# Condensed Matter Physics in the City 2021: Video Talks

## Contributed talks by junior conference attendees:

Video talks are listed in order of submission. Submission is now closed.

The winner for this year’s CMPC video competition is Aprem Joy from the University of Cologne.

Our panel was extremely impressed with the quality of the talks throughout, and we would like to particularly give mentions to two runner-up talks by Victor Drouin-Touchette from Rutgers and Youngjoon Choi from Caltech.

Sergueï Tchoumakov (Institut Néel)

co-authors: B. Bujnowksi, J. Noky, J. Gooth, A. G. Grushin and J. Cayssol

(Université de Bordeaux, Max Planck Institute)

In Weyl semimetals the location of linear band crossings, the Weyl cones, is not bound to any high symmetry point of the Brillouin zone, unlike the Dirac nodes in graphene. This flexibility is advantageous for valleytronics, where information is encoded in the valleys of the band structure when intervalley scattering is weak.  However, if numerous Weyl cones coexist the encoded information can decohere rapidly because of band mixing.  Here, we investigate how the helical iso-spin texture of Weyl cones affects valleytronics in heterojunctions of Weyl materials, and show how the chirality of this iso-spin texture can serve to encode information.

Tamaghna Hazra (Rutgers University)

https://youtu.be/q5yXjjSborY

co-authors: Piers Coleman (Rutgers University)

We show how Oshikawa’s theorem for the Fermi surface volume of the Kondo lattice can be extended to the SU(N) symmetric case. By extending the theorem, we are able to show that the mechanism of Fermi surface expansion seen in the large N mean-field theory is directly linked to the expansion of the Fermi surface in a spin-1/2 Kondo lattice. This linkage enables us to interpret the expansion of the Fermi surface in a Kondo lattice as a fractionalization of the local moments into heavy electrons. Our method allows extension to a pure U(1) spin liquid, where we find the volume of the spinon Fermi surface by applying a spin-twist, analogous to Oshikawa’s flux insertion. Lastly, we discuss the possibility of interpreting the FL* phase characterised by a small Fermi surface in the absence of symmetry breaking, as a non-topological coexistence of such a U(1) spin liquid and an electronic Fermi liquid.

Pye Ton How (NCTS, Hsinchu, Taiwan)

https://youtu.be/9uHRPB0bjtY

The identification of unconventional superconductors characterized by multi-component order parameter remains a difficult task.  We discuss specifically the case of MxBi2Se3.  Its nematic superconductivity scenario calls for a so-called “pinning field” that explicitly breaks the lattice symmetry in the normal phase.  One important (but so far unobserved) consequence is that the superconducting transition should be split into two branches by this pinning field, as is well-established in similar cases.  We discuss the shear modulus anomaly across such a pair of split transitions.  In particular, we predict that the shear modulus c_66 *vanishes* at the lower transition.

Julien Barrier (University of Manchester)

co-authors: Piranavan Kumaravadivel, Roshan Krishna-Kumar, V.I. Fal’ko, A. K. Geim, A.I. Berdyugin

In quantizing magnetic fields, graphene superlattices exhibit a complex fractal spectrum often referred to as the Hofstadter butterfly. It can be viewed as a collection of Landau levels that arise from quantization of Brown-Zak minibands recurring at rational (p/q) fractions of the magnetic flux quantum per superlattice unit cell. Here we show that, in graphene-on-boron-nitride superlattices, Brown-Zak fermions can exhibit mobilities above 10⁶ cm²/Vs and the mean free path exceeding several micrometers. The exceptional quality of our devices allows us to show that Brown-Zak minibands are 4q times degenerate and all the degeneracies (spin, valley and mini-valley) can be lifted by exchange interactions below 1K. We also found negative bend resistance at 1/q fractions for electrical probes placed as far as several micrometers apart. The latter observation highlights the fact that Brown-Zak fermions are Bloch quasiparticles propagating in high fields along straight trajectories, just like electrons in zero field.

### Anupam Saha (AKPC Mahavidyalaya, Bengai, India)

co-authors: Dr. Satyaki Kar (AKPC Mahavidyalaya, Bengai, India)

Quantum oscillation measurements are crucial for nodal line semimetals in determining electronic Berry phases and thus topological nature of its magnetic oscillations. Here we study a continuum model of a nodal line semimetal under strong magnetic fi eld and report the characteristics of the Landau level spectra and the fluctuations in the Fermi level as the  field is varied through. We demonstrate the growth of quantum oscillation with  field strength as well as its constant in period when plotted against 1/B from the obtained results on magnetization. Furthermore we  find that the density of states which show series of peaks in succession witness bifurcation of such peaks due to Zeeman contributions. Those bifurcations, however, are discernible only if the electron/quasiparticle effective mass is considerably smaller than its free value, which usually happens in these systems. We compare the Zeeman splitting with the low lying nodal line semimetal spectra as m* is arti cially tuned from its free to low values. However, experimental results indicate a manyfold increase in the Lande g factor which again increases the the Zeeman contribution. We produce analytical results on NLSM spectra and DOS with parameters similar to that obtained experimentally for ZrSiS, a NLSM compound and discuss its implications on the quantum oscillations.

### Aprem P. Joy (University of Cologne)

co-author: Achim Rosch

A vison is an excitation of the Kitaev spin liquid which carries a $\mathbb Z_2$ gauge flux. While immobile in the pure Kitaev model, it becomes a dynamical degree of freedom in perturbed Kitaev models. We study an isolated vison in the gapless phase of the Kitaev honeycomb model in the presence of small external magnetic fields $h$ and offdiagonal exchange interactions $\Gamma$. In the ferromagnetic Kitaev model, the dressed vison obtains a dispersion linear in $\Gamma$ and $h$ and a rapidly diverging mobility $\mu \sim 1/T^4$. In contrast, in the antiferromagnetic Kitaev model an interference effect precludes the coherent propagation and the dynamics is dominated by incoherent Majorana-assisted hopping processes with a temperature independent in a high temperature regime. This explains the striking difference of ferromagnetic and antiferromagnetic Kitaev models in the presence of perturbations.

### Glenn Wagner (Oxford University)

https://youtu.be/kaTu1jG3Z6o

co-authors: Yves H Kwan (Oxford), Tomohiro Soejima (Berkeley), Michael P Zaletel (Berkeley), Steven H Simon (Oxford), Siddharth A Parameswaran (Oxford), Nick Bultinck (Oxford)

We study magic angle graphene in the presence of both strain and particle-hole symmetry breaking due to non-local inter-layer tunneling. We perform a self-consistent Hartree-Fock study that incorporates these effects alongside realistic interaction and substrate potentials, and explore a comprehensive set of competing orders including those that break translational symmetry at arbitrary wavevectors. We find that at all non-zero integer fillings very small strains, comparable to those measured in scanning tunneling experiments, stabilize a fundamentally new type of time-reversal symmetric and spatially non-uniform order. This order, which we dub the ‘incommensurate Kekulé spiral’ (IKS) order, spontaneously breaks both the emergent valley-charge conservation and moir\’e translation symmetries, but preserves a modified translation symmetry — which simultaneously shifts the spatial coordinates and rotates the angle which characterizes the spontaneous inter-valley coherence. We discuss the phenomenological and microscopic properties of this order. We argue that our findings are consistent with all experimental observations reported so far, suggesting a unified explanation of the global phase diagram in terms of the IKS order.

### Pavel A. Volkov (Rutgers University)

https://youtu.be/7p84NBe2Pj8

co-authors: M. Ye 1, H. Lohani 2, I. Feldman 2, A. Kanigel 2, G. Blumberg 1,3 1Rutgers University 2 Technion, Haifa 3 NICPB, Tallin

Excitonic insulator (EI) is a phase driven by Coulomb attraction between electrons and holes leading to a proliferation of particle-hole pairs. However, excitonic insulators break lattice symmetries, raising the question of whether a particular transition is excitonic or structural. I will show how electronic Raman scattering can be used to elucidate the transition origin in the Ta2Ni(Se1-xSx)5 family of candidate materials. In particular, I will demonstrate the excitonic-driven transition at low x transforming into a structural transition at x=1. The study reveals a quantum phase transition of an excitonic insulator masked by a preemptive structural order.  arXiv:2104.07032; arXiv:2102.07912; npj Quantum Materials 6:52 (2021)

### Yu-Ping Lin (University of Colorado Boulder)

https://youtu.be/1C4s__5HCb8

co-author: Rahul Nandkishore

I show that the charge density waves can realize a host of unconventional phenomena at the Van Hove singularity on the hexagonal lattices. Under the interplay of real and imaginary orders, a Haldane-model phase diagram arises from the 3Q complex orders at three nesting momenta. The Chern insulators explain transparently the experimental observations in the kagome metals AV3Sb5 with A = K, Rb, Cs. Meanwhile, the 3Q charge bond orders can realize the higher-order topological insulators. In-gap corner states arise in the energy spectra, where fractional corner charges appear under the corner filling anomaly. More intriguing phenomena are expected under further explorations.   Reference: [1] Y.-P. Lin and R. M. Nandkishore, arXiv:2104.02725 [2] Y.-P. Lin, arXiv:2106.09717

### Wen-Han Kao (University of Minnesota, Twin Cities)

https://youtu.be/eRYkn6TjTM0

co-authors: Johannes Knolle, Gábor B. Halász, Roderich Moessner, Natalia B. Perkins Technische Universität München, Oak Ridge National Laboratory, Max-Planck-Institut für Physik komplexer Systeme

Since 2006, the Kitaev honeycomb model has attracted significant attention by its exactly solvable quantum spin-liquid state with fractionalized Majorana excitations [1] and the possible materialization in spin-orbit assisted Mott insulators [2]. Recently, the 5d-electron compound H3LiIr2O6 has shown to be a strong candidate of Kitaev materials considering the absence of magnetic order and the signature of low-energy fermionic excitations [3]. In this work [4], we demonstrate that random vacancies in Kitaev spin liquid can give rise to a remarkable pile up of low-energy states and its relevance to the experimental observations. Physically, vacancies can originate from different sources such as non-magnetic impurities or rare regions with very weak magnetic couplings. We show numerically that the vacancy effect is readily detectable even at low vacancy concentrations and that it is not very sensitive neither to the nature of vacancies nor to different flux backgrounds. Furthermore, we show that the site disorder leads to more pronounced localization effect and density of states, compared to the bond disorder and thermal-flux disorder [5].

[1] A. Kitaev, Ann. Phys. 321, 2 (2006). [2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009). [3] K. Kitagawa, T. Takayama, Y. Matsumoto, A. Kato, R. Takano,Y. Kishimoto, S. Bette, R. Dinnebier, G. Jackeli, and H. Takagi, Nature 554, 341 (2018). [4] W.-H. Kao, J. Knolle, G. B. Halász, R. Moessner, N. B. Perkins, Phys. Rev. X 11, 011034 (2021) [5] W.-H. Kao and N. B. Perkins, Ann. Phys. (2021), doi: 10.1016/j.aop.2021.168506.

### Marcin Szyniszewski (University College London)

co-authors: Oliver Lunt, Arijeet Pal

Entanglement transitions in quantum dynamics present a novel class of phase transitions in non-equilibrium systems. When a many-body quantum system undergoes hybrid quantum dynamics, consisting of unitary evolution interspersed with monitored random measurements, the steady-state can exhibit a phase transition between volume- and area-law entanglement. It is known that for monitored Clifford circuits in one dimension, their critical properties are close to those of 2D classical percolation. In this work we show more evidence of this correspondence in 1+1D and 2+1D circuits using a graph-state based simulation algorithm where the entanglement structure is encoded in an underlying graph, providing access to the geometric structure of entanglement. We locate the critical point using the tripartite information, revealing area-law entanglement scaling at criticality in two dimensions. We also find that critical exponents of entanglement clusters do not match those of surface percolation exponents, but rather seem to be close to the bulk exponents of percolation in one lower dimension. Our results shed more light on the relation and possible mapping between monitored Clifford models and percolating networks.

### Shiyu Deng (University of Cambridge)

https://youtu.be/XvHZJ7txl2E

co-authors: Emilio Artacho, Matthew J Coak, David M Jarvis, Hayrullo Hamidov, Cheng Liu, Charles R S Haines, Chris J Pickard, Andrew R Wildes, Siddharth S Saxena Univ of Warwick, Institut Laue-Langevin Control of dimensionality in condensed matter continues to reveal novel quantum phenomena and effects. TMPX3 (e.g. FePS3) have proven to be ideal examples where structural, magnetic and electronic properties evolve into novel states when their dimensionality is tuned with pressure. At ambient pressure, they are two-dimensional van-der-Waals antiferromagnets, and Mott or charge-transfer insulators. Our recent studies [1-3] have reported dimensionality crossover related pressure-induced insulator-to-metal transitions and novel magnetic phases. There are also reports of superconductivity in a related member of this family of compounds. To further understand the structure and physical property evolution with pressure, we have performed a random structure search using first-principles calculations at high pressures and DFT+U studies to elucidate relationship between structural transitions, magnetism and electronic properties. Our computational explorations into pressure tuned TMPX3 are expected to guide the discovery of novel phases and superconductivity in these van-der-Waals systems.

[1] C. R. S. Haines, et al., Phys. Rev. Lett. 121, 266801 (2018). [2] M. J. Coak, et al., J. Phys. Condens. Matter 32, 124003 (2020). [3] M.J. Coak, et al., Phys. Rev. X 11, 011024 (2021).

### Yves Kwan (University of Oxford)

https://youtu.be/vW5e8WvDzxM

We consider magic-angle twisted bilayer graphene (TBG) at filling ν=+3, where experiments have observed a robust quantized anomalous Hall effect. This has been attributed to the formation of a valley- and spin-polarized Chern insulating ground state that spontaneously breaks time-reversal symmetry, and is stabilized by a hexagonal boron nitride (hBN) substrate. We identify three different types of domain wall, and study their properties and energetic selection mechanisms via theoretical arguments and Hartree-Fock calculations adapted to deal with inhomogeneous moiré systems. We comment on the implications of these results for transport and scanning probe experiments.

### Artem Shesterikov  (Royal Holloway, University of London)

https://youtu.be/lVu7je2bOXk

co-authors: Rais Shaikhaidarov, Ilya Antonov, Tobias Lindstrom, Teresa Hönigl-Decrinis, Jacob Dunstan, Vladimir Antonov, Oleg Astafiev National Physical Laboratory

The  aim  of  our  research  was  to  develop  and  characterise  a  superconducting  alu-minium Transmon qubit, with a long coherence time, of more than 1μs.  The Transmonqubit is well known as a scalable qubit architecture, so it can be used for the design ofa universal quantum processor.  Despite, the Transmon qubits with high decoherencetime  are already  well  developed  and  used  by  many  companies  and  research  groups;however,  we  propose  some  novel features  in  the  design,  and  fabrication  technologywhich enable us to get to the front line of development in this field.We  have  managed  to  design  and  study  a  method  for  fabrication  of  Xmon  qubitwith the coherence time on the order of microseconds.  We use Manhattan Josephsonjunctions with some novel improvements.  Additionally, a smooth angle patches evapo-ration was performed.  Specific steps for further improvement of the coherence time arediscussed and elaborated upon.  The Xmon qubit will be used as a building block fora 4-qubit annealing processor.  In the future this processor can be used in simulationsof complex biomolecular systems such as chlorophyll pigment [1] and proteins.

### Rahul Kumar (School of Advanced Materials and Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India)

https://youtu.be/hAYxTiQgPIo

co-authors: Premakumar Yanda and Prof. A. Sundaresan

We present the detailed structural and magnetic properties of  Li 2 Mn 3 O 7  from powder x-ray diffraction (XRD), dc susceptibility, heat capacity, ac susceptibility, thermoremanent magnetization, magnetic memory, and exchange bias effect. Rietveld refinement of XRD data reveals that this compound has a rhombhohedral structure composed of a layered triangular lattice. Onset of spin-glass transition was confirmed by dc magnetization and ac susceptibility measurements. Dynamic scaling laws were used to analyze and classify the glassy behavior of the compound. Magnetic field dependence of irreversible temperature follows the Almeida-Thouless line, which is characteristic for an Ising spin-glass system. Fitting of the frequency-dependent freezing temperature with a power law results in  z ν ′ = 4.06 ± 0.06 , which indicates the critical exponent of the sluggish spin dynamics and  τ 0 = 4.2 × 10 − 8  s is a characteristic time scale for a single spin-flip. Further evidence of cluster-glass behavior comes from the frequency dependence of the freezing temperature fitted with the Vogel-Fulcher law, which considers interaction between bigger magnetic entities. Values of fitting parameters are  E a / k B = 27.62  K and  T 0 = 9.57  K, which confirm cluster-glass behavior. The presence of magnetic relaxation below freezing temperature and the magnetic memory effect confirms the nonequilibrium dynamics of the system through a number of metastable states. Moreover, observation of the exchange bias effect reflects the presence of intrinsic phase inhomogeneity. These results indicate that the triangular lattice causes a disordered ground state as a result of competing exchange interactions. Link for the paper published in Phys. Rev. B: https://doi.org/10.1103/PhysRevB.103.214427

### Youngjoon Choi (Caltech)

co-authors: Hyunjin Kim, Yang Peng, Alex Thomson, Cyprian Lewandowski, Robert Polski, Yiran Zhang, Harpreet Arora, Kenji Watanabe, Takashi Taniguchi, Jason Alicea, Stevan Nadj-Perge Caltech, NIMS

Magic-angle twisted bilayer graphene (MATBG) shows variety of correlated phenomena owing to strong electronic interactions. We report newly discovered correlation-driven topological phases that develop in the finite magnetic fields. We identify Chern numbers of these topological phases by a novel technique that enables scanning tunneling microscopy (STM) to measure a local Landau fan diagram. Direct comparison to the corresponding spectroscopy, one of the advantages of our method, reveals that these topological phases arise from the exchange-like interaction acting on the Hofstadter sub-bands. The phases only appear at the small range of twist angles around the magic angle, highlighting the importance of correlations. Our experiments not only add to understandings of strong interactions in MATBG, but also open up a new direction of utilizing STM to discover new phases in other gate-tunable materials.

### Victor Drouin-Touchette (Rutgers University) – Doping the multiorbital Hund’s coupled impurity: an exploration of non-Fermi liquid ground states

https://youtu.be/hI1BUalbg18