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Higgs spectroscopy of superconductors : a new method to identify the superconducting gap symmetry Cheng, Nathan
Abstract
In this thesis we study the response of a BCS superconductor to an external ultra-fast terahertz electromagnetic field, which we choose so as to mimic the setup in a pump-probe experiment. We begin by considering an optical experimental setup and demonstrate that in an optical pump-probe experiment, the superconducting amplitude Higgs mode can be excited and measured with ultra-fast terahertz pump pulses. Moreover, for an anistropic d-wave superconductor, there are two Higgs mode, one at the usual 2∆ energy and one with a lower energy. The latter can be used to differentiate the d-wave symmetry from isotropic s-wave, by varying the polarization of the pump relative to the sample. For a linearly polarized pump with a vector potential aligned along a d-wave node we find only a single Higgs mode, while for a direction along an antinode we find two Higgs modes. Next, we consider an angle resolved photoemission spectroscopy (ARPES) experiment and derive a new set of equations of motion, for which we can analyze the two-time nonequilibrium Green’s functions. We show that the Higgs mode can also be studied in an ARPES pump-probe experiment. Moreover, we show how an ARPES pump-probe experiment can be used to differentiate between different momentum-dependent nonequilibrium Higgs modes. Our results suggest that in a d-wave superconductor, the second low-energy Higgs mode is of osculating, B1g character, which corresponds to a symmetry breaking along the d-wave nodal lines. Further study of the role of momentum symmetry breaking promises to provide deeper insight into generating new nonequilibrium states.
Item Metadata
Title |
Higgs spectroscopy of superconductors : a new method to identify the superconducting gap symmetry
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Creator | |
Publisher |
University of British Columbia
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Date Issued |
2018
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Description |
In this thesis we study the response of a BCS superconductor to an external ultra-fast terahertz electromagnetic field, which we choose so as to mimic the setup in a pump-probe experiment.
We begin by considering an optical experimental setup and demonstrate that in an optical pump-probe experiment, the superconducting amplitude Higgs mode can be excited and measured with ultra-fast terahertz pump pulses. Moreover, for an anistropic d-wave superconductor, there are two Higgs mode, one at the usual 2∆ energy and one with a lower energy. The latter can be used to differentiate the d-wave symmetry from isotropic s-wave, by varying the polarization of the pump relative to the sample. For a linearly polarized pump with a vector potential aligned along a d-wave node we find only a single Higgs mode, while for a direction along an antinode we find two Higgs modes.
Next, we consider an angle resolved photoemission spectroscopy (ARPES) experiment and derive a new set of equations of motion, for which we can analyze the two-time nonequilibrium Green’s functions. We show that the Higgs mode can also be studied in an ARPES pump-probe experiment. Moreover, we show how an ARPES pump-probe experiment can be used to differentiate between different momentum-dependent nonequilibrium Higgs modes. Our results suggest that in a d-wave superconductor, the second low-energy Higgs mode is of osculating, B1g character, which corresponds to a symmetry breaking along the d-wave nodal lines. Further study of the role of momentum symmetry breaking promises to provide deeper insight into generating new nonequilibrium states.
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Genre | |
Type | |
Language |
eng
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Date Available |
2018-08-29
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Provider |
Vancouver : University of British Columbia Library
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Rights |
Attribution-NonCommercial-NoDerivatives 4.0 International
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DOI |
10.14288/1.0371605
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URI | |
Degree | |
Program | |
Affiliation | |
Degree Grantor |
University of British Columbia
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Graduation Date |
2018-09
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Campus | |
Scholarly Level |
Graduate
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Rights URI | |
Aggregated Source Repository |
DSpace
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Rights
Attribution-NonCommercial-NoDerivatives 4.0 International