Condensed Matter Seminars

Date: December 8, 2023

Speaker: Petr Stepanov, University of Notre Dame

Title: Quantum Heavy Fermion Simulator in Moiré Materials

Host: Cyprian Lewandowski

Location: MagLab B 101 at 3:00 PM-4:00 PM

Abstract: The unexpected discovery of superconductivity in magic angle twisted bilayer graphene immediately generated a wave of intense theoretical and experimental research attracted by its rich phase diagram, which seemingly resembles ones of copper-oxide high-temperature superconductors. Originated in the low-energy narrow electronic bands, a family of magic angle graphene compounds hosts a collection of exotic phases including but not limited to superconductivity, correlated insulators, topological and magnetic orders. Compared to other strongly-correlated systems, 2D multilayers offer a unique opportunity to tune the charge carrier density in situ and adjust system properties in other ways (for example, by alternating the distance to the gate or varying the dielectric environment), thus offering a potentially faster progress in understanding the underlying microscopic mechanisms governing its strong correlations. While the seemingly disagreeing electronic transport and scanning tunneling microscopy experiments brought up a controversy about the locality of the Wannier orbitals in these materials, a definitive experimental evidence merging two patterns together has been much coveted. Here I discuss on the first local thermoelectric measurements in the flat electronic bands of the twisted symmetric trilayer graphene (TSTG). We use a cryogenic near-field optical microscope with an oscillating atomic force microscopy (AFM) tip irradiated by infrared photons to create a nanoscopic hot spot in the planar samples. We observe a breakdown of the non-interacting Mott formalism at low temperatures (~10 K) signaling an importance of the electronic interactions in PV generation. Explained by the interacting topological heavy-fermion model, our data suggest a spatial variation of the interaction strength dependent on the local twist angle. These experimental findings provide the first evidence of heavy fermion behavior in the topological flat bands of moiré graphene and epitomize an avenue to apply local thermoelectric measurements to other strongly correlated materials in the disorder-free limit.

Date: October 27, 2023

Speaker: Milan Orlita, Laboratoire National des Champs Magnétiques Intenses, CNRS, Grenoble, France

Title: Magneto-optics of van-der-Waals antiferromagnets

Host: Mykhaylo Ozerov

Location: MagLab B 101 at 3:00 PM-4:00 PM

Abstract: Infrared and THz magneto-spectroscopy serves as a relevant tool for investigating a wide range of low-energy excitations in solids. During my presentation, I will provide a succinct overview of research activities employing this experimental technique combined with the high-magnetic-field facility at the LNCMI in Grenoble. My talk will be primarily focused on recent results obtained on layered antiferromagnets, particularly those belonging to the FePS3 family. These investigations have revealed a surprisingly complex magneto-optical response which goes beyond the conventional picture of classical antiferromagnetic resonance. We interpret it in terms of magnon-polarons and magnon excitations with high angular momenta.

Date: October 20, 2023

Speaker: Gang Cao, University of Colorado at Boulder

Title: Control of chiral orbital currents in a colossal magnetoresistance material

Host: Peng Xiong

Location: MagLab B 101 at 3:00 PM-4:00 PM

Abstract: Colossal magnetoresistance is an extraordinary enhancement of the electric conductivity in the presence of a magnetic field, an important property of matter that has been studied for decades.

It is conventionally associated with a magnetic-field-induced spin polarization, which drastically reduces spin scattering, thus electric resistance. Our earlier studies uncover an intriguing exception to this rule in that the electric resistivity in a magnetic insulator is reduced by up to 7 orders of magnitude only when a spin polarization is absent [1]. Here I report a newly identified quantum state in a honeycomb material where chiral orbital currents flowing along edges of crystal unit cells dictate electric conductivity, providing a key element driving the novel colossal magnetoresistance [2] and a current-sensitive Hall effect [3]. The control of the exotic quantum state, along with implications of this discovery, will be presented and discussed after a brief review of conventional colossal magnetoresistance and loop currents in other materials.

References:

1. Colossal magnetoresistance via avoiding fully polarized magnetization in ferrimagnetic insulator Mn₃Si₂Te₆, Yifei Ni, Hengdi Zhao, Yu Zhang, Bing Hu, Itamar Kimchi and Gang Cao, Letter of Phys. Rev. B 103, L161105 (2021); DOI:10.1103/PhysRevB.103.L161105
2. Control of chiral orbital currents in a colossal magnetoresistance material, Yu Zhang, Yifei Ni, Hengdi Zhao, Sami Hakani, Feng Ye, Lance DeLong, Itamar Kimchi, and Gang Cao, Nature 611, 467-472 (2022); DOI: 10.1038/s41586-022-05262-3
3. Current-sensitive Hall effect in a chiral-orbital-current state, Yu Zhang, Yifei Ni, Pedro Schlottmann, Rahul Nandkishore, Lance E. DeLong, and Gang Cao, https://arxiv.org/abs/2309.06610

Date: October 13, 2023

Speaker: Adrian E Feiguin, Northeastern University

Title: Beyond perturbation theory: multi-photon and non-radiative recombination effects in spectroscopies

Host: Kun Yang

Location: Virtual(Zoom) 3:00 PM-4:00 PM

Abstract: The conventional calculation of scattering cross sections relies on a treatment based on timedependent perturbation theory that provides formulation in terms of Green’s functions in the frequency domain. In equilibrium, it boils down to evaluating a simple spectral function equivalent to Fermi’s golden rule, which can be solved efficiently by a number of numerical methods. However, away from equilibrium, the resulting expressions require a full knowledge of the excitation spectrum and eigenvectors to account for all the possible allowed transitions and intermediate states, a seemingly unsurmountable complication. We have recently presented a new paradigm to overcome these hurdles[1-3]: we explicitly introduce the scattering particles (neutron, electron, photon, positron) and simulate the full scattering event by solving the time-dependent Schrödinger equation. The spectrum is recovered by measuring the momentum and energy lost by the scattered particles, akin an actual energy-loss experiment. I here show how these ideas can be generalized to study multi-photon processes such as coincidence ARPES, and the interplay between radiative and non-radiative recombination channels in X-ray spectroscopies.

References:

[1] Krissia Zawadzki, Luhang Yang, Adrian E. Feiguin; Phys. Rev. B 102, 235141 (2020).

[2] Krissia Zawadzki, Adrian Feiguin; Phys. Rev. B 100, 195124 (2019).

[3] Krissia Zawadski, Alberto Nocera, and Adrian E. Feiguin arXiv: 1905.08166

Date: October 6, 2023

Speaker: Xiao-Xiao Zhang, University of Florida

Title: Optical probing of electronic and spin states in 2D quantum materials

Host: Luis Balicas

Location: MagLab B 101 at 3:00 PM-4:00 PM

Abstract: View Xiao-Xiao Zhang abstract and Bio

Date: August 24, 2023

Speaker: Rong Cong, Brown University and Center for Intergrated Nanotechnology (CINT)

Title: Orbital glass induced unconventional magnetic order in a correlated insulator

Host: Arneil Reyes

Location: MagLab B 101 at 11:00 AM-12:00 PM

Abstract: Intertwined electronic and orbital degrees of freedom in strongly correlated materials with strong spin-orbit-coupling has shown to give rise to many exotic quantum phenomena, especially to emergent quantum phases and transitions [1]. Coupling between the spin and orbit degrees of freedom can lead to two different viable order parameters: magnetic (spin) order and multi-polar (orbital) order. In this talk, I will present our recent direct observation of an orbital glass state caused by such a coupling in the 5d¹ relativistic Mott insulator Ba₂NaOsO₆ by implementing a technique that is able to independently probe interactions of different symmetries [2]. I will talk about simultaneous measurements of the fluctuations in the spin and orbital order using a Hahn echo sequence, giving an unprecedented view into the nature of a complex spin-orbit phase transition. We observe short-range quadrupolar order up to room temperature and a dramatic increase of orbital fluctuations near the phase transition. The presence of quadrupolar fluctuations well above the phase transition solves a long-standing puzzle of missing entropy in this material. In the end of the talk, I will transition the focus and discuss our most recent progress in using resistive detected magnetic resonance techniques to probe spin properties in 2D quantum materials and phases.

[1] Witczak-Krempa, William, et al. "Correlated quantum phenomena in the strong spin-orbit regime." Annu. Rev. Condens. Matter Phys. 5.1 (2014): 57-82.

[2] Carr, Stephen, et al. "Multi-modal spectroscopy of phase transitions." Phys. Rev. B 108, 054421 (2023)

Date: August 22, 2023

Speaker: Shengzhi Zhang, National MagLab, Los Alamos National Laboratory

Title: Electronic and magnetic phase diagrams of quantum spin liquid candidates under high magnetic fields

Host: Arneil Reyes

Location: MagLab B 101 at 11:00 AM-12:00 PM

Abstract: One crucial step toward understanding a compound is establishing its phase diagram via parameter tuning, such as temperature (T) and magnetic field (H). By examining these phase diagrams, we gain valuable insights into the underlying physics behind the phases. Additionally, examining the intrinsic spin dynamics is indispensable to further achieve a microscopic understanding of the compound's behaviors within each phase. In this talk, I will present our recent work on constructing comprehensive T-H phase diagrams of a potential Kiatev quantum spin liquid candidate, Na2Co2TeO6. Using five different techniques including magnetization, specific heat, dielectric constant, magnetostriction, and magnetocaloric effect measurements, I have identified four distinct phases, among which one exhibits the characteristics of a potential field-induced quantum spin liquid phase. Moreover, I successfully resolved a long-standing debate concerning the magnetic structure (zigzag vs. triple-Q) for the low-field phase utilizing a symmetry-sensitive electric polarization measurement. In the end, I will propose a couple of quantum spin liquid candidates where nuclear magnetic resonance measurement will play an important role in identifying the nature of their high-field phases.

Date: August 11, 2023

Speaker: Raquel D. Ribeiro, Iowa State University

Title: La₂Ni₇ – A Small Moment System with AFM Order

Host: Greg S Boebinger

Location: MagLab B 101 at 3:00 PM-4:00 PM

Abstract: Small moment magnetic systems attract attention as their magnetic ordering temperatures can often be tuned towards a quantum phase transition, using non-thermal parameters, to study exotic physical phenomena such as unconventional superconductivity, heavy Fermi liquid, etc. La2Ni7 presents a series of AFM phase transitions at 61.0K, 56.5K, and 42.2K with a low saturated moment of 0.12μB/Ni. I will highlight the main results of recent collaborative works in La2Ni7 single crystal. Analysis of M(T), M(H), ρ(T) and ρ(H) data allow for the construction of anisotropic H-T phase diagrams. Neutron diffraction studies have found the propagation vectors and magnetic moment directions of the three magnetically ordered phases. ARPES data reveal several electron and hole pockets that have hexagonal symmetry. NMR also shed light onto the nature of the magnetic order and how it impacts the bandstructure. DFT calculations show reasonable agreement experimental data, demonstrating their applicability to itinerant antiferromagnet systems.

Date: August 10, 2023

Speaker: Xiaoling (Cocoa) Wang, California State University East Bay

Title: Untangling the unconventional coupling between charge density wave order and superconductivity in kagome superconductors

Host: Arneil Reyes, CMS - DC Field

Location: MagLab B 101 at 11:00 AM-12:00 PM

Abstract: The investigation of the interplay between superconductivity and other ordered states, such as charge density wave (CDW) state, constitutes a fundamental aspect of condensed matter physics. The discovery of the layered kagome metal AV₃Sb₅ (A=K, Rb, Cs) offers a new experimental platform for exploring this competition.

AV₃Sb₅ lattices exhibit both a non-conventional CDW order (TCDW ∼ 80 − 104 K) and a topological superconducting ground state (TC ∼ 0.9 − 2.5 K). In this talk, I will present our investigation on the superstructure deformation in the crystal lattice associated with the CDW order, utilizing Nuclear Magnetic Resonance (NMR) spectroscopy. Our observations shed light on the formation and configuration of CDW states and indicate a new avenue for further exploring the electronic structure evolution of various instabilities of kagome superconductors.

Date: April 21, 2023

Speaker: Long Ju, MIT

Title: Electron correlation and topology in rhombohedral multilayer graphene

Host: Cyprian Lewandowski

Location: MagLab B 101 at 3:00 PM-4:00 PM

Abstract: Rhombohedral stacked multilayer graphene is an ideal platform to search for correlated electron phenomena, due to its pair of flat bands touching at zero energy and further tunability by an electric field. Furthermore, its valley-dependent Berry phase at zero energy points to possible topological states when the isospin symmetry is broken by electron correlation. In this talk, I will first show our efforts on the optical spectroscopy study of trilayer graphene/hBN moire superlattice. We observed optical signatures of flat moire band formation and Mott insulator due to band splitting at half-filling. Then I will talk about DC transport measurements of pentalayer graphene without any moire superlattices. We observed a plethora of correlated and topological states including a ferro-valleytronic half-metal, a correlated insulating state and Chern insulator states. Our results point to a new direction of exploring electron correlation and topology in natural 2D crystals where the layer number plays a critical role.

Date: March 3, 2023

Speaker: Jerome K. Hyun, Department of Chemistry and Nanoscience, Ewha Womans University, Korea

Title: Modulating light scattering and absorption for active structural colors

Host: Hanwei Gao (Physics)

Location: MagLab B 101 at 2:30 PM-4:30 PM

Abstract: Structural colors refer to colors arising from the scattering and interference of light by judiciously designed nanostructures. To be viable candidates for display or smart colorimetric sensing applications, their responses must be dynamic. To this end, our lab has worked on designs that can be tuned through a suite of external stimuli including electricity, humidity, and chemical vapor. Unlike plasmonic designs that require the excitation of surface plasmons, our approach mainly relies on photonic responses such as Mie resonances and dielectric waveguide-array modes which free our system from ohmic damping and the interdependence between wavelength and intensity. We also extend our approach to the microwave regime where unprecedentedly thin electromagnetic wave absorbers are realized.

Date: February 17, 2023

Speaker: Sergio de la Barrera, Massachusetts Institute of Technology

Title: Superconductivity in an engineered moiré quasiperiodic system

Host: Cyprian Lewandowski

Location: MagLab B 101 at 3:00 PM-4:00 PM

Abstract: View abstract

Date: February 3, 2023

Speaker: Pavan Hosur, University of Houston

Title: Superconductor vortices in Weyl semimetals

Host: Hitesh Changlani

Location: MagLab B 101 at 3:00 PM-4:30 PM

Abstract: View abstract