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Superconductivity and Bosonic Metal States of FeSe/SrTiO3.

Record Type:	Language materials, manuscript : Monograph/item
Title/Author:	Superconductivity and Bosonic Metal States of FeSe/SrTiO3./
Author:	Li, Yanan.
Description:	1 online resource (141 pages)
Notes:	Source: Dissertations Abstracts International, Volume: 85-03, Section: B.
Contained By:	Dissertations Abstracts International85-03B.
Subject:	Electrons. -
Online resource:	click for full text (PQDT)
ISBN:	9798380258463

Superconductivity and Bosonic Metal States of FeSe/SrTiO3.
Li, Yanan.

Superconductivity and Bosonic Metal States of FeSe/SrTiO3. - 1 online resource (141 pages)

Source: Dissertations Abstracts International, Volume: 85-03, Section: B.

Thesis (Ph.D.)--The Pennsylvania State University, 2023.

Includes bibliographical references

When a single atomic layer (single unit cell, UC) of FeSe is grown on SrTiO3 (FeSe/STO), its superconducting transition temperature (Tc) and superconducting gap show a striking increase compared to their values in bulk FeSe (as large as an eightfold increase). This makes the FeSe/STO an attractive model system for exploring the potential of interface engineering by molecular beam epitaxy (MBE) growth in complex material design. Since the discovery of this phenomenon in 2012, researchers have been working to replicate, comprehend, and extend the discovery of enhanced superconductivity in FeSe/STO with great enthusiasm and scrutiny. In this thesis, we report a combined MBE growth, in-situ and ex-situ transport study of FeSe/STO.We first investigate the capping layer influence of the superconductivity in FeSe/STO by growing FeSe films capped with three different materials: insulating FeTe, nonmetallic Te, and metallic Zr. The in-situ and ex-situ transport measurements show that FeTe acts as an optimal cap that has minimal impact on the intrinsic Tc of pristine FeSe/STO, while the transfer of holes from the Te cap completely suppresses the superconductivity and results in insulating behavior. We then studied the angular dependence of the in-plane critical field of FeSe/STO capped with FeTe. An isotropic in-plane critical field is obtained, providing new insights into and setting constraints on the nature of superconductivity in FeSe/STO.Next, we carried out a study of anomalous metal and bosonic strange metal behavior in disordered FeSe/STO capped with FeTe. Initially, two-dimensional bosonic systems with Cooper pairs were considered to exist in one of two ground states, condensed in a superconductor or localized in an insulator. However, experiments on disordered two-dimensional superconducting films revealed that a metallic state disrupts the direct transition between the superconductor and the insulator. The key signature of this metallic state, which is known as an 'anomalous metal,' is the saturated resistance at low temperature. In our study, we tuned the strength of disorder in FeSe/STO by offsetting the growth parameter from optimal conditions for high critical temperature superconductivity and by patterning the film into Josephson junction arrays. When FeSe/STO is weakly disordered, we found an anomalous metal state with the highest critical temperature amongst all the previously reported two-dimensional superconducting systems. The critical temperature of the anomalous metal state is found to decrease with increasing disorder strength.With increasing disorder strength, we found a new metallic state of Cooper pairs different from the anomalous metal. In this state, the resistance shows a linear dependence on temperature (T-linear). In fermionic systems, the T-linear resistance deviates from the conventional T2 resistance in the low temperature regime described by Fermi liquid theory; this is known as a 'strange metal.' Different from the fermionic strange metal, the T-linear resistance we observed shows a much larger slope, a suppressed Hall coefficient, and clear h/2e quantum oscillations of Cooper pairs, indicating the bosonic nature of the T-linear resistance. We thus named the T-linear resistance behavior of Cooper pairs a 'bosonic strange metal.' The slope of the T-linear resistance in the bosonic strange metal increases with increasing disorder strength.

Electronic reproduction.
Ann Arbor, Mich. :
ProQuest,
2024

Mode of access: World Wide Web

ISBN: 9798380258463Subjects--Topical Terms:

580456
Electrons.
Subjects--Index Terms:

Superconducting systemsIndex Terms--Genre/Form:

554714
Electronic books.

Superconductivity and Bosonic Metal States of FeSe/SrTiO3.
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520 $a When a single atomic layer (single unit cell, UC) of FeSe is grown on SrTiO3 (FeSe/STO), its superconducting transition temperature (Tc) and superconducting gap show a striking increase compared to their values in bulk FeSe (as large as an eightfold increase). This makes the FeSe/STO an attractive model system for exploring the potential of interface engineering by molecular beam epitaxy (MBE) growth in complex material design. Since the discovery of this phenomenon in 2012, researchers have been working to replicate, comprehend, and extend the discovery of enhanced superconductivity in FeSe/STO with great enthusiasm and scrutiny. In this thesis, we report a combined MBE growth, in-situ and ex-situ transport study of FeSe/STO.We first investigate the capping layer influence of the superconductivity in FeSe/STO by growing FeSe films capped with three different materials: insulating FeTe, nonmetallic Te, and metallic Zr. The in-situ and ex-situ transport measurements show that FeTe acts as an optimal cap that has minimal impact on the intrinsic Tc of pristine FeSe/STO, while the transfer of holes from the Te cap completely suppresses the superconductivity and results in insulating behavior. We then studied the angular dependence of the in-plane critical field of FeSe/STO capped with FeTe. An isotropic in-plane critical field is obtained, providing new insights into and setting constraints on the nature of superconductivity in FeSe/STO.Next, we carried out a study of anomalous metal and bosonic strange metal behavior in disordered FeSe/STO capped with FeTe. Initially, two-dimensional bosonic systems with Cooper pairs were considered to exist in one of two ground states, condensed in a superconductor or localized in an insulator. However, experiments on disordered two-dimensional superconducting films revealed that a metallic state disrupts the direct transition between the superconductor and the insulator. The key signature of this metallic state, which is known as an 'anomalous metal,' is the saturated resistance at low temperature. In our study, we tuned the strength of disorder in FeSe/STO by offsetting the growth parameter from optimal conditions for high critical temperature superconductivity and by patterning the film into Josephson junction arrays. When FeSe/STO is weakly disordered, we found an anomalous metal state with the highest critical temperature amongst all the previously reported two-dimensional superconducting systems. The critical temperature of the anomalous metal state is found to decrease with increasing disorder strength.With increasing disorder strength, we found a new metallic state of Cooper pairs different from the anomalous metal. In this state, the resistance shows a linear dependence on temperature (T-linear). In fermionic systems, the T-linear resistance deviates from the conventional T2 resistance in the low temperature regime described by Fermi liquid theory; this is known as a 'strange metal.' Different from the fermionic strange metal, the T-linear resistance we observed shows a much larger slope, a suppressed Hall coefficient, and clear h/2e quantum oscillations of Cooper pairs, indicating the bosonic nature of the T-linear resistance. We thus named the T-linear resistance behavior of Cooper pairs a 'bosonic strange metal.' The slope of the T-linear resistance in the bosonic strange metal increases with increasing disorder strength.
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