# Condensed Matter Physics in All the Cities: Student Video Talks

Students may submit contributed prerecorded video talks. Submission is now closed.

The conference prize for the best talk, along with the opportunity to present live on Thursday 25 June 2020 to Ms Alexandra Ziolkowska from the University of Oxford, for her talk on Yang-Baxter Integrable Lindblad Equations, see listing number 17, below.

## Student contributed talks

1. Md Shafayat Hossain (Princeton University)
Bloch ferromagnetism of composite fermions
2. Glenn Wagner (University of Oxford)
Transport in bilayer graphene
3. Callum Stevens and Luke Pritchard Cairns (University of Edinburgh)
Composition Dependence of the Superconducting Properties of UTe2
4. Camilla Di Mino (UCL)
Neutron Diffraction Study on Methanol Solutions of Alkenes: Weak Hydrogen Bonding
5. Sam Garratt (Oxford)
Goldstone modes in the emergent gauge fields of a frustrated magnet
6. Chris Oliver (Birmingham)
A Quantum Hall Model for Photons Using Time as a Synthetic Dimension
7. Mostafa Ragragui (Faculty of Science Rabat)
Topological materials for Smart cities 2020
8. Adil Elkhou (Mohammed V University)
Magnetic Skyrmions
(abstract)
9. Marie-Therese Huebsch (University of Tokyo, RIKEN)
A Benchmark of Predicting Magnetic Structures using a Combination of the Cluster Multipole Expansion and LSDA
(abstract)
10. Tamaghna Hazra (The Ohio State University)
Upper bounds on superfluid stiffness and superconducting critical temperature
(abstract)
11. Yu-Ping Lin (University of Colorado Boulder)
Dual Haldane sphere and quantized band geometry in chiral multifold fermions
12. Attila Szabó (University of Cambridge)
Neural network wave functions and the sign problem
13. Daniel Muñoz-Segovia (Donostia International Physics Center, Spain)
Many-body effects in nodal-line semimetals: Correction to the optical conductivity
14. Anita Halder (S. N. Bose National Centre for Basic Sciences)
Prediction of Magnetic Double Perovskite: ab-initio and Machine Learning Approach
15. Harry Lane (University of Edinburgh)
Soliton bound states in large spin anisotropic antiferromagnets
16. Rebecca M. Smith (University of St Andrews)
Quantum criticality in ferroelectric tetragonal tungsten bronze potassium lithium tantalate
17. Aleksandra Ziolkowska (University of Oxford)
18. Loi Nguyen (Princeton University)
A Quantum Spin Liquid Candidate
19. Sreejith Chulliparambil (Dresden)
Microscopic Models for Kitaev’s Sixteenfold way of Anyon theories.
20. Victor Drouin-Touchette (Rutgers)
Exploring the multiorbital Hund’s coupled impurity
21. Shi Feng (Ohio State University)
Magnetic phase transitions in quantum spin-orbital liquids
22. Jennifer Reid (University of Waterloo)
Evidence for charge neutral excitations in bulk SmB6

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## Abstracts

#### 1. Md Shafayat Hossain

Princeton University

Bloch ferromagnetism of composite fermions

The ground state of a dilute system of electrons has long been a topic of theoretical fascination, because physics here is governed by strong correlations. In 1929, Felix Bloch predicted that the ground state of such a system should be an itinerant ferromagnet, known as Bloch ferromagnet. Here via direct measurements of Fermi wavevector, we report the observation of Bloch ferromagnetism, signaled by an abrupt, interaction-driven transition to full magnetization in an interacting composite fermion (CF) Fermi sea at the half-filled lowest Landau level. Our findings finally confirm the presence of Bloch ferromagnetism, which has eluded experimental observations for nearly a century. Our findings highlight the role of inter-CF interaction contrasting the long-believed picture of non-interacting CFs and pave the way for future research on strongly correlated CF phases.

#### 2. Glenn Wagner

University of Oxford

Transport in bilayer graphene

In this talk, I will discuss hydrodynamic electron transport in bilayer graphene. In the hydrodynamic regime, electron-electron scattering dominates over momentum-relaxing scattering. I will use two complementary frameworks to study this problem: the quantum Boltzmann equation and the two-fluid model. This leads to the prediction of two signatures of hydrodynamic transport: the Wiedemann-Franz law violation at charge neutrality and the sharp increase in the electrical conductivity away from charge neutrality. We compare our theoretical results to available experimental data on suspended bilayer graphene and find good agreement with both the quantum Boltzmann equation and the two-fluid model.

#### 3. Callum Stevens and Luke Pritchard Cairns

University of Edinburgh

Composition Dependence of the Superconducting Properties of UTe2

A better understanding of the synthesis conditions, composition and physical properties of UTe2 are required to interpret previously reported unconventional superconductivity. Here we report how the superconducting properties of single crystals depend on the ratio of elements present in their synthesis by chemical vapour transport. We have obtained crystals with the highest reported ambient pressure Tc and a larger superconducting heat capacity jump from a growth with a U:Te ratio different from that widely used in the literature. For these crystals, the ratio of residual heat capacity in the superconducting state to that of the normal state, γ∗/γN , is significantly lower than 0.5, reported elsewhere. An upturn in the heat capacity below 200 mK is also reduced compared to other studies and is well described by a Schottky anomaly and residual Sommerfeld term rather than quantum critical behaviour.

#### 4. Camilla Di Mino

University College London

Neutron Diffraction Study on Methanol Solutions of Alkenes: Weak Hydrogen Bonding

Neutron diffraction has been used to elucidate the interaction between carbon atoms double bonded in alkenes and the OH group in methanol. Different computational models have been used to fit the neutron data providing subtle differences in structure.

#### 5. Sam Garratt

University of Oxford

Goldstone modes in the emergent gauge fields of a frustrated magnet

We consider magnon excitations in the spin-glass phase of a geometrically frustrated antiferromagnet, focusing on the nearest-neighbour pyrochlore-lattice Heisenberg model with exchange randomness. The low-energy degrees of freedom in this system are represented by three copies of a U(1) emergent gauge field, related by global spin-rotation symmetry. We show that the Goldstone modes associated with spin-glass order are excitations of these gauge fields, and that the standard theory of Goldstone modes in Heisenberg spin glasses (due to Halperin and Saslow) must be modified in this setting.

#### 6. Chris Oliver

University of Birmingham

A Quantum Hall Model for Photons Using Time as a Synthetic Dimension

A number of different works have demonstrated versions of the quantum Hall effect for photons. Some of these approaches make use of synthetic dimensions to overcome the fact that photons do not carry charge, and hence cannot feel a magnetic field. In this work, we demonstrate theoretically how to use time as a synthetic spatial dimension to realise a variant of the quantum Hall effect for photons, the coupled wire model. This makes use of a well-established experimental platform, coupled waveguide arrays. This work has exciting future prospects in exploring the interplay of topology and weak interactions through the use of nonlinear optical media.

#### 7. Mostafa Ragragui

Faculty of Science Rabat

Topological materials for Smart cities 2020

Search for new topological materials became a very hot topic in the field of both condensed matter and materials science. These materials hosting novel linear responses in the bulk and gapless states at the edge for 2D or surface for 3D materials, these gapless surface states can be a key to resolve the problem of increasing of consumption of energy in the world and to address challenges in other areas to build the next smart cities. Using a first principle calculation such as DFT, or machine learning we can the investigate properties of these materials and for designing new functional materials. In this talk, I will discuss ideas in the literature on how to design new topological materials with strong spin-orbit coupling, and some of the suggested materials where it has been proposed to exist.

Mohammed V University

Magnetic Skyrmions

Magnetic skyrmions are topologically stable spin configurations, which generally come from chiral interactions known as Dzyaloshinskii Moriya interactions, this interaction is one of the very important and necessary mechanisms to generate the skyrmion. We also studied magnetic skyrmions in a family of Janus van derWaals, MnSTe, MnSeTe, VSeTe and MnSSe monolayers.

#### 9. Marie-Therese Huebsch

University of Tokyo, RIKEN

A Benchmark of Predicting Magnetic Structures using a Combination of the Cluster Multipole Expansion and LSDA

The cluster multipole (CMP) expansion for magnetic structures [1] provides a scheme to systematically generate candidate magnetic structures specifically including noncollinear magnetic configurations adapted to the crystal symmetry of a given material. A comparison with the experimental data collected on MAGNDATA [2] shows that the most stable magnetic configurations in nature are a linear combination of only few CMPs. Furthermore, a high-throughput generalized local spin-density approximation (LSDA) calculation, in which each candidate magnetic structure was considered as an initial guess, was performed using VASP [3]. We benchmark whether CMP+LSDA reproduces the experimental magnetic configurations.
[1] M.-T. Suzuki 𝘦𝘵 𝘢𝘭., Phys. Rev. B 𝟵𝟵, 174407 (2019).
[2] S. V. Gallego 𝘦𝘵 𝘢𝘭., J. Appl. Cryst. 𝟰𝟵, 1941–1956 (2016).
[3] D. Hobbs, G. Kresse and J. Hafner, Phys. Rev. B. 𝟲𝟮, 11 556 (2000); G. Kresse and J. Hafner, Phys. Rev. B 𝟰𝟳 , 558 (1993); ibid. 𝟰𝟵 , 14 251 (1994).

#### 10. Tamaghna Hazra

The Ohio State University

Upper bounds on superfluid stiffness and superconducting critical temperature

Understanding the material parameters that control the superconducting transition temperature $T_c$ is a problem of fundamental importance. In many novel superconductors, phase fluctuations determine $T_c$, rather than the collapse of the pairing amplitude. We derive rigorous upper bounds on the superfluid phase stiffness for multi-band systems, valid in any dimension. This in turn leads to an upper bound on $T_c$ in 2D, which holds irrespective of pairing mechanism, interaction strength, or order-parameter symmetry. Our bound is particularly useful for the strongly correlated regime of low-density and narrow-band systems, where mean field theory fails. For a simple parabolic band in 2D with Fermi energy $E_F$, we find that $k_BT_c \leq \ef/8$, an exact result that has direct implications for the 2D BCS-BEC crossover in ultra-cold Fermi gases. Applying our multi-band bound to monolayer FeSe on SrTiO$_3$ and to magic-angle twisted bilayer graphene, we find that band structure results constrain the maximum $T_c$ to be close to the experimentally observed value. Following our work, this prescription has also been used (https://dx.doi.org/10.1103/PhysRevResearch.2.013219) to estimate an upper bound on $T_c$ in the newly discovered nickelate superconductors. Finally, we discuss the question of deriving rigorous upper bounds on $T_c$ in 3D.
“Bounds on the Superconducting Transition Temperature: Applications to Twisted Bilayer Graphene and Cold Atoms”,
T. Hazra, N. Verma, M. Randeria,
Phys. Rev. X 9, 031049 (2019) (https://doi.org/10.1103/PhysRevX.9.031049)

#### 11. Yu-Ping Lin

Dual Haldane sphere and quantized band geometry in chiral multifold fermions

I show that the chiral multifold fermions present a dual Haldane sphere problem in momentum space. Owing to the Berry monopole at the degenerate point, a dual Landau level emerges in the trace of quantum metric, with which a quantized geometric invariant is defined through a surface integration. I further demonstrate potential manifestations in the measurable physical observables. With a lower bound derived for the finite spread of Wannier functions, anomalous phase coherence is identified accordingly for the flat band superconductivity.

#### 12. Attila Szabó

University of Cambridge

Neural network wave functions and the sign problem

Neural quantum states (NQS) are a promising approach to study many-body quantum physics. However, they face a major challenge when applied to lattice models: Neural networks struggle to converge to ground states with a nontrivial sign structure. In this talk, I present a neural network architecture with a simple, explicit, and interpretable phase ansatz, which can robustly represent such states and achieve state-of-the-art variational energies for both conventional and frustrated antiferromagnets. In the latter case, this approach uncovers low-energy states that exhibit the Marshall sign rule and are therefore inconsistent with the expected ground state. Such states are a likely cause of the obstruction for NQS-based variational Monte Carlo to access the true ground states of these systems, and may have a more general significance for numerical techniques for frustrated quantum magnets.

#### 13. Daniel Muñoz-Segovia

Donostia International Physics Center, Spain

Many-body effects in nodal-line semimetals: Correction to the optical conductivity

Long-range Coulomb electron-electron interaction might have important effects on the physical observables in topological semimetals with vanishing density of states at the band touching due to the weak screening. In this talk, I will present our result for the first-order perturbative correction to the optical conductivity in a clean three-dimensional (3D) nodal-line semimetal (NLSM), which is based on the analogy between two-dimensional Dirac fermions and 3D NLSMs. I will first review the analogous problem of the interaction correction to the optical conductivity in graphene, where there was a long-standing controversy in the past. I will then discuss the optical conductivity of the 3D NLSMs both in the noninteracting limit and in the interacting case, comparing them to the corresponding ones in graphene and Weyl semimetals. Finally, I will briefly comment on the possibility of experimentally measuring our result.

#### 14. Anita Halder

S. N. Bose National Centre for Basic Sciences

Prediction of Magnetic Double Perovskite: ab-initio and Machine Learning Approach

We employed machine learning (ML) algorithm for screening of new stable double perovskite candidates among the possible combination of 3d transition metal at B site and 4d/5d at B’ site which are generally found to be magnetic. Our study predicted 33 new compounds, among which the electronic and magnetic properties of 25 compounds were studied extensively in this work. We used genetic algorithm to predict their ground state structures. Our analysis predicted 21 compounds to be magnetic with a wide range of magnetic and electronic properties, 7 compounds were predicted as ferromagnetic half-metal, 6 compounds as ferrimagnetic insulator, 5 compounds as antiferromagnetic insulator, 2 compounds as ferromagetic metal and one compound was found to be uncommon example of antiferromagnetic metal. Our computational study is expected to help in discovering new magnetic double perovskites.

#### 15. Harry Lane

University of Edinburgh

Soliton bound states in large spin anisotropic antiferromagnets

It has long been known that the low-energy dynamics of spin systems are well-described by classical linear spin-wave theory. This is particularly true in the case of systems with a large spin moment, where one can expand in powers of 1/S. For large-spin antiferromagnets, linear spin wave theory predicts dispersive magnon modes which are gapped in the presence of anisotropy. Such spectra are found time and again in the literature, but can the quantum nature of spin give rise to exotic excitations that are not well described by linear spin-wave theory?
Using a path integral approach I will show that the movement of domain walls in an anisotropic large-spin antiferromagnetic chain can be described by solitons. In the presence of coupling to other chains, the frustration induced by this soliton propagation leads to a nonlinear confinement potential that can support bound states. I will then present a possible experimental example of bound states that may be described by nonlinear soliton confinement.

#### 16. Rebecca M. Smith

University of St Andrews

Quantum criticality in ferroelectric tetragonal tungsten bronze potassium lithium tantalate

Quantum criticality is known in ferroelectrics by tuning the properties by changing the isotopes or chemical composition. In this work, it was discovered that potassium lithium tantalate (KLT) is a relaxor-type ferroelectric in new ceramics samples. Therefore allowing that frequency could be used as a tuning parameter for the quantum criticality in this system. This suggestion was investigated by using the empirical Vogel-Fulcher relationship to the dielectric data, which resulted in the dipole freezing temperature below absolute zero. However, confirming the exponent and comparing this to literature values is not possible with these ceramics due to ceramic dispersion. Therefore KLT is a possible quantum critical system using frequency as a tuning parameter.

#### 17. Aleksandra Ziolkowska

University of Oxford

We consider Lindblad equations for one dimensional fermionic models and quantum spin chains. By employing a (graded) super-operator formalism we identify a number of Lindblad equations than can be mapped onto non-Hermitian interacting Yang-Baxter integrable models. Employing Bethe Ansatz techniques we show that the late-time dynamics of some of these models is diffusive.

#### 18. Loi Nguyen

Princeton University

A Quantum Spin Liquid Candidate

Ba4NbIr3O12, a material with a triangular planar geometry of Ir3O12 trimers, is described. Magnetic susceptibility measurements show it to be paramagnetic with no magnetic ordering down to 1.8 K despite the Curie-Weiss temperature (θCW) of −13 K. The material has a very low effective magnetic moment (μeff ) of 0.80 μB/f.u. To look at the lower temperature behavior, the specific heat (Cp ) measured down to 50 mK shows the linear upturn under 0 and 1 T applied magnetic fields. This upturn gets suppressed by higher fields. These results indicate that Ba4NbIr3O12 is a potential candidate for hosting the quantum spin liquid state at low temperatures.

#### 19. Sreejith Chulliparambil

Technical University, Dresden

Microscopic Models for Kitaev’s Sixteenfold way of Anyon theories.

I introduce the microscopic models for the Kitaev’s Sixteenfold way classification of topological orders for a Z2 gauge theory coupled to weakly interacting or free Majorana fermions with an integer spectral Chern number. I also present the exact solution to the family of models. By adding suitable perturbations, one can achieve appropriate Chern numbers and characterize the topological order using anyonic topological spin and topological ground state degeneracy, in the phases of interest. I also briefly comment on the spin-orbital realization of the models.

#### 20. Victor Drouin-Touchette

Rutgers University

Exploring the multiorbital Hund’s coupled impurity

Motivated by the relevance of Hund’s coupling in iron-based superconductors, we revisit the problem of a multiorbital Anderson impurity with Hund’s interaction. Using large-N and Schwinger boson techniques, we study the ground state and thermodynamic properties of this system in both integer and mixed valence regimes. The physics is characterized by the interplay of Hund’s coupling, which tends to form large moments by aligning the spins of the impurity, and the Kondo effect, which leads to the low-temperature screening of the moments. In the integer valence regime, we confirm the formation of large moments, which eventually become screened at an exponentially reduced Kondo temperature due to the so-called Schrieffer effect, and compare to previous renormalization group studies [1]. In the mixed valence regime, the Hund’s coupling gives rise to interesting physics: we explore the presence of a non-Fermi-liquid ground state and the possibility that the large moments can generate retarded on-site pairing correlations for the conduction electrons. [1] Nevidomskyy, A.H. and Coleman, P. “Kondo resonance narrowing in d-and f-electron systems.” Phys. Rev. Lett 103.14 (2009): 147205. *We acknowledge support by the DOE, Basic Energy Sciences grant DE-FG02-99ER45790, and the Quebec FRQNT.

#### 21. Shi Feng

The Ohio State University

Magnetic phase transitions in quantum spin-orbital liquids

We investigate the spin and orbital correlations of a superexchange model with spin S=1 and orbital L=1 relevant for 5d4 transition metal Mott insulators, using exact diagonalization and density matrix renormalization group (DMRG). For spin-orbit coupling λ=0, the orbitals are in an entangled state that is decoupled from the spins. We find two phases with increasing λ: (I) the S2 phase with two peaks in the structure factor for λ≤λc1≈0.34J where J is the ferromagnetic exchange; and, (II) the S1 phase for λc1≤λ≤λc2≈1.2J with emergent antiferromagnetic correlations. Both S1 and S2 phases are shown to exhibit power law correlations, indicative of a gapless spectrum. Upon increasing λ≥λc2 leads to a product state of local spin-orbital singlets that exhibit exponential decay of correlations, indicative of a gapped phase. We obtain insights into the phases from the well-known Uimin-Lai-Sutherland (ULS) model in an external field that provides an approximate description of our model within mean field theory.

#### 22. Jennifer Reid

University of Waterloo

Evidence for charge neutral excitations in bulk SmB6

Thermal conductivity measurements have been conducted on high quality single crystals of SmB6 over a temperature range spanning three decades from T~0.05 – 50 K and in magnetic field up to B = 12 T. In the ultra low temperature regime, T < 1 K, the zero field conductivity for samples with the lowest levels of disorder, scales with sample size demonstrating that the conductivity is due to phonons in the boundary scattering limit. Samples with increased disorder show a conductivity below this limit. Based on this observation, the observed field induced increase in conductivity must be generated by additional excitations, which are charge neutral because of the absence of any corresponding change in the charge conductivity. The conductivity of these field induced neutral excitations is unchanged whether the magnetic field is directed perpendicular or parallel to the heat current direction.