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Double-Crucible Vertical Bridgman Technique for Stoichiometry-Controlled Chalcogenide Crystal Growth
Authors:
Yingdong Guan,
Suguru Yoshida,
Jairo Obando-Guevara,
Seng Huat Lee,
Heike Pfau,
Zhiqiang Mao
Abstract:
Precise stoichiometry control in single-crystal growth is essential for both technological applications and fundamental research. However, conventional growth methods often face challenges such as non-stoichiometry, compositional gradients, and phase impurities, particularly in non-congruent melting systems. Even in congruent melting systems like Bi2Se3, deviations from the ideal stoichiometric co…
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Precise stoichiometry control in single-crystal growth is essential for both technological applications and fundamental research. However, conventional growth methods often face challenges such as non-stoichiometry, compositional gradients, and phase impurities, particularly in non-congruent melting systems. Even in congruent melting systems like Bi2Se3, deviations from the ideal stoichiometric composition can lead to significant property degradation, such as excessive bulk conductivity, which limits its topological applications. In this study, we introduce the double-crucible vertical Bridgman (DCVB) method, a novel approach that enhances stoichiometry control through the combined use of continuous source material feeding, traveling-solvent growth, and liquid encapsulation, which suppresses volatile element loss under high pressure. Using Bi2Se3 as a model system, we demonstrate that crystals grown via DCVB exhibit enhanced stoichiometric control, significantly reducing defect density and achieving much lower carrier concentrations compared to those produced by conventional Bridgman techniques. Moreover, the continuous feeding of source material enables the growth of large crystals. This approach presents a promising strategy for synthesizing high-quality, large-scale crystals, particularly for metal chalcogenides and pnictides that exhibit challenging non-congruent melting behaviors.
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Submitted 10 April, 2025; v1 submitted 13 March, 2025;
originally announced March 2025.
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Observation of Two Cascading Screening Processes in an Iron-based Superconductor
Authors:
Ming-Hua Chang,
Steffen Backes,
Donghui Lu,
Nicolas Gauthier,
Makoto Hashimoto,
Guan-Yu Chen,
Hai-Hu Wen,
Sung-Kwan Mo,
Zhi-Xun Shen,
Roser Valenti,
Heike Pfau
Abstract:
Understanding how renormalized quasiparticles emerge in strongly correlated electron materials provides a challenge for both experiment and theory. It has been predicted that distinctive spin and orbital screening mechanisms drive this process in multiorbital materials with strong Coulomb and Hund's interactions. Here, we provide the experimental evidence of both mechanisms from angle-resolved pho…
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Understanding how renormalized quasiparticles emerge in strongly correlated electron materials provides a challenge for both experiment and theory. It has been predicted that distinctive spin and orbital screening mechanisms drive this process in multiorbital materials with strong Coulomb and Hund's interactions. Here, we provide the experimental evidence of both mechanisms from angle-resolved photoemission spectroscopy on RbFe$_2$As$_2$. We observe that the emergence of low-energy Fe 3$d_{xy}$ quasiparticles below 90K is tied to spin screening. A second process changes the spectral weight at high energies up to room temperature. Supported by theoretical calculations we attribute it to orbital screening of Fe 3d atomic excitations. These two cascading screening processes drive the temperature evolution from a bad metal to a correlated Fermi liquid.
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Submitted 30 July, 2025; v1 submitted 8 March, 2025;
originally announced March 2025.
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Analysis methodology of coherent oscillations in time- and angle-resolved photoemission spectroscopy
Authors:
Nicolas Gauthier,
Hadas Soifer,
Jonathan A. Sobota,
Heike Pfau,
Edbert J. Sie,
Aaron M. Lindenberg,
Zhi-Xun Shen,
Patrick S. Kirchmann
Abstract:
Oscillatory signals from coherently excited phonons are regularly observed in ultrafast pump-probe experiments on condensed matter samples. Electron-phonon coupling implies that coherent phonons also modulate the electronic band structure. These oscillations can be probed with energy and momentum resolution using time- and angle-resolved photoemission spectroscopy (trARPES) which reveals the orbit…
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Oscillatory signals from coherently excited phonons are regularly observed in ultrafast pump-probe experiments on condensed matter samples. Electron-phonon coupling implies that coherent phonons also modulate the electronic band structure. These oscillations can be probed with energy and momentum resolution using time- and angle-resolved photoemission spectroscopy (trARPES) which reveals the orbital dependence of the electron-phonon coupling for a specific phonon mode. However, a comprehensive analysis remains challenging when multiple coherent phonon modes couple to multiple electronic bands. Complex spectral line shapes due to strong correlations in quantum materials add to this challenge. In this work, we examine how the frequency domain representation of trARPES data facilitates a quantitative analysis of coherent oscillations of the electronic bands. We investigate the frequency domain representation of the photoemission intensity and \tred{the first moment of the energy distribution curves}. Both quantities provide complimentary information and are able to distinguish oscillations of binding energy, linewidth and intensity.We analyze a representative trARPES dataset of the transition metal dichalcogenide WTe$_2$ and construct composite spectra which intuitively illustrate how much each electronic band is affected by a specific phonon mode. We also show how a linearly chirped probe pulse can generate extrinsic artifacts that are distinct from the intrinsic coherent phonon signal.
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Submitted 25 February, 2025;
originally announced February 2025.
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Dispersion kinks from electronic correlations in an unconventional iron-based superconductor
Authors:
Ming-Hua Chang,
Steffen Backes,
Donghui Lu,
Nicolas Gauthier,
Makoto Hashimoto,
Guan-Yu Chen,
Hai-Hu Wen,
Sung-Kwan Mo,
Roser Valenti,
Heike Pfau
Abstract:
The attractive interaction in conventional BCS superconductors is provided by a bosonic mode. However, the pairing glue of most unconventional superconductors is unknown. The effect of electron-boson coupling is therefore extensively studied in these materials. A key signature are dispersion kinks that can be observed in the spectral function as abrupt changes in velocity and lifetime of quasipart…
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The attractive interaction in conventional BCS superconductors is provided by a bosonic mode. However, the pairing glue of most unconventional superconductors is unknown. The effect of electron-boson coupling is therefore extensively studied in these materials. A key signature are dispersion kinks that can be observed in the spectral function as abrupt changes in velocity and lifetime of quasiparticles. Here, we show the existence of two kinks in the unconventional iron-based superconductor RbFe$_2$As$_2$ using angle-resolved photoemission spectroscopy (ARPES) and dynamical mean field theory (DMFT). In addition, we observe the formation of a Hubbard band multiplet due to the combination of Coulomb interaction and Hund's rule coupling in this multiorbital systems. We demonstrate that the two dispersion kinks are a consequence of these strong many-body interactions. This interpretation is in line with a growing number of theoretical predictions for kinks in various general models of correlated materials. Our results provide a unifying link between iron-based superconductors and different classes of correlated, unconventional superconductors such as cuprates and heavy-fermion materials.
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Submitted 30 October, 2024;
originally announced October 2024.
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Uniaxial stress effect on the electronic structure of quantum materials
Authors:
Na Hyun Jo,
Elena Gati,
Heike Pfau
Abstract:
Uniaxial stress has proven to be a powerful experimental tuning parameter for effectively controlling lattice, charge, orbital, and spin degrees of freedom in quantum materials. In addition, its ability to manipulate the symmetry of materials has garnered significant attention. Recent technical progress to combine uniaxial stress cells with quantum oscillation and angle-resolved photoemission tech…
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Uniaxial stress has proven to be a powerful experimental tuning parameter for effectively controlling lattice, charge, orbital, and spin degrees of freedom in quantum materials. In addition, its ability to manipulate the symmetry of materials has garnered significant attention. Recent technical progress to combine uniaxial stress cells with quantum oscillation and angle-resolved photoemission techniques allowed to study the electronic structure as function of uniaxial stress. This review provides an overview on experimental advancements in methods and examines studies on diverse quantum materials, encompassing the semimetal WTe2, the unconventional superconductor Sr2RuO4, Fe-based superconductors, and topological materials.
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Submitted 2 May, 2024;
originally announced May 2024.
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Spectroscopic evidence for topological band structure in FeTe$_{0.55}$Se$_{0.45}$
Authors:
Y. -F. Li,
S. -D. Chen,
M. Garcia-Diez,
M. I. Iraola,
H. Pfau,
Y. -L. Zhu,
Z. -Q. Mao,
T. Chen,
M. Yi,
P. -C. Dai,
J. A. Sobota,
M. Hashimoto,
M. G. Vergniory,
D. -H. Lu,
Z. -X. Shen
Abstract:
FeTe$_{0.55}$Se$_{0.45}$(FTS) occupies a special spot in modern condensed matter physics at the intersections of electron correlation, topology, and unconventional superconductivity. The bulk electronic structure of FTS is predicted to be topologically nontrivial thanks to the band inversion between the $d_{xz}$ and $p_z$ bands along $Γ$-$Z$. However, there remain debates in both the authenticity…
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FeTe$_{0.55}$Se$_{0.45}$(FTS) occupies a special spot in modern condensed matter physics at the intersections of electron correlation, topology, and unconventional superconductivity. The bulk electronic structure of FTS is predicted to be topologically nontrivial thanks to the band inversion between the $d_{xz}$ and $p_z$ bands along $Γ$-$Z$. However, there remain debates in both the authenticity of the Dirac surface states (DSS) and the experimental deviations of band structure from the theoretical band inversion picture. Here we resolve these debates through a comprehensive ARPES investigation. We first observe a persistent DSS independent of $k_z$. Then, by comparing FTS with FeSe which has no band inversion along $Γ$-$Z$, we identify the spectral weight fingerprint of both the presence of the $p_z$ band and the inversion between the $d_{xz}$ and $p_z$ bands. Furthermore, we propose a reconciling band structure under the framework of a tight-binding model preserving crystal symmetry. Our results highlight the significant influence of correlation on modifying the band structure and make a strong case for the existence of topological band structure in this unconventional superconductor.
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Submitted 22 August, 2023; v1 submitted 7 July, 2023;
originally announced July 2023.
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Expanding the momentum field of view in angle-resolved photoemission systems with hemispherical analyzers
Authors:
Nicolas Gauthier,
Jonathan A. Sobota,
Heike Pfau,
Alexandre Gauthier,
Hadas Soifer,
Maja D. Bachmann,
Ian R. Fisher,
Zhi-Xun Shen,
Patrick S. Kirchmann
Abstract:
In photoelectron spectroscopy, the measured electron momentum range is intrinsically related to the excitation photon energy. Low photon energies $<10$ eV are commonly encountered in laser-based photoemission and lead to a momentum range that is smaller than the Brillouin zones of most materials. This can become a limiting factor when studying condensed matter with laser-based photoemission. An ad…
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In photoelectron spectroscopy, the measured electron momentum range is intrinsically related to the excitation photon energy. Low photon energies $<10$ eV are commonly encountered in laser-based photoemission and lead to a momentum range that is smaller than the Brillouin zones of most materials. This can become a limiting factor when studying condensed matter with laser-based photoemission. An additional restriction is introduced by widely used hemispherical analyzers that record only electrons photoemitted in a solid angle set by the aperture size at the analyzer entrance. Here, we present an upgrade to increase the effective solid angle that is measured with a hemispherical analyzer. We achieve this by accelerating the photoelectrons towards the analyzer with an electric field that is generated by a bias voltage on the sample. Our experimental geometry is comparable to a parallel plate capacitor and, therefore, we approximate the electric field to be uniform along the photoelectron trajectory. With this assumption, we developed an analytic, parameter-free model that relates the measured angles to the electron momenta in the solid and verify its validity by comparing with experimental results on the charge density wave material TbTe$_3$. By providing a larger field of view in momentum space, our approach using a bias potential considerably expands the flexibility of laser-based photoemission setups.
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Submitted 16 December, 2021; v1 submitted 19 August, 2021;
originally announced August 2021.
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Anisotropic quasiparticle coherence in nematic BaFe$_2$As$_2$ studied with strain-dependent ARPES
Authors:
H. Pfau,
S. D. Chen,
M. Hashimoto,
N. Gauthier,
C. R. Rotundu,
J. C. Palmstrom,
I. R. Fisher,
S. -K. Mo,
Z. -X. Shen,
D. Lu
Abstract:
The hallmark of nematic order in iron-based superconductors is a resistivity anisotropy but it is unclear to which extent quasiparticle dispersions, lifetimes and coherence contribute. While the lifted degeneracy of the Fe $d_{xz}$ and $d_{yz}$ dispersions has been studied extensively, only little is known about the two other factors. Here, we combine in situ strain tuning with ARPES and study the…
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The hallmark of nematic order in iron-based superconductors is a resistivity anisotropy but it is unclear to which extent quasiparticle dispersions, lifetimes and coherence contribute. While the lifted degeneracy of the Fe $d_{xz}$ and $d_{yz}$ dispersions has been studied extensively, only little is known about the two other factors. Here, we combine in situ strain tuning with ARPES and study the nematic response of the spectral weight in BaFe$_2$As$_2$. The symmetry analysis of the ARPES spectra demonstrates that the $d_{xz}$ band gains quasiparticle spectral weight compared to the $d_{yz}$ band for negative antisymmetric strain $Δε_{yy}$ suggesting the same response inside the nematic phase. Our results are compatible with a different coherence of the $d_{xz}$ and $d_{yz}$ orbital within a Hund's metal picture. We also discuss the influence of orbital mixing.
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Submitted 10 April, 2021;
originally announced April 2021.
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Quantum well states in fractured crystals of the heavy fermion material CeCoIn$_5$
Authors:
Nicolas Gauthier,
Jonathan A. Sobota,
Makoto Hashimoto,
Heike Pfau,
Dong-Hui Lu,
Eric D. Bauer,
Filip Ronning,
Patrick S. Kirchmann,
Zhi-Xun Shen
Abstract:
Quantum well states appear in metallic thin films due to the confinement of the wave function by the film interfaces. Using angle-resolved photoemission spectroscopy, we unexpectedly observe quantum well states in fractured single crystals of CeCoIn$_5$. We confirm that confinement occurs by showing that these states' binding energies are photon-energy independent and are well described with a pha…
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Quantum well states appear in metallic thin films due to the confinement of the wave function by the film interfaces. Using angle-resolved photoemission spectroscopy, we unexpectedly observe quantum well states in fractured single crystals of CeCoIn$_5$. We confirm that confinement occurs by showing that these states' binding energies are photon-energy independent and are well described with a phase accumulation model, commonly applied to quantum well states in thin films. This indicates that atomically flat thin films can be formed by fracturing hard single crystals. For the two samples studied, our observations are explained by free-standing flakes with thicknesses of 206 and 101 Å. We extend our analysis to extract bulk properties of CeCoIn$_5$. Specifically, we obtain the dispersion of a three-dimensional band near the zone center along in-plane and out-of-plane momenta. We establish part of its Fermi surface, which corresponds to a hole pocket centered at $Γ$. We also reveal a change of its dispersion with temperature, a signature that may be caused by the Kondo hybridization.
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Submitted 14 September, 2020; v1 submitted 29 June, 2020;
originally announced June 2020.
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Tuning time and energy resolution in time-resolved photoemission spectroscopy with nonlinear crystals
Authors:
Alexandre Gauthier,
Jonathan A. Sobota,
Nicolas Gauthier,
Ke-Jun Xu,
Heike Pfau,
Costel Rotundu,
Zhi-Xun Shen,
Patrick S. Kirchmann
Abstract:
Time- and angle-resolved photoemission spectroscopy is a powerful probe of electronic band structures out of equilibrium. Tuning time and energy resolution to suit a particular scientific question has become an increasingly important experimental consideration. Many instruments use cascaded frequency doubling in nonlinear crystals to generate the required ultraviolet probe pulses. We demonstrate h…
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Time- and angle-resolved photoemission spectroscopy is a powerful probe of electronic band structures out of equilibrium. Tuning time and energy resolution to suit a particular scientific question has become an increasingly important experimental consideration. Many instruments use cascaded frequency doubling in nonlinear crystals to generate the required ultraviolet probe pulses. We demonstrate how calculations clarify the relationship between laser bandwidth and nonlinear crystal thickness contributing to experimental resolutions and place intrinsic limits on the achievable time-bandwidth product. Experimentally, we tune time and energy resolution by varying the thickness of nonlinear $β$-BaB$_2$O$_4$ crystals for frequency up-conversion, providing for a flexible experiment design. We achieve time resolutions of 58 to 103 fs and corresponding energy resolutions of 55 to 27 meV.
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Submitted 6 August, 2020; v1 submitted 13 June, 2020;
originally announced June 2020.
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Low work function in the 122-family of iron-based superconductors
Authors:
H. Pfau,
H. Soifer,
J. A. Sobota,
A. Gauthier,
C. R. Rotundu,
J. C. Palmstrom,
I. R. Fisher,
G. -Y. Chen,
H. -H. Wen,
Z. -X. Shen,
P. S. Kirchmann
Abstract:
We determine the work functions of the iron arsenic compounds $A$Fe$_2$As$_2$ ($A=\mathrm{Ca, Ba, Cs}$) using photoemission spectroscopy to be 2.7 eV for CaFe$_2$As$_2$, 1.8 eV for BaFe$_2$As$_2$, and 1.3 eV for CsFe$_2$As$_2$. The work functions of these 122 iron-based superconductors track those of the elementary metal $A$ but are substantially smaller. The most likely explanation of this observ…
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We determine the work functions of the iron arsenic compounds $A$Fe$_2$As$_2$ ($A=\mathrm{Ca, Ba, Cs}$) using photoemission spectroscopy to be 2.7 eV for CaFe$_2$As$_2$, 1.8 eV for BaFe$_2$As$_2$, and 1.3 eV for CsFe$_2$As$_2$. The work functions of these 122 iron-based superconductors track those of the elementary metal $A$ but are substantially smaller. The most likely explanation of this observation is that the cleaving surface exposes only half an $A$-layer. The low work function and good photoemission cross section of BaFe$_2$As$_2$ and CsFe$_2$As$_2$ enable photoemission even from a common white LED light.
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Submitted 18 February, 2020;
originally announced February 2020.
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Momentum Dependence of the Nematic Order Parameter in Iron-Based Superconductors
Authors:
H. Pfau,
S. D. Chen,
M. Yi,
M. Hashimoto,
C. R. Rotundu,
J. C. Palmstrom,
T. Chen,
P. -C. Dai,
J. Straquadine,
A. Hristov,
R. J. Birgeneau,
I. R. Fisher,
D. Lu,
Z. -X. Shen
Abstract:
The momentum dependence of the nematic order parameter is an important ingredient in the microscopic description of iron-based high-temperature superconductors. While recent reports on FeSe indicate that the nematic order parameter changes sign between electron and hole bands, detailed knowledge is still missing for other compounds. Combining angle-resolved photoemission spectroscopy (ARPES) with…
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The momentum dependence of the nematic order parameter is an important ingredient in the microscopic description of iron-based high-temperature superconductors. While recent reports on FeSe indicate that the nematic order parameter changes sign between electron and hole bands, detailed knowledge is still missing for other compounds. Combining angle-resolved photoemission spectroscopy (ARPES) with uniaxial strain tuning, we measure the nematic band splitting in both FeSe and BaFe$_2$As$_2$ without interference from either twinning or magnetic order. We find that the nematic order parameter exhibits the same momentum dependence in both compounds with a sign change between the Brillouin center and the corner. This suggests that the same microscopic mechanism drives the nematic order in spite of the very different phase diagrams.
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Submitted 7 August, 2019;
originally announced August 2019.
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The Nematic Energy Scale and the Missing Electron Pocket in FeSe
Authors:
Ming Yi,
Heike Pfau,
Yan Zhang,
Yu He,
Han Wu,
Tong Chen,
Zirong Ye,
Makoto Hashimoto,
Rong Yu,
Qimiao Si,
Dung-Hai Lee,
Pengcheng Dai,
Zhi-Xun Shen,
Donghui Lu,
Robert J. Birgeneau
Abstract:
Superconductivity emerges in proximity to a nematic phase in most iron-based superconductors. It is therefore important to understand the impact of nematicity on the electronic structure. Orbital assignment and tracking across the nematic phase transition prove to be challenging due to the multiband nature of iron-based superconductors and twinning effects. Here, we report a detailed study of the…
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Superconductivity emerges in proximity to a nematic phase in most iron-based superconductors. It is therefore important to understand the impact of nematicity on the electronic structure. Orbital assignment and tracking across the nematic phase transition prove to be challenging due to the multiband nature of iron-based superconductors and twinning effects. Here, we report a detailed study of the electronic structure of fully detwinned FeSe across the nematic phase transition using angle-resolved photoemission spectroscopy. We clearly observe a nematicity-driven band reconstruction involving dxz, dyz, and dxy orbitals. The nematic energy scale between dxz and dyz bands reaches a maximum of 50 meV at the Brillouin zone corner. We are also able to track the dxz electron pocket across the nematic transition and explain its absence in the nematic state. Our comprehensive data of the electronic structure provide an accurate basis for theoretical models of the superconducting pairing in FeSe.
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Submitted 7 December, 2019; v1 submitted 11 March, 2019;
originally announced March 2019.
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Electronic structure of the quadrupolar ordered heavy-fermion compound YbRu2Ge2 measured by angle-resolved photoemission
Authors:
H. Pfau,
E. W. Rosenberg,
P. Massat,
B. Moritz,
M. Hashimoto,
D. Lu,
I. R. Fisher,
Z. -X. Shen
Abstract:
We studied the electronic structure of the heavy fermion compound Yb(Ru$_{1-x}$Rh$_{x}$)$_2$Ge$_2$ with $x=0$ and nominally $x=0.125$ using ARPES and LDA calculations. We find a valence band structure of Yb corresponding to a non-integer valence close to $3+$. The three observed crystal electric field levels with a splitting of 32 and 75 meV confirm the suggested configuration with a quasi-quartet…
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We studied the electronic structure of the heavy fermion compound Yb(Ru$_{1-x}$Rh$_{x}$)$_2$Ge$_2$ with $x=0$ and nominally $x=0.125$ using ARPES and LDA calculations. We find a valence band structure of Yb corresponding to a non-integer valence close to $3+$. The three observed crystal electric field levels with a splitting of 32 and 75 meV confirm the suggested configuration with a quasi-quartet ground state. The experimentally determined band structure of the conduction electrons with predominantly Ru $4d$ character is well reproduced by our calculations. YbRu$_2$Ge$_2$ undergoes a non-magnetic phase transition into a ferroquadrupolar ordered state below 10.2\,K and then to an antiferromagnetically ordered state below 6.5\,K. A small hole Fermi surface shows nesting features in our calculated band structure and its size determined by ARPES is close to the magnetic ordering wave vector found in neutron scattering. The transitions are suppressed when YbRu$_2$Ge$_2$ is doped with 12.5% Rh. The electron doping leads to a shift of the band structure and successive Lifshitz transitions.
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Submitted 28 February, 2019;
originally announced March 2019.
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Detailed band structure of twinned and detwinned BaFe$_2$As$_2$ studied with angle-resolved photoemission spectroscopy
Authors:
H. Pfau,
C. R. Rotundu,
J. C. Palmstrom,
S. D. Chen,
M. Hashimoto,
D. Lu,
A. F. Kemper,
I. R. Fisher,
Z. -X. Shen
Abstract:
We study the band structure of twinned and detwinned BaFe$_2$As$_2$ using angle-resolved photoemission spectroscopy (ARPES). The combination of measurements in the ordered and normal state along four high-symmetry momentum directions $Γ$/Z--X/Y enables us to identify the complex reconstructed band structure in the ordered state in great detail. We clearly observe the nematic splitting of the…
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We study the band structure of twinned and detwinned BaFe$_2$As$_2$ using angle-resolved photoemission spectroscopy (ARPES). The combination of measurements in the ordered and normal state along four high-symmetry momentum directions $Γ$/Z--X/Y enables us to identify the complex reconstructed band structure in the ordered state in great detail. We clearly observe the nematic splitting of the $d_{xz}$ and $d_{yz}$ orbitals as well as folding due to magnetic order with a wave vector of $(π,π,π)$. We are able to assign all observed bands. In particular we suggest an assignment of the electron bands different from previous reports. The high quality spectra allow us to achieve a comprehensive understanding of the band structure of BaFe$_2$As$_2$.
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Submitted 10 January, 2019;
originally announced January 2019.
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Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu$_2$Si$_2$ versus YbRh$_2$Si$_2$
Authors:
M. Smidman,
O. Stockert,
J. Arndt,
G. M. Pang,
L. Jiao,
H. Q. Yuan,
H. A. Vieyra,
S. Kitagawa,
K. Ishida,
K. Fujiwara,
T. C. Kobayashi,
E. Schuberth,
M. Tippmann,
L. Steinke,
S. Lausberg,
A. Steppke,
M. Brando,
H. Pfau,
U. Stockert,
P. Sun,
S. Friedemann,
S. Wirth,
C. Krellner,
S. Kirchner,
E. M. Nica
, et al. (3 additional authors not shown)
Abstract:
In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu$_2$Si$_2$, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP).…
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In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu$_2$Si$_2$, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu$_2$Si$_2$. In YbRh$_2$Si$_2$, superconductivity appears to be suppressed at $T\gtrsim~10$ mK by AF order ($T_N$ = 70 mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at $T_A$ slightly above 2 mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at $T_c$ = 2 mK. Like the pressure - induced QCP in CeRhIn$_5$, the magnetic field - induced one in YbRh$_2$Si$_2$ is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-$T$ unconventional heavy - fermion superconductors and other families of unconventional superconductors with higher $T_c$s, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon.
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Submitted 22 August, 2018; v1 submitted 7 August, 2018;
originally announced August 2018.
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Cascade of magnetic field induced Lifshitz transitions in the ferromagnetic Kondo lattice material YbNi4P2
Authors:
H. Pfau,
R. Daou,
S. Friedemann,
S. Karbassi,
S. Ghannadzadeh,
R. Kuechler,
S. Hamann,
A. Steppke,
D. Sun,
M. Koenig,
A. P. Mackenzie,
K. Kliemt,
C. Krellner,
M. Brando
Abstract:
A ferromagnetic quantum critical point is thought not to exist in two and three-dimensional metallic systems yet is realized in the Kondo lattice compound YbNi4(P,As)2, possibly due to its one-dimensionality. It is crucial to investigate the dimensionality of the Fermi surface of YbNi4P2 experimentally but common probes such as ARPES and quantum oscillation measurements are lacking. Here, we studi…
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A ferromagnetic quantum critical point is thought not to exist in two and three-dimensional metallic systems yet is realized in the Kondo lattice compound YbNi4(P,As)2, possibly due to its one-dimensionality. It is crucial to investigate the dimensionality of the Fermi surface of YbNi4P2 experimentally but common probes such as ARPES and quantum oscillation measurements are lacking. Here, we studied the magnetic field dependence of transport and thermodynamic properties of YbNi4P2. The Kondo effect is continuously suppressed and additionally we identify nine Lifshitz transitions between 0.4 and 18 T. We analyze the transport coefficients in detail and identify the type of Lifshitz transitions as neck or void type to gain information on the Fermi surface of YbNi4P2. The large number of Lifshitz transitions observed within this small energy window is unprecedented and results from the particular flat renormalized band structure with strong 4f-electron character shaped by the Kondo lattice effect.
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Submitted 4 October, 2017; v1 submitted 19 December, 2016;
originally announced December 2016.
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Superconducting gap structure of the skutterudite LaPt4Ge12 probed by specific heat and thermal transport
Authors:
Heike Pfau,
Michael Nicklas,
Ulrike Stockert,
Roman Gumeniuk,
Walter Schnelle,
Andreas Leithe-Jasper,
Yuri Grin,
Frank Steglich
Abstract:
We investigated the superconducting order parameter of the filled skutterudite LaPt4Ge12, with a transition temperature of Tc = 8.3 K. To this end, we performed temperature and magnetic-field dependent specific-heat and thermal-conductivity measurements. All data are compatible with a single superconducting s-wave gap. However, a multiband scenario cannot be ruled out. The results are discussed in…
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We investigated the superconducting order parameter of the filled skutterudite LaPt4Ge12, with a transition temperature of Tc = 8.3 K. To this end, we performed temperature and magnetic-field dependent specific-heat and thermal-conductivity measurements. All data are compatible with a single superconducting s-wave gap. However, a multiband scenario cannot be ruled out. The results are discussed in the context of previous studies on the substitution series Pr1-xLaxPt4Ge12. They suggest compatible order parameters for the two end compounds LaPt4Ge12 and PrPt4Ge12. This is not consistent with a single s-wave gap in LaPt4Ge12 considering previous reports of unconventional and/or multiband superconductivity in PrPt4Ge12.
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Submitted 24 August, 2016;
originally announced August 2016.
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Heavy fermion and Kondo lattice behavior in the itinerant ferromagnet CeCrGe3
Authors:
Debarchan Das,
T. Gruner,
H. Pfau,
U. B. Paramanik,
U. Burkhardt,
C. Geibel,
Z. Hossain
Abstract:
Physical properties of polycrystalline CeCrGe$_{3}$ and LaCrGe$_{3}$ have been investigated by x-ray absorption spectroscopy, magnetic susceptibility $χ(T)$, isothermal magnetization M(H), electrical resistivity $ρ(T)$, specific heat C($T$) and thermoelectric power S($T$) measurements. These compounds are found to crystallize in the hexagonal perovskite structure (space group \textit{P6$_{3}$/mmc}…
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Physical properties of polycrystalline CeCrGe$_{3}$ and LaCrGe$_{3}$ have been investigated by x-ray absorption spectroscopy, magnetic susceptibility $χ(T)$, isothermal magnetization M(H), electrical resistivity $ρ(T)$, specific heat C($T$) and thermoelectric power S($T$) measurements. These compounds are found to crystallize in the hexagonal perovskite structure (space group \textit{P6$_{3}$/mmc}), as previously reported. The $ρ(T)$, $χ(T)$ and C($T$) data confirm the bulk ferromagnetic ordering of itinerant Cr moments in LaCrGe$_{3}$ and CeCrGe$_{3}$ with $T_{C}$ = 90 K and 70 K respectively. In addition a weak anomaly is also observed near 3 K in the C($T$) data of CeCrGe$_{3}$. The T dependences of $ρ$ and finite values of Sommerfeld coefficient $γ$ obtained from the specific heat measurements confirm that both the compounds are of metallic character. Further, the $T$ dependence of $ρ$ of CeCrGe$_{3}$ reflects a Kondo lattice behavior. An enhanced $γ$ of 130 mJ/mol\,K$^{2}$ together with the Kondo lattice behavior inferred from the $ρ(T)$ establish CeCrGe$_{3}$ as a moderate heavy fermion compound with a quasi-particle mass renormalization factor of $\sim$ 45.
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Submitted 7 February, 2014;
originally announced February 2014.
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Evidence for a Kondo destroying quantum critical point in YbRh2Si2
Authors:
Frank Steglich,
Heike Pfau,
Stefan Lausberg,
Peijie Sun,
Ulrike Stockert,
Manuel Brando,
Sven Friedemann,
Cornelius Krellner,
Christoph Geibel,
Steffen Wirth,
Stefan Kirchner,
Elihu Abrahams,
Qimiao Si
Abstract:
The heavy-fermion metal YbRh$_{2}$Si$_{2}$ is a weak antiferromagnet below $T_{N} = 0.07$ K. Application of a low magnetic field $B_{c} = 0.06$ T ($\perp c$) is sufficient to continuously suppress the antiferromagnetic (AF) order. Below $T \approx 10$ K, the Sommerfeld coefficient of the electronic specific heat $γ(T)$ exhibits a logarithmic divergence. At $T < 0.3$ K, $γ(T) \sim T^{-ε}$ (…
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The heavy-fermion metal YbRh$_{2}$Si$_{2}$ is a weak antiferromagnet below $T_{N} = 0.07$ K. Application of a low magnetic field $B_{c} = 0.06$ T ($\perp c$) is sufficient to continuously suppress the antiferromagnetic (AF) order. Below $T \approx 10$ K, the Sommerfeld coefficient of the electronic specific heat $γ(T)$ exhibits a logarithmic divergence. At $T < 0.3$ K, $γ(T) \sim T^{-ε}$ ($ε: 0.3 - 0.4$), while the electrical resistivity $ρ(T) = ρ_{0} + aT$ ($ρ_{0}$: residual resistivity). Upon extrapolating finite-$T$ data of transport and thermodynamic quantities to $T = 0$, one observes (i) a vanishing of the "Fermi surface crossover" scale $T^{*}(B)$, (ii) an abrupt jump of the initial Hall coefficient $R_{H}(B)$ and (iii) a violation of the Wiedemann Franz law at $B = B_{c}$, the field-induced quantum critical point (QCP). These observations are interpreted as evidence of a critical destruction of the heavy quasiparticles, i.e., propagating Kondo singlets, at the QCP of this material.
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Submitted 27 September, 2013;
originally announced September 2013.
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Thermal and Electrical Transport across a Magnetic Quantum Critical Point
Authors:
H. Pfau,
S. Hartmann,
U. Stockert,
P. Sun,
S. Lausberg,
M. Brando,
S. Friedemann,
C. Krellner,
C. Geibel,
S. Wirth,
S. Kirchner,
E. Abrahams,
Q. Si,
F. Steglich
Abstract:
A quantum critical point (QCP) arises at a continuous transition between competing phases at zero temperature. Collective excitations at magnetic QCPs give rise to metallic properties that strongly deviate from the expectations of Landau's Fermi liquid description, the standard theory of electron correlations in metals. Central to this theory is the notion of quasiparticles, electronic excitations…
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A quantum critical point (QCP) arises at a continuous transition between competing phases at zero temperature. Collective excitations at magnetic QCPs give rise to metallic properties that strongly deviate from the expectations of Landau's Fermi liquid description, the standard theory of electron correlations in metals. Central to this theory is the notion of quasiparticles, electronic excitations which possess the quantum numbers of the bare electrons. Here we report measurements of thermal and electrical transport across the field-induced magnetic QCP in the heavy-fermion compound YbRh$_2$Si$_2$. We show that the ratio of the thermal to electrical conductivities at the zero-temperature limit obeys the Wiedemann-Franz (WF) law above the critical field, $B_c$. This is also expected at $B < B_c$, where weak antiferromagnetic order and a Fermi liquid phase form below 0.07 K ($B = 0$). However, at the critical field the low-temperature electrical conductivity suggests a non-Fermi-liquid ground state and exceeds the thermal conductivity by about 10%. This apparent violation of the WF law provides evidence for an unconventional type of QCP at which the fundamental concept of Landau quasiparticles breaks down. These results imply that Landau quasiparticles break up, and that the origin of this disintegration is inelastic scattering associated with electronic quantum critical fluctuations. Our finding brings new insights into understanding deviations from Fermi-liquid behaviour frequently observed in various classes of correlated materials.
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Submitted 3 July, 2013;
originally announced July 2013.
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Interplay between Kondo suppression and Lifshitz transitions in YbRh$_2$Si$_2$ at high magnetic fields
Authors:
H. Pfau,
R. Daou,
S. Lausberg,
H. R. Naren,
M. Brando,
S. Friedemann,
S. Wirth,
T. Westerkamp,
U. Stockert,
P. Gegenwart,
C. Krellner,
C. Geibel,
G. Zwicknagl,
F. Steglich
Abstract:
We investigate the magnetic field dependent thermopower, thermal conductivity, resistivity and Hall effect in the heavy fermion metal YbRh2Si2. In contrast to reports on thermodynamic measurements, we find in total three transitions at high fields, rather than a single one at 10 T. Using the Mott formula together with renormalized band calculations, we identify Lifshitz transitions as their origin…
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We investigate the magnetic field dependent thermopower, thermal conductivity, resistivity and Hall effect in the heavy fermion metal YbRh2Si2. In contrast to reports on thermodynamic measurements, we find in total three transitions at high fields, rather than a single one at 10 T. Using the Mott formula together with renormalized band calculations, we identify Lifshitz transitions as their origin. The predictions of the calculations show that all experimental results rely on an interplay of a smooth suppression of the Kondo effect and the spin splitting of the flat hybridized bands.
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Submitted 20 June, 2013; v1 submitted 27 February, 2013;
originally announced February 2013.
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Thermoelectric transport across the metamagnetic transition of CeRu2Si2
Authors:
Heike Pfau,
Ramzy Daou,
Manuel Brando,
Frank Steglich
Abstract:
We have measured the thermopower across the metamagnetic transition of the heavy fermion compound CeRu2Si2 at temperatures down to 0.1K and magnetic fields up to 11.5T. We find a large negative enhancement of the thermopower on crossing the metamagnetic field, as well as a sudden change in slope. We argue that this is consistent with the Zeeman-driven deformation of the Fermi surface through a top…
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We have measured the thermopower across the metamagnetic transition of the heavy fermion compound CeRu2Si2 at temperatures down to 0.1K and magnetic fields up to 11.5T. We find a large negative enhancement of the thermopower on crossing the metamagnetic field, as well as a sudden change in slope. We argue that this is consistent with the Zeeman-driven deformation of the Fermi surface through a topological transition. The field dependence of the thermopower highlights the discrepancy between thermodynamic and transport properties across the metamagnetic transition.
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Submitted 17 January, 2012; v1 submitted 6 October, 2011;
originally announced October 2011.
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Ferromagnetic Quantum Criticality in the Quasi-One-Dimensional Heavy Fermion Metal YbNi4P2
Authors:
C Krellner,
S Lausberg,
A Steppke,
M Brando,
L Pedrero,
H Pfau,
S Tencé,
H Rosner,
F Steglich,
C Geibel
Abstract:
We present a new Kondo-lattice system, YbNi4P2, which is a clean heavy-fermion metal with a severely reduced ferromagnetic ordering temperature at T_C=0.17K, evidenced by distinct anomalies in susceptibility, specific-heat, and resistivity measurements. The ferromagnetic nature of the transition, with only a small ordered moment of ~0.05mu_B, is established by a diverging susceptibility at T_C wit…
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We present a new Kondo-lattice system, YbNi4P2, which is a clean heavy-fermion metal with a severely reduced ferromagnetic ordering temperature at T_C=0.17K, evidenced by distinct anomalies in susceptibility, specific-heat, and resistivity measurements. The ferromagnetic nature of the transition, with only a small ordered moment of ~0.05mu_B, is established by a diverging susceptibility at T_C with huge absolute values in the ferromagnetically ordered state, severely reduced by small magnetic fields. Furthermore, YbNi4P2 is a stoichiometric system with a quasi-one-dimensional crystal and electronic structure and strong correlation effects which dominate the low temperature properties. This is reflected by a stronger-than-logarithmically diverging Sommerfeld coefficient and a linear-in-T resistivity above T_C which cannot be explained by any current theoretical predictions. These exciting characteristics are unique among all correlated electron systems and make this an interesting material for further in-depth investigations.
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Submitted 22 August, 2011;
originally announced August 2011.