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Wednesday, January 17th 2018

1:25 pm:

Our understanding of crystal growth fundamentals and processes are advanced when the synergy between mathematical models and novel experiments is exploited. We present recent examples of how modeling and experiments together have enabled the identification of fundamental mechanisms important during the growth of bulk crystals from the melt.

We first discuss how microgravity experiments, carried out via sounding rockets, motivated a reexamination of classical theories for foreign particle engulfment during crystal growth. Via the development and application of rigorous numerical models, we were able, for the first time in over a decade of research on this system, to quantitatively describe data on the engulfment of SiC (silicon carbide) particles during the growth of crystalline silicon. Moreover, model results identified previously unascertained mechanisms responsible for the behavior of this system and, via this insight, provided insight for analytical derivation of a new scaling law for the dependence of critical engulfment velocity on particle size.

We finish with an overview of exciting, new research that employs neutron imaging to directly “see,” in operando, the bulk growth of scintillator crystals during a gradient-freeze process. We argue that the synergies of “seeing” via both models and neutron imaging will improve our fundamental understanding and provide for a closed-loop approach for optimizing the growth of large, single crystals from the melt.

This research was supported in part by NASA NNX10AR70G, DOE/NNSA DE-NA0002514, DOE/NNSA/DNN R&D (LBNL subcontract AC0205CH11231); no official endorsement should be inferred.

Wednesday, January 24th 2018

1:30 pm:

Recent progress in synthesizing insulators with a new type of dominant spin-exchange interaction, known as the Kitaev interaction, has opened new possibilities for experimentally realizing spin liquid compounds. Among the distinctive features of these spin liquids is the possibility that they can harbour protected gapless boundary modes which carry spin but not charge. This possibility raises a challenge of how best to detect these chargeless boundary modes. I will discuss two possibilities — Raman scattering and the heat capacity — as well as what such measurements can reveal about the bulk phase.

Wednesday, January 31st 2018

1:30 pm:

Tunneling spectroscopy measurements on one-dimensional superconducting hybrid materials have revealed signatures of Majorana fermions which are the edge states of a bulk topological superconducting phase. We couple strong spin-orbit semiconductor InSb nanowires to conventional superconductors (NbTiN, Al) to obtain additional signatures of Majorana fermions and to explore the magnetic-field driven topological phase transition. Specifically, we map out the phase diagram of the topological phase in the space of Zeeman energy and chemical potential, and investigate the apparent closing and re-opening of the superconducting gap. We investigate how the topological superconducting phase would manifest in finite size systems, by electrostatically splitting the wire into segments of varied length. By chaining up several segments of a nanowire, we are realizing a quantum simulator of the Kitaev chain with tunable on-site energies and couplings between the sites, a step towards quantum simulation with semiconductor nanostructures.

Tuesday, February 6th 2018

1:30 pm:

Wednesday, February 7th 2018

1:30 pm:

The generation of an effective attraction between electrons out of the bare Coulomb repulsion is a long sought-after goal of the condensed matter community. In this talk I will introduce a pairing mechanism between holes in the dilute limit of doped frustrated Mott insulators. We will see that magnons provide a strong glue in the infinitely repulsive limit of the triangular lattice Hubbard model. The strongly attractive hole-magnon interaction is a manifestation of a “counter-Nagaoka” theorem: the single-hole kinetic energy is minimized for an antiferromagnetically ordered state. We will demonstrate that the resulting hole-magnon attraction is strong enough to bind a second hole and to form a hole-hole-magnon three-body bound state. Remarkably, the binding energy of this “composite Cooper pair” is rather strong, while its effective mass still has a moderate value, giving rise to relatively high transition temperature for superconductivity in the dilute limit. I will discuss a few interesting consequences of this new mechanism for unconventional superconductivity.

Work done in collaboration with Shangshun Zhang (University of Tennessee) and Wei Zhu (Los Alamos National Laboratory).

Wednesday, February 14th 2018

1:25 pm:

I will discuss new experimental results and insights into the physics of cuprate high-temperature superconductors, providing an overarching framework for understanding these materials.

Motivated by transport measurements, I will consider an inhomogeneous Mott-like (de)localization model wherein exactly one hole per copper-oxygen unit is gradually delocalized with increasing doping and temperature. The model comprehensively captures pivotal unconventional experimental results, including the temperature and doping dependence of the pseudogap phenomenon, the strange-metal linear temperature dependence of the planar resistivity, and the doping dependence of the superfluid density. The simple model greatly demystifies the cuprate phase diagram, and points to a local superconducting pairing mechanism involving the (de)localized hole. The spatial inhomogeneity of the localization gap is thus expected to cause a distribution of superconducting gaps as well, leading to superconducting percolation. Accordingly, for several representative cuprates the superconducting diamagnetism, nonlinear conductivity, and paraconductivity exhibit an unusual temperature dependence above Tc that is captured by a simple percolation model. The results show that that intrinsic, universal gap inhomogeneity is highly relevant to understanding the properties of the cuprates.

Wednesday, February 21st 2018

1:25 pm:

In normal metals, the electron's mean free path is much larger than its wavelength, allowing a semiclassical treatment of transport. Conversely, whenever scattering is so strong that the mean free path becomes comparable to the electron's wavelength, the concept of a quasiparticle becomes ill defined, and a new theoretical framework is needed. I will introduce a family of lattice models for interacting electrons that can be solved exactly in the limit of a large number of interacting electron flavors and/or phonon modes. Depending on details, these models exhibit either "resistivity saturation" at high temperatures to a value of the order of the quantum of resistance, or "bad metallic behavior" where the resistivity grows without bound with increasing temperature. Translationally invariant higher-dimensional generalizations of the Sachdev-Ye-Kitaev model can capture a variety of phenomena arising purely from electron-electron interactions, including local criticality, non-Fermi liquid, and marginal Fermi liquid behavior. I will describe the implications of these results for the problem of non-quasiparticle transport at large, local quantum criticality, and fundamental bounds on dissipation rates in quantum systems.

Wednesday, February 28th 2018

1:25 pm:

Wednesday, March 7th 2018

1:30 pm:

Wednesday, March 21st 2018

1:25 pm:

Photons carry spin angular momentum when circularly or elliptically polarized. During light-matter interaction, transfer of angular momentum induces optical torque. Here, we demonstrate the measurement of the spin angular momentum of photons propagating in a silicon waveguide and the use of optical torque to actuate rotational motion of an optomechanical device. We show that the sign and magnitude of the optical torque are determined by the photon polarization states that are synthesized on the chip. Our study reveals the mechanical effect of photon’s polarization degree of freedom and demonstrates its control in integrated photonic devices.

Tuesday, March 27th 2018

3:35 pm:

Waves with a hyperbolic dispersion relation are exotic yet surprisingly widespread phenomena that occur in anisotropic media with internal resonances. Such media have been investigated in numerous fields, ranging from condensed matter physics to plasma physics to optics to fluid dynamics and geophysics. Hyperbolic waves can be found in magnetic materials, in both usual and topological insulators, in superconductors, as well as in our oceans, beaches, atmosphere, and space. The characteristic lengths and frequencies of such waves vary vastly, from atomic to cosmic. However, they all exhibit certain common attributes, such as strict directionality, diverging density of states, and anomalous reflection. This talk will contain a primer on hyperbolic materials, a recipe for the death ray, and a report on our nano-optics studies of hyperbolic phonon-polaritons in new quasi-2D materials such as graphene and hexagonal boron nitride.

References:

1. L. V. Brown et al, “Nanoscale Mapping and Spectroscopy of Nonradiative Hyperbolic Modes in Hexagonal Boron Nitride Nanostructures,” Nano Lett. 18, 1628 (2018).

2. A. J. Giles et al., "Imaging of Anomalous Internal Reflections of Hyperbolic Phonon-Polaritons in Hexagonal Boron Nitride," Nano Lett. 16, 3858 (2016).

3. S. Dai et al., “Subdiffractional focusing and guiding of polaritonic rays in a natural hyperbolic material,” Nature Comms 6, 6963 (2015).

4. S. Dai et al., “Tunable Phonon Polaritons in Atomically Thin van der Waals Crystals of Boron Nitride”, Science 343, 1125 (2014).

Wednesday, March 28th 2018

2:30 pm:

Design, discovery, growth and characterization of novel materials is at the heart of New Materials Physics. One of the key steps is deciding what materials to study or try to grow. In this talk I will try to enunciate and elaborate the motivations for making/studying specific compounds. Many examples from current research will be touched upon and discussed. Humor of all types will be used to lighten the load and make the time fly by.

Wednesday, April 4th 2018

1:25 pm:

Non-Fermi liquids are exotic metallic states which do not support well defined quasiparticles. Due to strong quantum fluctuations and the presence of extensive gapless modes near the Fermi surface, it has been difficult to understand universal low-energy properties of non-Fermi liquids. In this talk I will discuss recent progress made on field theories for non-Fermi liquids. Based on a dimensional regularization scheme which tunes the co-dimension of Fermi surface, critical exponents that control scaling behaviors of physical observables can be computed in controlled ways. The systematic expansion provides important insight into strongly interacting non-Fermi liquids. This allows us to find the non-perturbative solution for the strange metal realized at the antiferromagnetic quantum critical point in 2+1 dimensions, and predict the exact critical exponents that can be experimentally tested in layered systems.

Wednesday, April 11th 2018

1:25 pm:

Wednesday, April 18th 2018

1:25 pm:

The iron pnictides represent a new family of unconventional superconductors in which superconductivity appears in close proximity to a magnetically ordered phase. In this talk I will review the magnetic order of the iron pnictides and discuss recent discoveries in the magnetic phase diagram, focusing on the reorientation of magnetic moments observed to occur in hole-doped compounds. Considering the impact of magnetic fluctuations on the phase diagram I will argue that even the modest spin-orbit coupling observed in iron pnictides has important consequences and cannot be neglected.

Wednesday, April 25th 2018

1:15 pm:

Wednesday, May 9th 2018

1:30 pm:

Tuesday, May 15th 2018

11:00 am:

The recent discovery of topological semimetals, which possess distinct electron-band crossing with non-trivial topological characteristics, has stimulated intense research interest. By extending the notion of symmetry-protected band crossing into one of the simplest magnetic groups, namely by including the symmetry of time-reversal followed by space-inversion, we predict the existence of topological magnon-band crossing in three-dimensional (3D) antiferromagnets. The crossing takes on the forms of Dirac points and nodal lines, in the presence and absence, respectively, of the conservation of the total spin along the ordered moments. In a concrete example of a Heisenberg spin model for a “spin-web” compound, we theoretically demonstrate the presence of Dirac magnons over a wide parameter range using the linear spin-wave approximation, and obtain the corresponding topological surface states [1].

Inelastic neutron scattering experiments have then been carried out to detect the bulk magnon-band crossing in a single-crystal sample. The highly interconnected nature of the spin lattice suppresses quantum fluctuations and facilitates our experimental observation, leading to remarkably clean experimental data and very good agreement with spin-wave calculations. The predicted topological band crossing is confirmed [2].

[1] K. Li et al., PRL 119, 247202 (2017).

[2] W. Yao et al., arXiv:1711.00632.

Wednesday, September 12th 2018

1:25 pm:

The field of iron-based high critical temperature superconductors continues attracting the attention of the Condensed Matter Physics community. I will briefly review the main ideas in this field and argue that electronic correlation effects cannot be neglected [1]. For this reason, in the main portion of the presentation I will focus on recent exciting results for the two-leg ladder compounds BaFe2S3 and BaFe2Se3.These are the only members of the iron-based family that were reported to become superconducting (at high pressure) without having iron layers in its crystal structure [2]. Theory becomes more accurate in 1D and numerically exact computational results for a two-orbital Hubbard model applied to ladders, as well as chains, will be discussed [3]. They reproduce the dominant magnetic order of BaFe2S3 and BaFe2Se3, and display intriguing indications of pairing tendencies upon doping at intermediate Hubbard couplings. I will speculate that this may be explained by the existence of unexpected preformed spin-singlets in the system [4]. Recent results for the dynamical spin structure factor of ladders S(q,) will also be briefly discussed, time allowing, and compared with neutron scattering data [5]. Novel optical modes are predicted.

[1] P. Dai et al., Nat. Phys. 8, 709 (2012), and references therein.

[2] H. Takahashi et al., Nat. Mater. 14, 1008 (2015); T. Yamauchi et al., PRL 115, 246402 (2015); J.-J. Ying et al., PRB 95, 241109 (R) (2017).

[3] N. D. Patel et al., PRB 94, 075119 (2016); PRB 96, 024520 (2017).

[4] N. D. Patel et al., in preparation.

[4] J. Herbrych et al., accepted in Nat. Communications 2018.

Wednesday, September 19th 2018

1:25 pm:

In a series of experiment on 2D electron gas at the surface of Bi, we have been able to probe a number of novel features of quantum Hall liquids for the first time. First, we have been able to use the scanning tunneling microscope (STM) to directly visualize Landau orbits in real space. This new technique has been used to show that the electronic states associated with the valley state on the surface of Bi form nematic quantum Hall liquids. [1] By tuning the magnetic field, we have been able to stabilize different type nematic fluids, and have been able uncover a ferroelectric quantum Hall liquid that forms when only one of the valley get occupied.[2] We are able to demonstrate that the formation of these quantum Hall phases are driven by electron-electron interaction. Finally, in the most recent experiment, we have been able to uncover domain walls between different nematic quantum Hall states and to direct image the 1D Luttinger liquids that form at such interfaces. This new type of Luttinger liquids can become metallic or insulating depending on the number of valley-textured edge modes.[3]

[1] B. Feldman et al. Science 354 6310 (2016).

[2] Randeria et al. Nature Physics, 14 1709 (2018)

[3] Randeria et al. in preparation. (2018)

Wednesday, October 3rd 2018

1:25 pm:

Wednesday, October 10th 2018

1:25 pm:

Hydrodynamical effects such as viscosity become significant In very clean materials and in ultracold atomic gases. We discuss how quantum mechanics and reduced dimensionality lead to new kinds of hydrodynamics and far-from-equilibrium transport. Examples include semiclassical kinetic theory (“generalized hydrodynamics”) and exact far-from-equilibrium results for some quantities in the XXZ model through expansion potentials. In many cases the predictions of theoretical approaches based on integrability can be checked, using in DMRG and other matrix product state algorithms. The talk finishes with a discussion of some experimental goals and challenges in 2D materials, including the possible observation of Hall viscosity in current experiments.

Wednesday, October 24th 2018

1:25 pm:

In all known fermionic superfluids, Cooper pairs are composed of spin-1/2 quasi-particles that pair to form either spin-singlet or spin-triplet bound states. The "spin" of a Bloch electron, however, is fixed by the symmetries of the crystal and the atomic orbitals from which it is derived, and in some cases can behave as if it were a spin-3/2 particle. The superconducting state of such a system allows pairing states to form beyond triplet, with higher spin quasi-particles combining to form quintet or even septet pairs. After reviewing experimental evidence for high-spin pairing in the exotic superconducting state of the half-Heusler compound YPtBi, I will introduce our recent work elucidating the influence of spin-orbit coupling on both the normal and superconducting states of this system

Wednesday, October 31st 2018

1:25 pm:

We have recently demonstrated an experimental platform to isolate 2D quantum materials that are unstable in the ambient environment. I will discuss our studies of the Weyl semimetal candidate, 1T’-MoTe2, and layered magnetic insulator, CrI3, in the atomically thin limit, made possible using this technique. In MoTe2, lowering dimensionality suppresses the inversion symmetric monoclinic phase, driving the Weyl ground state up to and beyond room temperature. The different electronic structure of thin samples is studied by magnetotransport measurements at low temperature. In CrI3, we observe a very large negative magnetoresistance effect that is quantitatively comparable to colossal magnetoresistance in the manganites. I will explain the origin of this effect and discuss some new opportunities for other 2D magnets.

Wednesday, November 7th 2018

1:25 pm:

The bulk conduction band of Si has six equivalent valleys. Strain in Si/SiGe heterostructures partially lifts the six-fold valley degeneracy by raising the energy of the four in-plane valleys. It is known that large electric fields can lift the degeneracy of the remaining two low-lying valleys. However, the measured valley splittings range from 10 – 300 μeV, suggesting that microscopic details such as interface roughness and disorder impact the valley splitting. In this lecture I will describe how microwave spectroscopy can be applied to probe valley states in silicon nanostructures [1]. In the first experiment, a cavity coupled Si double quantum dot is probed using microwave frequency photons. The transmission of the photons through the microwave cavity displays signatures that are consistent with the valley degree of freedom and the data can be modeled using cavity input-output theory [2]. We also use Landau-Zener interferometry to probe the low-lying energy level structure of a silicon double quantum dot. The observed Landau-Zener interference pattern persists down to low driving frequencies of 50 MHz, suggesting relatively long-lived charge coherence. Low-lying valley states result in a unique Landau-Zener interference pattern that is in contrast with measurements on conventional two-level charge qubits [3]. These new probes of valley states have high energy resolution and may be applied to other low energy degrees of freedom.

1. Burkard and Petta, PRB 94, 195305 (2016).

2. Mi, Peterfalvi, Burkard, and Petta, PRL 119, 176803 (2017).

3. Mi, Kohler, and Petta, PRB 98, 161404(R) (2018).

Tuesday, November 20th 2018

4:00 pm:

An electronic nematic order spontaneously breaks the rotation symmetry of the many body system, making various physical properties anisotropic. It has been observed in various systems, in particular the cuprate and iron-based high temperature superconductors. In the vicinity of a nematic quantum critical point — achieved by tuning some external parameter such as pressure or doping — the physics is described by that of low-frequency long-wavelength order parameter fluctuations coupled to a Fermi surface. However, due to the momentum-conserving nature of the induced electron-electron interaction, the temperature dependence of the resistivity near an Ising nematic QCP remains unclear. In this talk, we shed light on the problem by incorporating disorder and Umklapp process into the low-energy theory. Our work can be viewed as solving an extended Boltzmann equation, with a collision integral that accounts for complicated multi-particle scattering processes important near the QCP

Wednesday, November 28th 2018

1:25 pm:

Wednesday, December 12th 2018

1:25 pm:

When left unobserved, many-body quantum systems tend to evolve toward states of higher entanglement. Making a measurement, on the other hand, tends to reduce the amount of entanglement in a many-body system by collapsing one of its degrees of freedom. In this talk I discuss what happens when a many-body quantum system undergoes unitary evolution that is punctuated by a finite rate of projective measurements. Using numerical simulations and theoretical scaling arguments, we show that for a 1D spin chain there is a critical measurement rate separating two dynamical phases. At low measurement rate, the entanglement grows linearly with time, producing a volume-law entangled state at long times. When the measurement rate is higher than the critical value, however, the entanglement saturates to a constant as a function of time, leading to area-law entanglement. We map the dynamical behavior of the entanglement onto a problem of classical percolation, which allows us to obtain the critical scaling behavior near the transition. I briefly discuss generalizations of our result to higher dimensions, and its implications for the difficulty of simulating quantum systems on classical computers.

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