Jennifer E Hoffman Harvard U - "Visualizing the impurity states around Kondo holes in SmB6"

Rice Center for Quantum Materials

1 Dec 202131:07

EducationalLearning

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TLDRThe speaker expresses gratitude for the opportunity to discuss imaging in samarium hexaboride (SmB6), a topological insulator, after a two-year hiatus from travel and talks. The presentation delves into the material's controversies, particularly focusing on the surface state velocity, topological protection, and quantum oscillations. The speaker credits their graduate student and others for their contributions to the research. Using scanning tunneling microscopy (STM), they reveal the presence of heavy Dirac cones and discuss the discrepancies between STM results and other experimental techniques. The talk also explores the impact of samarium site defects, or 'condo holes,' on local charge fluctuations and their relation to quantum oscillations. The speaker suggests that these defects could explain the observed bulk quantum oscillations at the light band frequency, offering a new perspective on the material's properties and the ongoing debates within the field.

Takeaways

🙏 The speaker expresses gratitude to the organizers for the opportunity to speak after a long hiatus from traveling and giving talks.
🔬 The presentation focuses on imaging conduction holes in SmB6 (Samarium hexaboride), a material of interest due to its topological properties and related controversies.
🎓 Credit is given to graduate student Harry Perry and others for their significant contributions to the research conducted over the past five years.
🏆 Acknowledgement is made to funding bodies like the Moore Foundation and the National Science Foundation for their support of the research.
📈 The number of publications on SmB6 surged after 2010 due to debates on its properties, such as surface state velocity and topological protection.
🔍 STM (Scanning Tunneling Microscopy) was used to investigate the surface states of SmB6, providing high-resolution spatial and momentum space information.
📊 The STM results showed heavy Dirac cones with a Dirac point within the bulk hybridization gap, which was consistent with theoretical predictions but differed from other experimental techniques.
🤔 The speaker discusses the discrepancies between STM findings and other techniques, suggesting that spatial resolution might be a key factor in resolving these differences.
🌐 The research also explores the impact of condensation holes (condo holes) on the conduction electrons, suggesting that they can provide insights into the material's properties.
🧲 The study found that condo holes in SmB6 can lead to local charge fluctuations and affect the density of states, which could explain certain quantum oscillations observed in the material.
🤝 The speaker concludes by addressing some of the controversies and questions raised during the presentation, highlighting the importance of understanding the role of impurities and defects in SmB6.

Q & A

What is the main topic of the talk?
-The main topic of the talk is imaging condensation holes in samarium hexaboride with a focus on the controversies surrounding this material and its properties.
Who is credited for carrying the work through the last five years?
-Harry Perry, a graduate student, is credited for carrying the work through the last five years.
What is the significance of the number of publications increasing from 1955 to 2010 in the context of samarium hexaboride?
-The increase in the number of publications signifies the growing interest and controversies in the field, especially regarding the properties of samarium hexaboride, such as its low-temperature conductivity and whether it is a topological material.
What is STM and how does it contribute to understanding the properties of samarium hexaboride?
-STM stands for Scanning Tunneling Microscopy, a high-resolution technique that can measure the density of states in real space and momentum space, providing insights into the material's properties, such as the presence of surface states and their velocities.
What is the controversy regarding the surface state velocity and the Dirac point in samarium hexaboride?
-The controversy is about whether the surface state velocity and the location of the Dirac point are consistent with theoretical predictions or if they are influenced by impurity effects or other factors.
What does the speaker suggest about the surface of samarium hexaboride?
-The speaker suggests that the surface of samarium hexaboride is not easily cleaved and can have different terminations, which might affect the measurements and observations made by various techniques.
What is the role of the Moore Foundation and the National Science Foundation in this research?
-The Moore Foundation and the National Science Foundation provided funding and support for the research on samarium hexaboride.
What is the significance of the heavy Dirac cones observed in the STM measurements?
-The heavy Dirac cones observed in the STM measurements are significant because they provide evidence for the existence of surface states in samarium hexaboride and help resolve the controversy about their properties.
What is the purpose of the Fourier transform in the STM measurements?
-The Fourier transform is used to analyze the real space density of states data, allowing the researchers to observe the wave vector of the oscillations and infer the material's eigenstates.
How does the presence of condensation holes affect the properties of samarium hexaboride?
-The presence of condensation holes can cause local charge fluctuations and affect the hybridization strength, charge density, and even the magnetic susceptibility, which can have implications for the material's electronic properties.
What is the controversy regarding quantum oscillations in samarium hexaboride?
-The controversy is whether the observed quantum oscillations are due to impurity effects or if they indicate the presence of an exotic non-charged Fermi surface in the material.
What experimental techniques were used to measure the properties of samarium hexaboride?
-Techniques such as transport measurements, quantum oscillation work, and angle-resolved photoemission spectroscopy (ARPES) were used, along with STM for spatially resolved measurements.
What is the conclusion of the research on samarium hexaboride?
-The research concludes that samarium hexaboride has heavy fermion states with the heaviest ever seen, about 400 times the mass of the electron, and that condensation holes play a significant role in the material's properties.

Outlines

00:00

🎤 Speaker Expresses Gratitude and Introduces Topic

The speaker begins by thanking the organizers for the opportunity to speak, noting it's been over two years since they last traveled for a talk. They mention feeling a bit rusty and introduce the topic of imaging in SmB6, a material with ongoing scientific controversies. The speaker credits their graduate student Harry Perry and others for their contributions and acknowledges funding from the Moore Foundation and the National Science Foundation. They also highlight the significance of SmB6 due to its controversial nature in the scientific community.

05:02

🔬 Controversies in SmB6 Research and STM Findings

The speaker delves into the history of SmB6 research, discussing the debates surrounding its properties, particularly the anomalous low-temperature conductivity. They mention the pivotal 2010 research by Pierce, Cullman, and collaborators that classified SmB6 as a topological material. The speaker reviews their STM (Scanning Tunneling Microscopy) work conducted during the pandemic, explaining how STM provides high-resolution spatial and momentum space information. They discuss the findings related to the heavy Fermi surface states and the discrepancies with other experimental techniques like ARPES and quantum oscillation measurements.

10:02

📈 STM Data Analysis and Comparison with Theoretical Models

The speaker presents raw data from STM measurements on SmB6 samples, showing a heavy Dirac cone and comparing it with theoretical expectations. They discuss the velocities of the Dirac cones and their agreement with theoretical predictions, while also addressing discrepancies with other studies. The speaker explores the impact of surface terminations on STM measurements, suggesting that spatially averaged techniques might miss the heavy Dirac cones observed by STM due to different surface terminations.

15:03

🧠 Theoretical Insights into Condo Holes and Their Effects

The speaker shifts focus to condo holes, explaining their role in local charge fluctuations. They reference a 2011 theory by Dirk Moore and Jeremy Figgins, which describes how a vacancy in a condo lattice affects various parameters. The speaker discusses how the removal of an f electron from a fully hybridized situation leads to spatial oscillations in hybridization strength, charge density, and metallization, all occurring at the wave vector of the original conduction band before hybridization.

20:04

🔬 Experimental Evidence and STM Charge Sensitivity

The speaker discusses experimental requirements for observing the effects of condo holes, emphasizing the need for nanometer-scale spatial resolution, sub-milli electron volt energy resolution, and kelvin temperatures. They highlight STM's capabilities in this regard and introduce a new method for interpreting STM data to gain insights into charge density. The speaker uses uranium ruthenium silicene as a test case to demonstrate the method's effectiveness in revealing charge oscillations at the light band wave vector.

25:05

🕵️‍♂️ Investigating Quantum Oscillations and Bulk Insulator Behavior

The speaker addresses the ongoing controversy regarding quantum oscillations in SmB6, a material known to be a bulk insulator. They propose that condo holes could be responsible for the observed bulk quantum oscillations at the light band frequency. The speaker presents experimental data showing oscillations in the density of states and a rectification ratio that indicates charge fluctuations at the light band frequency, offering a potential explanation for the observed quantum oscillations.

30:06

🤔 Open Questions and Future Research Directions

In the final part of the script, the speaker engages in a Q&A session, addressing questions about NMR results, the behavior of condo holes in topological versus regular condensers, and the surface properties of SmB6. They also discuss the presence of aluminum inclusions in samples and their potential impact on quantum oscillations. The speaker acknowledges the complexity of the material and the need for further research to fully understand its properties.

Mindmap

Keywords

💡Imaging

Imaging in this context refers to the process of visually representing or 'imaging' atomic and electronic structures of materials, which is a central theme of the video. The speaker discusses using Scanning Tunneling Microscopy (STM) to image the surface states of samarium hexaboride (SmB6), a material with significant scientific interest due to its topological properties.

💡Samarium Hexaboride (SmB6)

Samarium Hexaboride, or SmB6, is a compound that has been a subject of extensive research due to its topological insulator properties. In the video, the speaker focuses on imaging the surface states of SmB6 and resolving controversies surrounding its electronic properties, such as the nature of its conductivity and the presence of heavy Dirac fermions.

💡Topological Material

A topological material is a type of material that exhibits unique electronic properties due to its topological order. In the script, the speaker mentions that SmB6 was identified as a topological material with a surface state, sparking increased research interest and controversy in the field.

💡Surface State Velocity

Surface state velocity refers to how fast the surface electrons in a material can move. The video discusses controversies regarding the velocity of surface states in SmB6, with different experimental results leading to debates about the material's properties.

💡Dirac Point

The Dirac point is a point in the band structure of a solid where the conduction and valence bands meet, leading to the formation of a Dirac cone. In the context of the video, the speaker talks about the location of the Dirac point in relation to the Fermi level and its implications for the topological protection of surface states.

💡Quantum Oscillations

Quantum oscillations are phenomena where physical properties of a system oscillate as a function of an external parameter, such as a magnetic field. The video discusses a controversy regarding whether the bulk quantum oscillations in SmB6 are due to impurity effects or an exotic non-charged Fermi surface.

💡Scanning Tunneling Microscopy (STM)

Scanning Tunneling Microscopy (STM) is a high-resolution imaging technique used to study the surface of materials at the atomic level. In the script, the speaker describes using STM to investigate the electronic properties of SmB6, providing insights into the material's surface states and resolving discrepancies in previous studies.

💡Quasi-Particle Interference (QPI)

Quasi-Particle Interference is a phenomenon observed in STM where the interference of incoming and outgoing electron waves can be analyzed to determine the material's electronic structure. The speaker mentions using QPI to extract information about the eigenstates of SmB6 and resolve the controversy about the heavy Dirac cones.

💡Fermi Level

The Fermi level is the energy level at which the highest occupied electron state is found in a material at absolute zero temperature. In the video, the Fermi level is discussed in relation to the hybridization gap and the presence of surface states in SmB6.

💡Hybridization Gap

A hybridization gap is an energy gap that opens up in a material's band structure due to the interaction between different electronic states. The script discusses how, in SmB6, the hybridization gap forms between the f states and the conduction band as temperature decreases, leading to the emergence of a topological surface state.

💡Condensation

In the context of the video, condensation refers to the process of electrons adjusting to the presence of certain defects, such as 'condo holes', which can lead to local charge fluctuations and affect the material's electronic properties. The speaker suggests that understanding these condensation effects can provide insights into the controversies surrounding SmB6.

Highlights

Speaker expresses gratitude for the opportunity to travel and present after a long hiatus.

Introduction of the topic: imaging condensation holes in SmB6 and a bonus discussion on uranium ruthenium toolican II.

Credit given to graduate student Harry Perry for significant contributions to the research over five years.

Acknowledgment of the Moore Foundation and the National Science Foundation for their support.

Discussion of the historical controversy surrounding SmB6's anomalous low-temperature conductivity.

Mention of the 2010 study by Pierce, Cullman, and collaborators that identified SmB6 as a topological material.

Overview of the recent controversies focusing on surface state velocity, the Dirac point, and quantum oscillations.

STM (Scanning Tunneling Microscopy) used to study the surface states of SmB6 and resolve discrepancies in previous studies.

Findings that SmB6's surface states are consistent with theoretical predictions and contradict earlier studies.

Explanation of how STM provides high energy resolution and real space and momentum space information.

Presentation of raw STM data showing heavy Dirac cones and their velocities.

Analysis of why other techniques might not be observing the same heavy Dirac cones as STM.

Discussion on the challenges of preparing SmB6 samples for STM due to its hardness.

STM's ability to resolve the controversy by identifying different surface terminations and their effects.

Introduction of the concept of condensation holes and their role in local charge fluctuations.

Theoretical background on the effects of removing an f-electron from a Kondo lattice.

Pedagogical explanation of how conduction electrons adjust to accommodate f-electron moments.

STM's capability to provide nanometer spatial resolution and its importance in studying condensation holes.

Use of STM to observe charge density oscillations around condensation holes in uranium ruthenium toolican II.

Proposal that condensation holes in SmB6 could explain the observed quantum oscillations at the light band frequency.

Discussion on the difference between density of states and charge density oscillations in relation to quantum oscillations.

Speaker's conclusion that STM provides crucial insights into the controversies surrounding SmB6.

Audience questions and speaker's responses on NMR results, the behavior of topological condensers, and surface charge states.

Final thoughts on the self-consistent picture provided by STM findings and their implications for quantum oscillations.

Transcripts

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Related Tags

Heavy Fermions Quantum Oscillations STM Imaging Smerian Hexaboride Condor Holes Topological Materials Surface States Hybridization Gap Scientific Research Materials Science