Berkeley CMT

Time/Venue Tuesday, March 14 at 2:00 pm Pacific Time in 325 Physics South and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Chris Waechtler (Moore Group)
Title Reservoir-assisted energy migration in hybrid quantum systems 
Abstract Recent developments in quantum technology have given us the ability to engineer composite quantum systems including components such as atomic, molecular, solid state and optical. These hybrid quantum systems represent ideal candidates for demonstrating novel and complex phenomena. As an example, it has been shown recently that an ensemble of negatively charged nitrogen-vacancy (NV-) centers in diamond coupled to a resonator exhibits superradiant decay – a collective effect where the radiation is enhanced by the presence of multiple emitters. It is essential we find new applications that benefit from the engineered nature of these hybrid systems. One such example could be in the transfer of energy and quantum correlations. The transport of energy through a network is fundamental to how both nature and current technologies operate. Generally, we think about a series of nodes being connected by channels that transport the energy through them. Motivated by the design of hybrid quantum systems, we will introduce an alternate approach, replacing our channels with engineered collective environments that interact with pairs of nodes. We will show how energy initialized at a specific location in the network can migrate to another network node—even though the environment may be at zero temperature. More importantly, we show that such energy migration occurs on timescales significantly faster than the relaxation rates associated with the single spins which further establishes this as a truly collective phenomenon. Our approach highlights the importance of being able to design and tailor the properties and symmetries of hybrid quantum systems. In doing so, we can illustrate new directions for the future of quantum technologies.

Time/Venue Monday, March 13 at 11 am in 325 Physics South and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Tomo Soejima (Zaletel Group)
Title Kitaev nanoribbon with boundary dephasing: Non-Hermitian free-fermion Hamiltonians and its application
Abstract Free fermion systems appear in various situations in physics. For example, it is known that the Kitaev honeycomb model [1] can be mapped to a model of free fermions coupled to a static gauge field by rewriting the spin operators in terms of Majorana fermions. Once the free-fermion Hamiltonian has been obtained, it is possible to derive information on many-body eigenstates from information on one-body eigenstates. On the other hand, non-Hermitian Hamiltonians appear in various situations, such as effective models for dissipative systems. The problem when treating non-Hermitian free-fermion systems is that there may be exceptional points where the Hamiltonian becomes non-diagonalizable.
In the first part, using the Jordan canonical form, we develop a general theory on the generalized eigenstates of a Non-Hermitian free-fermion Hamiltonian. We prove an important lemma conjectured by Prosen [2] and show that the dimensions of generalized eigenspaces appear as coefficients of a polynomial called a Gaussian polynomial.
In the second part, we consider the Kitaev nanoribbon (a one-dimensional version of the Kitaev honeycomb model) with boundary dephasing as a concrete example. The projection theorem known for the Hermitian case [3] is applicable to the non-Hermitian Hamiltonian describing the system. The physical spectrum can then be obtained by applying the standardization techniques discussed in Part I. Using this, we calculate a quantity characterizing the relaxation time called the Liouvillian gap, and show numerically and analytically that for the parameter Jx=Jy, the Liouvillian gap is proportional to the inverse square of the system size and is Liouvillian gapless in the thermodynamic limit.
[1] A. Kitaev, Ann. Phys. 321, 2 (2006).
[2] T. Prosen, J. Stat. Mech. 2010, P07020 (2010)
[3] F. L. Pedrocchi et al., Phys. Rev. B 84, 165414 (2011).

Title Liouvillian gap and single spin-flip dynamics in the dissipative Fermi-Hubbard model
Abstract Recent progress in experiments on ultra-cold atoms has made it possible to realize well-controlled open many-body quantum systems, where couplings to the environment are possible resources to engineer novel non-equilibrium states. For example, the Fermi-Hubbard model with SU(𝑁) spin symmetry is realized with alkaline-earth-like atoms, and well-controlled two-body loss can be induced by photoassociation [1]. Motivated by these experiments, the dynamics of the Fermi-Hubbard model with two-body loss has been studied theoretically by various means. However, most previous studies focused on one-dimensional systems, and results in higher dimensions are still limited.In this talk, we present two results for the SU(𝑁) Fermi-Hubbard model on a 𝑑-dimensional hypercubic lattice with two-body loss [2]. First, we obtain the Liouvillian gap in closed form for any 𝑑 and 𝑁 in the unit filling, which characterizes the relaxation time to the steady state. Interestingly, it is universal in the sense that it does not depend on the spatial dimension 𝑑. Second, we investigate the dynamics of a ferromagnetic initial state with a single spin flip both analytically and numerically. Then we show that a crossover from power-law to exponential decay in time occurs as the dissipation strength varies.
[1] B.Yanetal.,Nature501,521(2013),K.Sponseleeetal.,QuantumSci.Technol.4,014002(2019), K. Honda et al., Phys. Rev. Lett. 130, 063001 (2023).
[2] H. Yoshida and H. Katsura, arXiv:2209.03743 (2022).

Time/Venue Wednesday, February 15 at 2:00 pm PST in Physics South 402
Host Bob Birgeneau
Speaker 1 Zhi-Qiang Gao (Dung-Hai Lee Group)
Title Is the Coherence Peak Modulation in Eu-1144 Iron Pnictide Superconductor Due to Pair Density Wave?
Abstract Coherence peak modulation has been detected in recent STM experiments performed in high critical temperature superconductor iron pnictide Eu-1144. Such a particle-hole symmetric modulation is observed to appear simultaneously with a ferromagnetic order. It is argued to be a pair density wave, and absence of other density wave orders further implies it not to be a secondary order of other density waves. In this talk I will introduce an impurity scattering scenario proposed to explain the experiments. I will show that in this nodeless superconductor, magnetic impurity scattering can induce the particle-hole symmetric coherence peak modulation in the presence of a ferromagnetic order or a Zeeman field, which is consistent with the experimental observations. Therefore, according to our theory, the coherence peak modulation in Eu-1144 is originated from impurity scattering, instead of a pair density wave.
Speaker 2 Tomo Soejima (Zaletel Group)
Title Can we see ground states of twisted bilayer graphene?
Abstract The Magic angle twisted bilayer graphene has been shown to exhibit a variety of strong-correlated behavior, most notably superconductivity and correlated insulating phenomenon.
There have been a plethora of theoretical proposals about the nature of such correlated insulating states, but there has not been any definitive experimental confirmation.
In this talk, I will showcase how atomically-resolved Scanning Tunneling Microscopy (STM) can be used to detect symmetry breaking in twisted bilayer graphene.
This can be used to differentiate between popular ground state candidates by directly probing their symmetry breaking. I will in particular on Kramers Intervalley Coherent (K-IVC) insulator and Incommensurate Kekule Spiral (IKS) state, which show drastically different symmetry properties

Time/Venue Tuesday, February 14 at 2:00 pm Pacific Time in 375 Physics North and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore/ Alp Sipahigil
Title Identification of Massively Entangled Many-Body States
Abstract Entanglement of quantum many-body systems is one of the key concepts in quantum science and technology. Some of the highly entangled many-body states, for example, Z₂ quantum spin liquids, have been prepared and controlled in meta-material platforms, including superconducting qubits and Rydberg atoms. In this talk, we discuss the significance and properties of massively entangled many-body states in quantum materials and how to identify them.

Time/Venue Monday, February 13 at 2:30 pm Pacific time in Physics North, Lecture Hall #3 and via Zoom: https://berkeley.zoom.us/j/93923628683
Host Joel Moore
Title Bulk photospin effect
Abstract We know, e.g., from the bulk photovoltaic effect, that nonlinear transport is sensitive to the geometry of the electron wave function but, is this also true for other observables such as nonlinear magnetization? We partially answer this question by computing the electric spin susceptibility of Bloch electrons to second order. We find that interband coherence effects (a hallmark of the quantum hall effect), yields a spin magnetization with linearly polarized. Usually only circularly polarized light is considered to produce photo magnetization via, e.g., inverse faraday effect. The standard spin orientation effects are recovered in appropriate limits within the formalism. Finally, the electric spin susceptibility of metals has contributions proportional to spin multipole moments of the Fermi sea that dominate the low frequency spin response.
URL: https://sites.google.com/site/benjaminmfregoso/
BIO:  PhD from UIUC with for Eduardo Fradkin, postdocs at Maryland and UCB, finally Asst. Prof. Kent State University since 2017.

Time/Venue Thursday, February 9 at at 1:30 pm in 325 Physics South and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meet James Analytis and Joel Moore
Title Gapless topology and electron correlations
Abstract Electrons in a crystal behave as waves that interfere with one another. Accordingly, an electronic system can have non-trivial properties when viewed through the lens of topology. At the same time, the electrons also exist as charged particles that repel one another, which raises the question of whether strong electrostatic interactions can cooperate with the wave nature of the electrons to produce correlated topological matter. For insulators, the answer is long known in affirmative as exemplified by the fractional quantum Hall effect. In the case of metallic systems, however, the question is open and pressing.
In this talk, I will outline the route we have taken from certain canonical correlation physics to metallic topology. I’ll show how correlation effects in the form of Kondo interactions produce emergent excitations that are subjected to the constraints of crystalline symmetry, leading to Weyl-Kondo semimetals [1,2]. The materials realization [2,3] and design – in both f-electron- [3] and flat-band-hosting d-electron-[4] based systems — will be summarized. Finally, I will present a class of correlation-driven topological states that lose quasiparticles through fractionalization [5]. Some implications about topology and correlation physics in general will be discussed.
[1] H.-H. Lai, S. E. Grefe et al, PNAS 115, 93 (2018). https://doi.org/10.1073/pnas.1715851115
[2] S. Dzsaber et al, PNAS 118, e2013386118 (2021). https://doi.org/10.1073/pnas.2013386118
[3] L. Chen et al, Nature Physics 18, 1341 (2022). https://doi.org/10.1038/s41567-022-01743-4
[4] L. Chen et al., arXiv:2212.08017. https://doi.org/10.48550/arXiv.2212.08017
      H. Hu et al., arXiv:2209.10396. https://doi.org/10.48550/arXiv.2209.10396
[5] H. Hu et al., arXiv:2110.06182.  https://doi.org/10.48550/arXiv.2110.06182

Time/Venue Wednesday, February 8 at 2 pm in Physics South 402 and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Joel Moore
Title Hydrodynamics and Hall viscosity in GaAs versus graphene
Abstract Motivated by Hall viscosity measurements in graphene sheets [1], we study hydrodynamic transport of electrons in a channel of finite width in external electric and magnetic fields. We consider electric charge densities in graphene varying from close to the Dirac point up to the Fermi liquid regime and compare the results with GaAs Fermi liquid regime [2]. We find in graphene two competing contributions to the hydrodynamic Hall and inverse Nernst (generation of transverse gradient of temperature in the channel) signals that originate from the Hall viscous and Lorentz force [3]. This competition leads to a non-linear dependence of the full signals on the magnetic field and even a cancellation at different critical field values for both signals. In particular, the hydrodynamic inverse Nernst signal in the Fermi liquid regime is dominated by the Hall viscous contribution [3]. We further show that a finite channel width leads to a suppression of the Lorenz ratio, while the magnetic field enhances this ratio [3]. All of these effects are predicted in parameter regimes accessible in experiments.
[1] A. I. Berdyugin, S. G. Xu, F. M. D. Pellegrino, R. Krishna Kumar, A. Principi, I. Torre, M. Ben Shalom, T. Taniguchi, K. Watanabe, I. V. Grigorieva, M. Polini, A. K. Geim and D. A. Bandurin, Science 364, 162 (2019).
[2]I. Matthaiakakis, D. Rodríguez Fernández, C. Tutschku, E. M. Hankiewicz, J. Erdmenger, and R. Meyer Phys. Rev. B 101, 045423 (2020).
[3] Z.-Y. Xian, S. Danz, D. Rodríguez Fernández, I. Matthaiakakis, C. Tutschku, R. L. Klees, J. Erdmenger, R. Meyer, E. M. Hankiewicz arXiv:2207.10528, submitted to PRL.

Time/Venue Friday, February 3 at 2:00 pm in 402 Physics South and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Dung-Hai Lee
Title Classification and construction of crystalline topological
superconductors and insulators in interacting fermion systems.
Abstract The construction and classication of crystalline symmetry
protected topological (SPT) phases in interacting bosonic and
fermionic systems have been intensively studied in the past few years.
Crystalline SPT phases are not only of conceptual importance, but also provide us great opportunities towards experimental realization since space group symmetries naturally exist for any realistic material. In this talk, I will discuss how to construct and classify crystalline topological
superconductors (TSC) and topological insulators (TI) in interacting
fermion systems. I will also discuss the relationship between internal symmetry protected SPT phases and crystalline symmetry protected SPT Phases.

Time/Venue Friday, February 3 at 11:00 am in 325 Physics South and via Zoom:
https://berkeley.zoom.us/j/99523499113pwd=REovb3pyam03WXQwbEhrU3dqNHZvdz09
Meeting ID: 995 2349 9113 Passcode: 600704
Host Ehud Altman
Title Quantum simulators: from the Fermi Hubbard model to quantum assisted NMR inference
Abstract I will discuss recent progress of optical lattice emulators of the Fermi Hubbard model, specifically the new availability of snapshots of many-body states with single particle resolution. I will review new insights from these experiments on the properties of doped Mott insulators, including the demonstration of magnetically mediated pairing. I will also present the idea of using quantum simulators to perform inference of NMR spectra for biological molecules. I will review a recent experimental realization of this algorithm on a quantum computer using trapped ions. Prospects for scaling this approach to solving practically relevant problems will be discussed.