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Phonon drag thermal Hall effect in metallic strontium titanate. Proc Natl Acad Sci U S A 2022; 119:e2201975119. [PMID: 35994652 PMCID: PMC9436374 DOI: 10.1073/pnas.2201975119] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
SrTiO3, a quantum paralectric, displays a detectable phonon thermal Hall effect (THE). Here, we show that the amplitude of the THE is extremely sensitive to stoichiometry. It drastically decreases upon substitution of a tiny fraction of Sr atoms with Ca, which stabilizes the ferroelectric order. It drastically increases by an even lower density of oxygen vacancies, which turn the system to a dilute metal. The enhancement in the metallic state exceeds by far the sum of the electronic and the phononic contributions. We explain this observation as an outcome of three features: 1) Heat is mostly transported by phonons; 2) the electronic Hall angle is extremely large; and 3) there is substantial momentum exchange between electrons and phonons. Starting from Herring's picture of phonon drag, we arrive to a quantitative account of the enhanced THE. Thus, phonon drag, hitherto detected as an amplifier of thermoelectric coefficients, can generate a purely thermal transverse response in a dilute metal with a large Hall angle. Our results reveal a hitherto-unknown consequence of momentum-conserving collisions between electrons and phonons.
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Maruhashi K, Takahashi KS, Bahramy MS, Shimizu S, Kurihara R, Miyake A, Tokunaga M, Tokura Y, Kawasaki M. Anisotropic Quantum Transport through a Single Spin Channel in the Magnetic Semiconductor EuTiO 3. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2020; 32:e1908315. [PMID: 32383210 DOI: 10.1002/adma.201908315] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/18/2019] [Revised: 03/12/2020] [Accepted: 04/14/2020] [Indexed: 06/11/2023]
Abstract
Magnetic semiconductors are a vital component in the understanding of quantum transport phenomena. To explore such delicate, yet fundamentally important, effects, it is crucial to maintain a high carrier mobility in the presence of magnetic moments. In practice, however, magnetization often diminishes the carrier mobility. Here, it is shown that EuTiO3 is a rare example of a magnetic semiconductor that can be desirably grown using the molecular beam epitaxy to possess a high carrier mobility exceeding 3000 cm2 V-1 s-1 at 2 K, while intrinsically hosting a large magnetization value, 7 μB per formula unit. This is demonstrated by measuring the Shubnikov-de Haas (SdH) oscillations in the ferromagnetic state of EuTiO3 films with various carrier densities. Using first-principles calculations, it is shown that the observed SdH oscillations originate genuinely from Ti 3d-t2g states which are fully spin-polarized due to their energetical proximity to the in-gap Eu 4f bands. Such an exchange coupling is further shown to have a profound effect on the effective mass and fermiology of the Ti 3d-t2g electrons, manifested by a directional anisotropy in the SdH oscillations. These findings suggest that EuTiO3 film is an ideal magnetic semiconductor, offering a fertile field to explore quantum phenomena suitable for spintronic applications.
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Affiliation(s)
- Kazuki Maruhashi
- Department of Applied Physics and Quantum Phase Electronics Center, University of Tokyo, Tokyo, 113-8656, Japan
| | - Kei S Takahashi
- RIKEN Center for Emergent Matter Science (CEMS), Wako, 351-0198, Japan
- PRESTO, Japan Science and Technology Agency (JST), Chiyoda-ku, Tokyo, 102-0075, Japan
| | - Mohammad Saeed Bahramy
- Department of Applied Physics and Quantum Phase Electronics Center, University of Tokyo, Tokyo, 113-8656, Japan
- RIKEN Center for Emergent Matter Science (CEMS), Wako, 351-0198, Japan
| | - Sunao Shimizu
- RIKEN Center for Emergent Matter Science (CEMS), Wako, 351-0198, Japan
| | - Ryosuke Kurihara
- Institute for Solid State Physics, University of Tokyo, Kashiwanoha, Kashiwa, 277-8581, Japan
| | - Atsushi Miyake
- Institute for Solid State Physics, University of Tokyo, Kashiwanoha, Kashiwa, 277-8581, Japan
| | - Masashi Tokunaga
- Institute for Solid State Physics, University of Tokyo, Kashiwanoha, Kashiwa, 277-8581, Japan
| | - Yoshinori Tokura
- Department of Applied Physics and Quantum Phase Electronics Center, University of Tokyo, Tokyo, 113-8656, Japan
- RIKEN Center for Emergent Matter Science (CEMS), Wako, 351-0198, Japan
| | - Masashi Kawasaki
- Department of Applied Physics and Quantum Phase Electronics Center, University of Tokyo, Tokyo, 113-8656, Japan
- RIKEN Center for Emergent Matter Science (CEMS), Wako, 351-0198, Japan
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Tomioka Y, Shirakawa N, Shibuya K, Inoue IH. Enhanced superconductivity close to a non-magnetic quantum critical point in electron-doped strontium titanate. Nat Commun 2019; 10:738. [PMID: 30760712 PMCID: PMC6374393 DOI: 10.1038/s41467-019-08693-1] [Citation(s) in RCA: 42] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/25/2018] [Accepted: 01/25/2019] [Indexed: 11/24/2022] Open
Abstract
Studies on quantum critical points (QCP) have focused on magnetic QCPs to date. Remarkable phenomena such as superconductivity due to avoided criticality have been discovered, but we focus here on the non-magnetic counterpart, i.e., the superconductivity of SrTiO3 regarded as being close to a ferroelectric QCP. Here we prepare high-quality Sr1-xLaxTi(16O1-z18Oz)3 single crystals without localisation at low temperatures, which allow us to systematically investigate the La substitution of Sr as an alternative to introducing oxygen vacancies. Analysis of our data based on a theoretical model predicts an appearance of the ferroelectric QCP around 3 × 1018 cm-3. Because of the QCP, the superconducting dome of Sr1-xLaxTiO3 can be raised upwards. Furthermore, remarkable enhancement of Tc (~0.6 K) is achieved by 18O exchange on the Sr1-xLaxTiO3 crystals. These findings provide a new knob for observing intriguing physics around the ferroelectric QCP.
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Affiliation(s)
- Yasuhide Tomioka
- National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 305-8565, Japan.
| | - Naoki Shirakawa
- National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 305-8565, Japan
| | - Keisuke Shibuya
- National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 305-8565, Japan
| | - Isao H Inoue
- National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, 305-8565, Japan.
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Chandra P, Lonzarich GG, Rowley SE, Scott JF. Prospects and applications near ferroelectric quantum phase transitions: a key issues review. REPORTS ON PROGRESS IN PHYSICS. PHYSICAL SOCIETY (GREAT BRITAIN) 2017; 80:112502. [PMID: 28752823 DOI: 10.1088/1361-6633/aa82d2] [Citation(s) in RCA: 21] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The emergence of complex and fascinating states of quantum matter in the neighborhood of zero temperature phase transitions suggests that such quantum phenomena should be studied in a variety of settings. Advanced technologies of the future may be fabricated from materials where the cooperative behavior of charge, spin and current can be manipulated at cryogenic temperatures. The progagating lattice dynamics of displacive ferroelectrics make them appealing for the study of quantum critical phenomena that is characterized by both space- and time-dependent quantities. In this key issues article we aim to provide a self-contained overview of ferroelectrics near quantum phase transitions. Unlike most magnetic cases, the ferroelectric quantum critical point can be tuned experimentally to reside at, above or below its upper critical dimension; this feature allows for detailed interplay between experiment and theory using both scaling and self-consistent field models. Empirically the sensitivity of the ferroelectric T c's to external and to chemical pressure gives practical access to a broad range of temperature behavior over several hundreds of Kelvin. Additional degrees of freedom like charge and spin can be added and characterized systematically. Satellite memories, electrocaloric cooling and low-loss phased-array radar are among possible applications of low-temperature ferroelectrics. We end with open questions for future research that include textured polarization states and unusual forms of superconductivity that remain to be understood theoretically.
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Affiliation(s)
- P Chandra
- Center for Materials Theory, Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, United States of America
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Behnia K, Aubin H. Nernst effect in metals and superconductors: a review of concepts and experiments. REPORTS ON PROGRESS IN PHYSICS. PHYSICAL SOCIETY (GREAT BRITAIN) 2016; 79:046502. [PMID: 27010481 DOI: 10.1088/0034-4885/79/4/046502] [Citation(s) in RCA: 44] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
The Nernst effect is the transverse electric field produced by a longitudinal thermal gradient in the presence of a magnetic field. At the beginning of this century, Nernst experiments on cuprates were analyzed assuming that: (i) the contribution of quasi-particles to the Nernst signal is negligible; and (ii) Gaussian superconducting fluctuations cannot produce a Nernst signal well above the critical temperature. Both these assumptions were contradicted by subsequent experiments. This paper reviews experiments documenting multiple sources of a Nernst signal, which, according to the Bridgman relation, measures the flow of transverse entropy caused by a longitudinal particle flow. Along the lines of Landauer's approach to transport phenomena, the magnitude of the transverse magneto-thermoelectric response is linked to the quantum of thermoelectric conductance and a number of material-dependent length scales: the mean free path, the Fermi wavelength, the de Broglie thermal wavelength and the superconducting coherence length. Extremely mobile quasi-particles in dilute metals generate a widely-documented Nernst signal. Fluctuating Cooper pairs in the normal state of superconductors have been found to produce a detectable Nernst signal with an amplitude conforming to the Gaussian theory, first conceived by Ussishkin, Sondhi and Huse. In addition to these microscopic sources, mobile Abrikosov vortices, mesoscopic objects simultaneously carrying entropy and magnetic flux, can produce a sizeable Nernst response. Finally, in metals subject to a magnetic field strong enough to truncate the Fermi surface to a few Landau tubes, each exiting tube generates a peak in the Nernst response. The survey of these well-established sources of the Nernst signal is a helpful guide to identify the origin of the Nernst signal in other controversial cases.
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Affiliation(s)
- Kamran Behnia
- Laboratoire de Physique et d'Etude des Matériaux, (Centre National de la Recherche Scientifique-Université Pierre et Marie Curie), Ecole Supérieure de Physique et de Chimie Industrielles, 75005 Paris, France
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