Ta$_{4}$Pd$_{3}$Te$_{16}$
Authors:
D. Chen,
P. Richard,
Z. -D. Song,
W. -L. Zhang,
S. -F. Wu,
W. H. Jiao,
Z. Fang,
G. -H. Cao,
H. Ding
Abstract:
We have performed polarized Raman scattering measurements on the newly discovered superconductor Ta$_{4}$Pd$_{3}$Te$_{16}$ ($T_c = 4.6$ K). We observe twenty-eight out of thirty-three Raman active modes, with frequencies in good accordance with first-principles calculations. Although most of the phonons observed vary only slightly with temperature and do not exhibit any asymmetric profile that wou…
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We have performed polarized Raman scattering measurements on the newly discovered superconductor Ta$_{4}$Pd$_{3}$Te$_{16}$ ($T_c = 4.6$ K). We observe twenty-eight out of thirty-three Raman active modes, with frequencies in good accordance with first-principles calculations. Although most of the phonons observed vary only slightly with temperature and do not exhibit any asymmetric profile that would suggest strong electron-phonon coupling, the linewidth of the A$_{g}$ phonon mode at 89.9 cm$^{-1}$ shows an unconventional increase with temperature decreasing, which is possibly due to a charge-density-wave transition or the emergence of charge-density-wave fluctuations below a temperature estimated to fall in the 140-200 K range.
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Submitted 20 May, 2015;
originally announced May 2015.
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Raman scattering investigation of large positive magnetoresistance material WTe$_2$
Authors:
W. -D. Kong,
S. -F. Wu,
P. Richard,
C. -S. Lian,
J. -T. Wang,
C. -L. Yang,
Y. -G. Shi,
H. Ding
Abstract:
We have performed polarized Raman scattering measurements on WTe$_2$, for which an extremely large positive magnetoresistance has been reported recently. We observe 5 A$_1$ phonon modes and 2 A$_2$ phonon modes out of 33 Raman active modes, with frequencies in good accordance with first-principles calculations. The angular dependence of the intensity of the peaks observed is consistent with the Ra…
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We have performed polarized Raman scattering measurements on WTe$_2$, for which an extremely large positive magnetoresistance has been reported recently. We observe 5 A$_1$ phonon modes and 2 A$_2$ phonon modes out of 33 Raman active modes, with frequencies in good accordance with first-principles calculations. The angular dependence of the intensity of the peaks observed is consistent with the Raman tensors of the $C_{2v}$ point group symmetry attributed to WTe$_2$. Although the phonon spectra suggest neither strong electron-phonon nor spin-phonon coupling, the intensity of the A$_1$ phonon mode at 160.6 cm$^{-1}$ shows an unconventional decrease with temperature decreasing, for which the origin remains unclear.
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Submitted 15 February, 2015; v1 submitted 26 January, 2015;
originally announced January 2015.
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Camelback-shaped band reconciles heavy electron behavior with weak electronic Coulomb correlations in superconducting TlNi2Se2
Authors:
N. Xu,
C. E. Matt,
P. Richard,
A. van Roekeghem,
S. Biermann,
X. Shi,
S. -F. Wu,
H. W. Liu,
D. Chen,
T. Qian,
N. C. Plumb,
M. Radovic,
Hangdong Wang,
Qianhui Mao,
Jianhua Du,
Minghu Fang,
J. Mesot,
H. Ding,
M. Shi
Abstract:
Using high-resolution photoemission spectroscopy and first-principles calculations, we characterize superconducting TlNi$_2$Se$_2$ as a material with weak electronic Coulomb correlations leading to a bandwidth renormalization of 1.4. We identify a camelback-shaped band, whose energetic position strongly depends on the selenium height. While this feature is universal in transition metal pnictides,…
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Using high-resolution photoemission spectroscopy and first-principles calculations, we characterize superconducting TlNi$_2$Se$_2$ as a material with weak electronic Coulomb correlations leading to a bandwidth renormalization of 1.4. We identify a camelback-shaped band, whose energetic position strongly depends on the selenium height. While this feature is universal in transition metal pnictides, in TlNi$_2$Se$_2$ it lies in the immediate vicinity of the Fermi level, giving rise to a pronounced van Hove singularity. The resulting heavy band mass resolves the apparent puzzle of a large normal-state specific heat coefficient (Phys. Rev. Lett. 112, 207001) in this weakly correlated compound.
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Submitted 22 December, 2014;
originally announced December 2014.
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Raman scattering investigation of superconducting Ba$_{2}$Ti$_{2}$Fe$_{2}$As$_{4}$O
Authors:
S. -F. Wu,
P. Richard,
W. -L. Zhang,
C. -S. Lian,
Y. -L. Sun,
G. -H. Cao,
J. -T. Wang,
H. Ding
Abstract:
We have performed polarized Raman scattering measurements on the newly discovered superconductor Ba$_{2}$Ti$_{2}$Fe$_{2}$As$_{4}$O$_{2}$ ($T_c = 21$ K). We observe seven out of eight Raman active modes, with frequencies in good accordance with first-principle calculations. The phonon spectra suggest neither strong electron-phonon nor spin-phonon coupling and vary only slightly with temperature, ex…
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We have performed polarized Raman scattering measurements on the newly discovered superconductor Ba$_{2}$Ti$_{2}$Fe$_{2}$As$_{4}$O$_{2}$ ($T_c = 21$ K). We observe seven out of eight Raman active modes, with frequencies in good accordance with first-principle calculations. The phonon spectra suggest neither strong electron-phonon nor spin-phonon coupling and vary only slightly with temperature, except for one E$_g$ mode associated with large displacements of As atoms near the Ti$_2$O planes. We also identify a small anomaly around 125 K in the linewidth of a A$_{1g}$ mode involving the same As atoms. Our results suggest that the transition at 125 K is most likely driven by electronic interactions taking place in the Ti$_2$O planes.
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Submitted 29 March, 2014;
originally announced March 2014.
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Coexistence of orbital degeneracy lifting and superconductivity in iron-based superconductors
Authors:
H. Miao,
L. -M. Wang,
P. Richard,
S. -F. Wu,
J. Ma,
T. Qian,
L. -Y. Xing,
X. -C. Wang,
C. -Q. Jin,
C. -P. Chou,
Z. Wang,
W. Ku,
H. Ding
Abstract:
In contrast to conventional superconducting (SC) materials, superconductivity in high-temperature superconductors (HTCs) usually emerges in the presence of other fluctuating orders with similar or higher energy scales, thus instigating debates over their relevance for the SC pairing mechanism. In iron-based superconductors (IBSCs), local orbital fluctuations have been proposed to be directly respo…
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In contrast to conventional superconducting (SC) materials, superconductivity in high-temperature superconductors (HTCs) usually emerges in the presence of other fluctuating orders with similar or higher energy scales, thus instigating debates over their relevance for the SC pairing mechanism. In iron-based superconductors (IBSCs), local orbital fluctuations have been proposed to be directly responsible for the structural phase transition and closely related to the observed giant magnetic anisotropy and electronic nematicity. However, whether superconductivity can emerge from, or even coexist with orbital fluctuations, remains unclear. Here we report the angle-resolved photoemission spectroscopy (ARPES) observation of the lifting of symmetry-protected band degeneracy, and consequently the breakdown of local tetragonal symmetry in the SC state of Li(Fe1-xCox)As. Supported by theoretical simulations, we analyse the doping and temperature dependences of this band-splitting and demonstrate an intimate connection between ferro-orbital correlations and superconductivity.
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Submitted 29 October, 2013; v1 submitted 17 October, 2013;
originally announced October 2013.
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Investigations of the EPR parameters for the tetrahedral [FeX_4]^- clusters in AgX (X=Cl, Br)
Authors:
B. -T. Song,
S. -Y. Wu,
M. -Q. Kuang,
Z. -H. Zhang
Abstract:
The EPR parameters g factors and the superhyperfine parameters for the tetrahedral [FeX_4]^- clusters in AgX (X=Cl, Br) are theoretically investigated from the perturbation formulas of these parameters for a 3d^5 ion under tetrahedra by considering both the crystal-field and charge transfer contributions. The related model parameters are quantitatively determined from the cluster approach in a uni…
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The EPR parameters g factors and the superhyperfine parameters for the tetrahedral [FeX_4]^- clusters in AgX (X=Cl, Br) are theoretically investigated from the perturbation formulas of these parameters for a 3d^5 ion under tetrahedra by considering both the crystal-field and charge transfer contributions. The related model parameters are quantitatively determined from the cluster approach in a uniform way. The g-shift Δg (=g-g_s, where g_s\approx 2.0023 is the spin only value) from the charge transfer contribution is opposite (positive) in sign and much larger in magnitude as compared with that from the crystal-field one. The importance of the charge transfer contribution increases rapidly with increasing the covalency and the spin-orbit coupling coefficient of the ligand and thus exhibits the order of AgCl<AgBr. The unpaired spin densities of the halogen ns, npσ and npπ orbitals are quantitatively determined from the related molecular orbital coefficients based on the cluster approach.
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Submitted 26 April, 2012;
originally announced April 2012.
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Surface optical phonon modes in ternary aligned crystalline InGaN-GaN multi-quantum well nanopillar arrays
Authors:
S. -E. Wu,
S. Dhara,
T. -H. Hsueh,
Y. -F. Lai,
C. -Y. Wang,
C. -P. Liu
Abstract:
The optical properties of focused ion beam-engraved perfectly aligned and spatially controlled multi-quantum-well InGaN-GaN nanopillars were investigated. Crystalline MQW nanopillars with a diameter of 30 to 95 nm and high aspect ratios showed a maximum of three-fold enhancement in emission intensity per unit active area. A light emitting contour map of Taiwan is drawn with a nanopillar structur…
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The optical properties of focused ion beam-engraved perfectly aligned and spatially controlled multi-quantum-well InGaN-GaN nanopillars were investigated. Crystalline MQW nanopillars with a diameter of 30 to 95 nm and high aspect ratios showed a maximum of three-fold enhancement in emission intensity per unit active area. A light emitting contour map of Taiwan is drawn with a nanopillar structure to demonstrate the site control of the technique adopted in the present study. Raman scattering studies were used to characterize the newly created surfaces. Unknown peaks in GaN and InGaN nanostructures are identified for surface optical (SO) phonon modes with proper assignments of wave vectors using multiple excitations, and the SO mode for the ternary phase is reported for the first time.
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Submitted 2 May, 2009;
originally announced May 2009.
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Phase diagram of a dilute fermion gas with density imbalance
Authors:
C. -H. Pao,
S. -T. Wu,
S. -K. Yip
Abstract:
We map out the phase diagram of a dilute two-component atomic fermion gas with unequal populations and masses under a Feshbach resonance. As in the case of equal masses, no uniform phase is stable for an intermediate coupling regime. For majority component heavier, the unstable region moves towards the BEC side. When the coupling strength is increased from the normal phase, there is an increased…
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We map out the phase diagram of a dilute two-component atomic fermion gas with unequal populations and masses under a Feshbach resonance. As in the case of equal masses, no uniform phase is stable for an intermediate coupling regime. For majority component heavier, the unstable region moves towards the BEC side. When the coupling strength is increased from the normal phase, there is an increased parameter space where the transition is into the FFLO state. The converse is true if the majority is light.
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Submitted 27 September, 2006;
originally announced September 2006.
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Reply to Comment on "Superfluid stability in the BEC-BCS crossover" by Sheehy and Radzihovsky
Authors:
C. -H. Pao,
S. -T. Wu,
S. -K. Yip
Abstract:
The reason behind the discrepancy between the phase diagrams of our earlier work [1] and the comment of Sheehy and Radzihovsky [2] is discussed. We show that, in contrast to what is claimed in [2], the requirement of positive susceptibility is sufficient to rule out states that are local maximum of the free energy (as a function of the order parameter $Δ$
The reason behind the discrepancy between the phase diagrams of our earlier work [1] and the comment of Sheehy and Radzihovsky [2] is discussed. We show that, in contrast to what is claimed in [2], the requirement of positive susceptibility is sufficient to rule out states that are local maximum of the free energy (as a function of the order parameter $Δ$
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Submitted 23 August, 2006;
originally announced August 2006.
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Resonant pairing between Fermions with unequal masses
Authors:
S. -T. Wu,
C. -H. Pao,
S. -K. Yip
Abstract:
We study the pairing between Fermions of different masses, especially at the unitary limit. At equal populations, the thermodynamic properties are identical with the equal mass case provided an appropriate rescaling is made. At unequal populations, for sufficiently light majority species, the system does not phase separate. For sufficiently heavy majority species, the phase separated normal phas…
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We study the pairing between Fermions of different masses, especially at the unitary limit. At equal populations, the thermodynamic properties are identical with the equal mass case provided an appropriate rescaling is made. At unequal populations, for sufficiently light majority species, the system does not phase separate. For sufficiently heavy majority species, the phase separated normal phase have a density larger than that of the superfluid. For atoms in harmonic traps, the density profiles for unequal mass Fermions can be drastically different from their equal-mass counterparts.
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Submitted 28 August, 2006; v1 submitted 25 April, 2006;
originally announced April 2006.