6 results on '"Charles Mielke"'
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2. Time-reversal symmetry broken by charge order in CsV_{3}Sb_{5}
- Author
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Rustem Khasanov, Debarchan Das, Ritu Gupta, Charles Mielke, III, Matthias Elender, Qiangwei Yin, Zhijun Tu, Chunsheng Gong, Hechang Lei, Ethan T. Ritz, Rafael M. Fernandes, Turan Birol, Zurab Guguchia, and Hubertus Luetkens
- Subjects
Physics ,QC1-999 - Abstract
The recently discovered vanadium-based kagome metals AV_{3}Sb_{5} (A = K, Rb, Cs) exhibit superconductivity at low temperatures and charge density wave (CDW) order at high temperatures. A prominent feature of the charge ordered state in this family is that it breaks time-reversal symmetry (TRSB), which is connected to the underlying topological nature of the band structure. In this work, a powerful combination of zero-field and high-field muon-spin rotation/relaxation is used to study the signatures of TRSB of the charge order in CsV_{3}Sb_{5}, as well as its anisotropic character. By tracking the temperature evolution of the in-plane and out-of-plane components of the muon-spin polarization, an enhancement of the internal field width sensed by the muon-spin ensemble was observed below T_{TRSB}=T_{CDW}≃95 K. Additional increase of the internal field width, accompanied by a change of the local field direction at the muon site from the ab plane to the c axis, was detected below T^{*}≃30 K. Remarkably, this two-step feature becomes well pronounced when a magnetic field of 8 T is applied along the crystallographic caxis, thus indicating a field-induced enhancement of the electronic response at the CDW transition. These results point to a TRSB in CsV_{3}Sb_{5} by charge order with an onset of ≃95 K, followed by an enhanced electronic response below ≃30 K. The observed two-step transition is discussed within the framework of different charge-order instabilities, which, in accordance with density functional theory calculations, are nearly degenerate in energy.
- Published
- 2022
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3. Nodeless electron pairing in CsV3Sb5-derived kagome superconductors
- Author
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Yigui Zhong, Jinjin Liu, Xianxin Wu, Zurab Guguchia, J.-X. Yin, Akifumi Mine, Yongkai Li, Sahand Najafzadeh, Debarchan Das, Charles Mielke, Rustem Khasanov, Hubertus Luetkens, Takeshi Suzuki, Kecheng Liu, Xinloong Han, Takeshi Kondo, Jiangping Hu, Shik Shin, Zhiwei Wang, Xun Shi, Yugui Yao, and Kozo Okazaki
- Subjects
Superconductivity (cond-mat.supr-con) ,Condensed Matter - Strongly Correlated Electrons ,Multidisciplinary ,Strongly Correlated Electrons (cond-mat.str-el) ,Condensed Matter - Superconductivity ,FOS: Physical sciences - Abstract
The newly discovered kagome superconductors represent a promising platform for investigating the interplay between band topology, electronic order, and lattice geometry. Despite extensive research efforts on this system, the nature of the superconducting ground state remains elusive. In particular, consensus on the electron pairing symmetry has not been achieved so far, in part owing to the lack of a momentum-resolved measurement of the superconducting gap structure. Here we report the direct observation of a nodeless, nearly isotropic, and orbital-independent superconducting gap in the momentum space of two exemplary CsV$_3$Sb$_5$-derived kagome superconductors -- Cs(V$_{0.93}$Nb$_{0.07}$)$_3$Sb$_5$ and Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$, using ultrahigh resolution and low-temperature angle-resolved photoemission spectroscopy (ARPES). Remarkably, such a gap structure is robust to the appearance or absence of charge order in the normal state, tuned by isovalent Nb/Ta substitutions of V. Moreover, we observe a signature of the time-reversal symmetry (TRS) breaking inside the superconducting state, which extends the previous observation of TRS-breaking CDW in the kagome lattice. Our comprehensive characterizations of the superconducting state provide indispensable information on the electron pairing of kagome superconductors, and advance our understanding of unconventional superconductivity and intertwined electronic orders.
- Published
- 2023
4. Isotropic single-gap superconductivity of elemental Pb
- Author
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Ritu Gupta, Debarchan Das, Dariusz Jakub Gawryluk, Rustem Khasanov, and Charles Mielke
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Physics ,Superconductivity ,Condensed matter physics ,Band gap ,Transition temperature ,Relaxation (NMR) ,Isotropy ,Muon spin spectroscopy ,Single crystal ,Quantum tunnelling - Abstract
The unconventional multigap superconductivity in elemental Pb were reported previously by the surface sensitive tunneling experiments as well as predicted by several theory works. To obtain bulk evidence for such multiple gap behavior, the thermodynamic critical-field ${B}_{\mathrm{c}}$ was measured along three different crystallographic directions ([100], [110], and [111]) in a high-quality Pb single crystal by means of muon spin rotation/relaxation. No difference in temperature evolution of ${B}_{\mathrm{c}}$ for all three directions was detected. The average reduced gap $\ensuremath{\alpha}=\mathrm{\ensuremath{\Delta}}/{k}_{\mathrm{B}}{T}_{\mathrm{c}}=2.312(3)$ ($\mathrm{\ensuremath{\Delta}}$ is the zero-temperature gap value, and ${T}_{\mathrm{c}}$ is the transition temperature) was further obtained by employing the phenomenological $\ensuremath{\alpha}$ model. Our results imply that the elemental Pb is an isotropic superconductor with a single energy gap.
- Published
- 2021
5. Time-reversal invariant and fully gapped unconventional superconducting state in the bulk of the topological compound Nb0.25Bi2Se3
- Author
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M. Z. Hasan, Charles Mielke, Jiaxin Yin, Kaya Kobayashi, Hubertus Luetkens, Debarchan Das, Kristin Willa, Zurab Guguchia, Ulrich Welp, Anthony A. Amato, Matthew Smylie, and Takeshi Takahashi
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Physics ,Superconductivity ,Magnetic moment ,Niobium ,chemistry.chemical_element ,02 engineering and technology ,Electronic structure ,Muon spin spectroscopy ,021001 nanoscience & nanotechnology ,Topology ,01 natural sciences ,symbols.namesake ,chemistry ,Condensed Matter::Superconductivity ,Topological insulator ,0103 physical sciences ,symbols ,Symmetry breaking ,van der Waals force ,010306 general physics ,0210 nano-technology - Abstract
Recently, the niobium (Nb) doped topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ layers, has been shown to exhibit both superconductivity with ${T}_{c}\ensuremath{\simeq}3$ K and topological surface states. Here we report on muon spin rotation experiments probing the temperature and field dependence of effective magnetic penetration depth ${\ensuremath{\lambda}}_{\mathrm{eff}}\left(T\right)$ in the layered topological superconductor candidate ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. The exponential temperature dependence of ${\ensuremath{\lambda}}_{\mathrm{eff}}^{\ensuremath{-}2}(T)$ at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature ${T}_{c}=2.9\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and the gap to ${T}_{c}$ ratio $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}=3.95(19)$. We also reveal that the ratio ${T}_{c}/{\ensuremath{\lambda}}_{\mathrm{eff}}^{\ensuremath{-}2}$ is comparable to those of unconventional superconductors, which hints at an unconventional pairing mechanism. Furthermore, time-reversal symmetry breaking was excluded in the superconducting state with sensitive zero-field $\ensuremath{\mu}\mathrm{SR}$ experiments. We hope the present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between superconductivity and the topologically nontrivial electronic structure of ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$.
- Published
- 2020
6. Bulk single-crystal growth of the theoretically predicted magnetic Weyl semimetals RAlGe ( R = Pr, Ce)
- Author
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Jonathan S. White, Ekaterina Pomjakushina, Charles Mielke, Pascal Puphal, Marisa Medarde, Tian Shang, Y. Soh, and Neeraj Kumar
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Materials science ,Physics and Astronomy (miscellaneous) ,Condensed matter physics ,Magnetism ,Order (ring theory) ,02 engineering and technology ,021001 nanoscience & nanotechnology ,01 natural sciences ,Magnetization ,Ferromagnetism ,Electrical resistivity and conductivity ,0103 physical sciences ,Antiferromagnetism ,General Materials Science ,Ideal (ring theory) ,010306 general physics ,0210 nano-technology ,Ground state - Abstract
We explore two methods for single-crystal growth of the theoretically proposed magnetic Weyl semimetals $R\mathrm{AlGe}$ ($R$ = Pr, Ce), which prove that a floating-zone technique, being both crucible- and flux-free, is crucial to obtain perfectly stoichiometric $R\mathrm{AlGe}$ crystals. In contrast, the crystals grown by a flux-growth technique tend to be Al-rich. We further present both structural and elemental analyses, along with bulk magnetization and electrical resistivity data on the crystals prepared by the floating-zone technique. Both systems with the intended 1:1:1 stoichiometry crystallize in the anticipated polar $I{4}_{1}md$ (No. 109) space group, although neither displays the theoretically expected ferromagnetic ground state. Instead PrAlGe displays a spin-glass-like transition below 16 K with an easy $c$ axis and CeAlGe has an easy-$ab$-plane antiferromagnetic order below 5 K. The grown crystals provide an ideal platform for microscopic studies of the magnetic field-tunable correlation physics involving magnetism and topological Weyl nodes.
- Published
- 2019
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