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Elahi MM, Vakili H, Zeng Y, Dean CR, Ghosh AW. Direct Evidence of Klein and Anti-Klein Tunneling of Graphitic Electrons in a Corbino Geometry. PHYSICAL REVIEW LETTERS 2024; 132:146302. [PMID: 38640364 DOI: 10.1103/physrevlett.132.146302] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/06/2023] [Revised: 10/10/2023] [Accepted: 02/01/2024] [Indexed: 04/21/2024]
Abstract
Transport measurement of electron optics in monolayer graphene p-n junction devices has been traditionally studied with negative refraction and chiral transmission experiments in Hall bar magnetic focusing setups. We show direct signatures of Klein (monolayer) and anti-Klein (bilayer) tunneling with a circular "edgeless" Corbino geometry made out of gated graphene p-n junctions. Noticeable in particular is the appearance of angular sweet spots (Brewster angles) in the magnetoconductance data of bilayer graphene, which minimizes head-on transmission, contrary to conventional Fresnel optics or monolayer graphene which show instead a sharpened collimation of transmission paths. The local maxima on the bilayer magnetoconductance plots migrate to higher fields with increasing doping density. These experimental results are in good agreement with detailed numerical simulations and analytical predictions.
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Affiliation(s)
- Mirza M Elahi
- Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, Virginia 22904, USA
| | - Hamed Vakili
- Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska 68588, USA
| | - Yihang Zeng
- Department of Physics, Columbia University, New York, New York 10027, USA
| | - Cory R Dean
- Department of Physics, Columbia University, New York, New York 10027, USA
| | - Avik W Ghosh
- Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, Virginia 22904, USA
- Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA
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Wang S, Zhong M, Liu H, Ju M. Coexistence of topological node surface and Dirac fermions in phonon-mediated superconductor YB 2C 2. Phys Chem Chem Phys 2024; 26:1454-1461. [PMID: 38113107 DOI: 10.1039/d3cp03678b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2023]
Abstract
The interaction between nontrivial topology and superconductivity in condensed matter physics has attracted tremendous research interest as it could give rise to exotic phenomena. Herein, based on first-principles calculations, we investigate the electronic structures, mechanical properties, topological properties, dynamic stability, electron-phonon coupling (EPC), and superconducting properties of the synthesized real material YB2C2. It is a tetragonal structure with P4/mbm symmetry and exhibits excellent stability. The calculated electronic band structures reveal that a zero-dimension (0D) Dirac point and two-dimensional (2D) nodal surface coexist near the Fermi level. A spin-orbit coupling (SOC) Dirac point with the topological Fermi arc is observed on the (001) surface. These nodal surfaces are protected by a two-fold screw axis and time-reversal symmetry. Based on the Bardeen-Cooper-Schrieffer theory, the superconducting transition temperature (Tc) in the range 1.25-4.45 K with different Coulomb repulsion constant μ* for YB2C2 is estimated to be consistent with previous experimental results. In addition, the EPC is mainly from the coupling between the dx2-y2 and dz2 orbitals of the Y atom and low-energy phonon modes. The presence of superconductivity and nontrivial topological surface state in YB2C2 suggests that it may be a candidate material for topological superconductors.
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Affiliation(s)
- Siqi Wang
- School of Physical Science and Technology, Southwest University, Chongqing 400715, China.
| | - Mingmin Zhong
- School of Physical Science and Technology, Southwest University, Chongqing 400715, China.
| | - Haibo Liu
- School of Physical Science and Technology, Southwest University, Chongqing 400715, China.
| | - Meng Ju
- School of Physical Science and Technology, Southwest University, Chongqing 400715, China.
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Stensberg J, Han X, Lee S, McGill SA, Paglione J, Takeuchi I, Kane CL, Wu L. Observation of the Superconducting Proximity Effect from Surface States in SmB_{6}/YB_{6} Thin Film Heterostructures via Terahertz Spectroscopy. PHYSICAL REVIEW LETTERS 2023; 130:096901. [PMID: 36930917 DOI: 10.1103/physrevlett.130.096901] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/10/2022] [Revised: 08/12/2022] [Accepted: 01/26/2023] [Indexed: 06/18/2023]
Abstract
The ac conduction of epitaxially grown SmB_{6} thin films and superconducting heterostructures of SmB_{6}/YB_{6} are investigated via time-domain terahertz spectroscopy. A two-channel model of thickness-dependent bulk states and thickness-independent surface states accurately describes the measured conductance of bare SmB_{6} thin films, demonstrating the presence of surface states in SmB_{6}. While the observed reductions in the simultaneously measured superconducting gap, transition temperature, and superfluid density of SmB_{6}/YB_{6} heterostructures relative to bare YB_{6} indicate the penetration of proximity-induced superconductivity into the SmB_{6} overlayer; the corresponding SmB_{6}-thickness independence between different heterostructures indicates that the induced superconductivity is predominantly confined to the interface surface state of the SmB_{6}. This study demonstrates the ability of terahertz spectroscopy to probe proximity-induced superconductivity at an interface buried within a heterostructure, and our results show that SmB_{6} behaves as a predominantly insulating bulk surrounded by conducting surface states in both the normal and induced-superconducting states in both terahertz and dc responses, which is consistent with the topological Kondo insulator picture.
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Affiliation(s)
- Jonathan Stensberg
- Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Xingyue Han
- Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Seunghun Lee
- Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA
- Department of Physics, Pukyong National University, Busan 48513, Republic of Korea
| | - Stephen A McGill
- National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA
| | - Johnpierre Paglione
- Maryland Quantum Materials Center, Department of Physics, University of Maryland, College Park, Maryland 20742, USA
| | - Ichiro Takeuchi
- Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA
- Maryland Quantum Materials Center, Department of Physics, University of Maryland, College Park, Maryland 20742, USA
| | - Charles L Kane
- Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
| | - Liang Wu
- Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
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Valence-skipping and quasi-two-dimensionality of superconductivity in a van der Waals insulator. Nat Commun 2022; 13:6938. [DOI: 10.1038/s41467-022-34726-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2021] [Accepted: 11/01/2022] [Indexed: 11/15/2022] Open
Abstract
AbstractValence fluctuation of interacting electrons plays a crucial role in emergent quantum phenomena in correlated electron systems. The theoretical rationale is that this effect can drive a band insulator into a superconductor through charge redistribution around the Fermi level. However, the root cause of such a fluctuating leap in the ionic valency remains elusive. Here, we demonstrate a valence-skipping-driven insulator-to-superconductor transition and realize quasi-two-dimensional superconductivity in a van der Waals insulator GeP under pressure. This is shown to result from valence skipping of the Ge cation, altering its average valency from 3+ to 4+, turning GeP from a layered compound to a three-dimensional covalent system with superconducting critical temperature reaching its maximum of 10 K. Such a valence-skipping-induced superconductivity with a quasi-two-dimensional nature in thin samples, showing a Berezinskii-Kosterlitz-Thouless-like character, is further confirmed by angle-dependent upper-critical-field measurements. These findings provide a model system to examine competing order parameters in valence-skipping systems.
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Aishwarya A, Cai Z, Raghavan A, Romanelli M, Wang X, Li X, Gu GD, Hirsbrunner M, Hughes T, Liu F, Jiao L, Madhavan V. Spin-selective tunneling from nanowires of the candidate topological Kondo insulator SmB 6. Science 2022; 377:1218-1222. [PMID: 36074835 DOI: 10.1126/science.abj8765] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Incorporating relativistic physics into quantum tunneling can lead to exotic behavior such as perfect transmission through Klein tunneling. Here, we probed the tunneling properties of spin-momentum-locked relativistic fermions by designing and implementing a tunneling geometry that uses nanowires of the topological Kondo insulator candidate samarium hexaboride. The nanowires are attached to the end of scanning tunneling microscope tips and used to image the bicollinear stripe spin order in the antiferromagnet Fe1.03Te with a Neel temperature of about 50 kelvin. The antiferromagnetic stripes become invisible above 10 kelvin concomitant with the suppression of the topological surface states in the tip. We further demonstrate that the direction of spin polarization is tied to the tunneling direction. Our technique establishes samarium hexaboride nanowires as ideal conduits for spin-polarized currents.
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Affiliation(s)
- Anuva Aishwarya
- Department of Physics and Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA
| | - Zhuozhen Cai
- Department of Physics and Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA
| | - Arjun Raghavan
- Department of Physics and Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA
| | - Marisa Romanelli
- Department of Physics and Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA
| | - Xiaoyu Wang
- National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA
| | - Xu Li
- State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, and School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China
| | - G D Gu
- Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA
| | - Mark Hirsbrunner
- Department of Physics and Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA
| | - Taylor Hughes
- Department of Physics and Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA
| | - Fei Liu
- State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Material and Technology, and School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China
| | - Lin Jiao
- Department of Physics and Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA.,National High Magnetic Field Laboratory, Florida State University, Tallahassee, FL 32310, USA
| | - Vidya Madhavan
- Department of Physics and Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA
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Hwang J, Lee S, Lee JE, Kang M, Ryu H, Joo HJ, Denlinger J, Park JH, Hwang C. Tunable Kondo Resonance at a Pristine Two-Dimensional Dirac Semimetal on a Kondo Insulator. NANO LETTERS 2020; 20:7973-7979. [PMID: 33104350 DOI: 10.1021/acs.nanolett.0c02751] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
The proximity of two different materials leads to an intricate coupling of quasiparticles so that an unprecedented electronic state is often realized at the interface. Here, we demonstrate a resonance-type many-body ground state in graphene, a nonmagnetic two-dimensional Dirac semimetal, when grown on SmB6, a Kondo insulator, via thermal decomposition of fullerene molecules. This ground state is typically observed in three-dimensional magnetic materials with correlated electrons. Above the characteristic Kondo temperature of the substrate, the electron band structure of pristine graphene remains almost intact. As temperature decreases, however, the Dirac Fermions of graphene become hybridized with the Sm 4f states. Remarkable enhancement of the hybridization and Kondo resonance is observed with further cooling and increasing charge-carrier density of graphene, evidencing the Kondo screening of the Sm 4f local magnetic moment by the conduction electrons of graphene at the interface. These findings manifest the realization of the Kondo effect in graphene by the proximity of SmB6 that is tuned by the temperature and charge-carrier density of graphene.
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Affiliation(s)
- Jinwoong Hwang
- Department of Physics, Pusan National University, Busan 46241, South Korea
- Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
| | - Seungseok Lee
- Center for Complex Phase Materials, Max Planck POSTECH/Korea Research Initiative, Pohang 37673, South Korea
- Division of Advanced Material Science, Pohang University of Science and Technology, Pohang 37673, South Korea
| | - Ji-Eun Lee
- Department of Physics, Pusan National University, Busan 46241, South Korea
- Center for Spintronics, Korea Institute of Science and Technology, Seoul 02792, South Korea
| | - Minhee Kang
- Department of Physics, Pusan National University, Busan 46241, South Korea
| | - Hyejin Ryu
- Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
- Center for Complex Phase Materials, Max Planck POSTECH/Korea Research Initiative, Pohang 37673, South Korea
- Center for Spintronics, Korea Institute of Science and Technology, Seoul 02792, South Korea
| | - Hyun-Jeong Joo
- Department of Physics, Pusan National University, Busan 46241, South Korea
| | - Jonathan Denlinger
- Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
| | - Jae-Hoon Park
- Center for Complex Phase Materials, Max Planck POSTECH/Korea Research Initiative, Pohang 37673, South Korea
- Division of Advanced Material Science, Pohang University of Science and Technology, Pohang 37673, South Korea
- Department of Physics, Pohang University of Science and Technology, Pohang 37673, South Korea
| | - Choongyu Hwang
- Department of Physics, Pusan National University, Busan 46241, South Korea
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Casas OE, Páez SG, Herrera WJ. A Green's function approach to topological insulator junctions with magnetic and superconducting regions. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2020; 32:485302. [PMID: 32894743 DOI: 10.1088/1361-648x/abafc9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/16/2020] [Accepted: 08/17/2020] [Indexed: 06/11/2023]
Abstract
This work presents a Green's function approach, originally implemented in graphene with well-defined edges, to the surface of a strong 3D topological insulator with a sequence of proximitized superconducting (S) and ferromagnetic (F) surfaces. This consists of the derivation of the Green's functions for each region by the asymptotic solutions method and their coupling by a tight-binding Hamiltonian with the Dyson equation to obtain the full Green's functions of the system. These functions allow the direct calculation of the momentum-resolved spectral density of states, the identification of subgap interface states and the derivation of the differential conductance for a wide variety of configurations of the junctions. We illustrate the application of this method for some simple systems with two and three regions, finding the characteristic chiral state of the quantum anomalous Hall effect at the NF interfaces, and chiral Majorana modes at the NS interfaces. Finally, we discuss some geometrical effects present in three-region junctions such as weak Fabry-Pérot resonances and Andreev bound states.
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Affiliation(s)
- Oscar E Casas
- Departamento de Física, Universidad Nacional de Colombia, Bogotá, Colombia
| | - Shirley Gómez Páez
- Departamento de Física, Universidad Nacional de Colombia, Bogotá, Colombia
- Departamento de Física, Universidad el Bosque, Bogotá, Colombia
| | - William J Herrera
- Departamento de Física, Universidad Nacional de Colombia, Bogotá, Colombia
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Blasi G, Taddei F, Arrachea L, Carrega M, Braggio A. Nonlocal Thermoelectricity in a Superconductor-Topological-Insulator-Superconductor Junction in Contact with a Normal-Metal Probe: Evidence for Helical Edge States. PHYSICAL REVIEW LETTERS 2020; 124:227701. [PMID: 32567914 DOI: 10.1103/physrevlett.124.227701] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/20/2019] [Revised: 02/05/2020] [Accepted: 05/15/2020] [Indexed: 06/11/2023]
Abstract
We consider a Josephson junction hosting a Kramers pair of helical edge states of a quantum spin Hall bar in contact with a normal-metal probe. In this hybrid system, the orbital phase, induced by a small magnetic field threading the junction known as a Doppler shift, combines with the conventional Josephson phase difference and originates an effect akin to a Zeeman field in the spectrum. As a consequence, when a temperature bias is applied to the superconducting terminals, a thermoelectric current is established in the normal probe. We argue that this purely nonlocal thermoelectric effect is a unique signature of the helical nature of the edge states coupled to superconducting leads and it can constitute a useful tool for probing the helical nature of the edge states in systems where the Hall bar configuration is difficult to achieve. We fully characterize thermoelectric response and performance of this hybrid junction in a wide range of parameters, demonstrating that the external magnetic flux inducing the Doppler shift can be used as a knob to control the thermoelectric response and the heat flow in a novel device based on topological junctions.
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Affiliation(s)
- Gianmichele Blasi
- NEST, Scuola Normale Superiore and Instituto Nanoscienze-CNR, I-56126 Pisa, Italy
| | - Fabio Taddei
- NEST, Scuola Normale Superiore and Instituto Nanoscienze-CNR, I-56126 Pisa, Italy
| | - Liliana Arrachea
- International Center for Advanced Studies, ECyT-UNSAM, Campus Miguelete, 25 de Mayo y Francia, 1650 Buenos Aires, Argentina
| | - Matteo Carrega
- NEST, Scuola Normale Superiore and Instituto Nanoscienze-CNR, I-56126 Pisa, Italy
| | - Alessandro Braggio
- NEST, Scuola Normale Superiore and Instituto Nanoscienze-CNR, I-56126 Pisa, Italy
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