Stress and crystal imperfections: Tools for the exploration of unconventional superconductivity via scanning tunneling microscopy

Olivares Rodriguez, Jorge

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https://hdl.handle.net/2142/117731 Copy

Description

Title

Stress and crystal imperfections: Tools for the exploration of unconventional superconductivity via scanning tunneling microscopy

Author(s)

Olivares Rodriguez, Jorge

Issue Date

2022-10-11

Director of Research (if dissertation) or Advisor (if thesis)

Madhavan, Vidya

Doctoral Committee Chair(s)

Mahmood, Fahad

Committee Member(s)

• Lorenz, Virginia
• Wagner, Lucas

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• superconductivity
• strain
• stress
• scanning probe microscopy
• scanning tunneling microscopy
• unconventional
• ripplocations
• detwinning
• iron superconductivity
• strontium ruthenate
• iron selenide
• Majorana
• defects
• crystal
• STM
• themal contraction
• passive strain device
• optimally doped iron selenide telluride
• iron telluride
• FeSe
• FeTe
• Fe(Se,Te)
• Sr2RuO4
• symmetry breaking stress
• thermal contraction
• strain STM

Abstract

Understanding unconventional superconductivity is one of the biggest challenges of condensed matter physics to date. Tackling this problem requires an understanding of the normal state from which it emerges and a direct path to probe the superconducting state itself directly. Scanning Tunneling Microscopy and Spectroscopy (STM/S) is uniquely suited for this challenge since it not only provides access to the electronic properties of materials directly, but grants us with the unique advantage of sub-angstrom resolution surface imaging and probing. Here we harness these advantages in order to study two relevant material sub-classes under unconventional superconductivity: the 11-iron chalchogenide series and Sr2RuO4. In this work, we show that crystal defects (more specifically one-dimensional defects) can be powerful allies in the search for novel physics. They can host bound states, break existing crystal symmetries or simply play passive roles on the surface. This list barely scratches the surface of what is possible and here we will explore some of the applications of such defects. In particular, we will show how propagating Majorana states can manifest within crystalline domain walls in the optimally doped Fe(Se,Te) alloy. Where we first use scanning tunneling spectroscopy to identify topological superconductivity at the surface of this material. We later demonstrate how certain domain walls can create a flat density of states inside the superconducting gap, a hallmark of linearly dispersing modes in one dimension. In an attempt to push the boundaries of what is possible under low temperature STM, we have developed a novel device capable of uniaxially straining a wide range of materials (from ductile metals to rigid ceramics) in a small footprint that simultaneously provides vertical access to the STM tip. Testing of the device through two independent methods sets our maximum compressive strain limit at 4K in the vicinity of 0.5%. This grants us an additional tuning parameter to investigate the interplay between lattice structure (e.g. crystal symmetry and orbital overlap) and electronic phenomena (including band structure and superconductivity). Several hydrostatic pressure and epitaxial strain studies have demonstrated that the electronic, magnetic, superconducting and structural properties in stoichiometric iron selenide (FeSe) are closely intertwinned. This makes FeSe an ideal candidate to study the effect of uniaxial strain among the iron superconductors. Our work in FeSe identifies how this twinned crystal relieves strain via twin boundary movement. While this effect has been known for decades in other (twinned) crystal systems, we believe it is the first time it has been observed under STM. While no evidence of homogeneous strain is found over the surface of the sample, we identified several extended one dimensional defects over the surface of the sample. We use STM to provide evidence that these are “ripple-like” structures similar to those identified in layered transition metal dichalcogenides. We show that there is electronic evidence for large tensile strain (in the order of 2 − 4%) at the apex of these ripples. Moreover, we show that for certain tensile strains, we can push the α’ hole band in FeSe completely below EF . That is one can effectively remove the hole-like pocket centered at the Γ point using uniaxial tensile strain, and thus the superconducting state we measure here emerges from the electron-like bands. In agreement with other measurements, we identify a 1 meV gap at the apex of these ripples. Hence we show that a Lifshitz transition can be induced in FeSe for large strains. Finally, motivated by recent stain measurements in Sr2RuO4 where an enhancement in T c was identified, we set out to perform a similar experiment under STM employing our novel device. Using the unique ability of STM to atomically image the surface of the sample, we identified several holes on the SrO surface. Through these topographic defects on the surface of strontium ruthenate, we first show that superconductivity seems to be suppressed on the surface layer. While the exact mechanism for this is suppression is unclear, the data suggests that this is limited to the top layer and thus the surface reconstruction may be involved. We then use these defects as windows into the layer below to directly probe the superconducting state of strontium ruthenate under stress. Here, we identify an enhancement of the superconducting state, where a strain of ≈ 0.4% increases the value of 2|∆| ≈ 750µV to ≈ 1.2mV, in agreement with thermodynamic measurements. However we find little enhancement in Hc2 and none in Tc. We suspect that the presence of scattering centers near our window to the “bulk-like” superconducting layer may be responsible for these discrepancies.

Graduation Semester

2022-12

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Jorge Olivares Rodriguez

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