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Wednesday, January 23rd 2019

1:30 pm:

Wednesday, February 6th 2019

1:25 pm:

A hallmark of the phase diagrams of correlated electronic systems is the existence of multiple electronic ordered states. In many cases, they cannot be simply described as independent competing phases, but instead display a complex intertwinement. A prime example of intertwined states is the case of primary and vestigial phases. While the former is characterized by a multi-component order parameter, the fluctuation-driven vestigial state is characterized by a composite order parameter formed by higher-order, symmetry-breaking combinations of the primary order parameter. This concept has been widely employed to elucidate nematicity in both iron-based and cuprate superconductors. In this talk, I will present a group-theoretical framework, supplemented by microscopic calculations, that extends this notion to a variety of phases, providing a general classification of vestigial orders of unconventional superconductors and density-waves. Electronic states with scalar and vector chiral order, spin-nematic order, Ising-nematic order, time-reversal symmetry-breaking order, and algebraic vestigial order emerge from this simple underlying principle. I will present a rich variety of possible phase diagrams involving the primary and vestigial orders, and discuss possible realizations of these exotic composite orders in different materials.

Wednesday, February 13th 2019

1:25 pm:

Wednesday, February 20th 2019

1:25 pm:

I will present a theory of magnetotransport phenomena related to the chiral anomaly in Weyl semimetals. I will show that conductivity, thermal conductivity, thermoelectric and the sound absorption coefficients exhibit strong and anisotropic magnetic field dependences. I will also discuss properties of magneto-plasmons and magneto-polaritons, whose existence is entirely determined by the chiral anomaly.

Wednesday, February 27th 2019

1:25 pm:

We determine the information scrambling rate due to electron-electron Coulomb interaction in graphene. The scrambling rate characterizes the growth of chaos and has been argued to give information about the thermalization and hydrodynamic transport coefficients of a many-body system. We discuss the scrambling rate at strong coupling, using a direct diagrammatic analysis and holographic methods and show that scrambling behaves similar to transport and energy relaxation rates. A weak coupling analysis, however, reveals that scrambling is in fact related to dephasing and single particle relaxation. Thus, while scrambling is obviously necessary for thermalization and quantum transport, it does generically not set the time scale for these processes.

Wednesday, March 6th 2019

1:25 pm:

Wednesday, March 13th 2019

1:25 pm:

The search for the enigmatic quantum spin liquid (QSL) state has switched into high gear in recent years. Amazing experimental progress has resulted in several highly promising QSL materials such as ZnCu3(OH)6Cl2, YbMgGaO4, and NaYbO2, to list just a few. All of these quasi-two-dimensional materials are characterized by a broad continuum of spin excitations observed in neutron scattering experiments. Unfortunately many, if not all, of these QSL candidates suffer from the presence of significant substitutional disorder which often tends to strongly broaden inelastic neutron spectra and thus calls into question the QSL interpretation of the experimental data. It is therefore incumbent upon the theoretical community to identify specific experimental signatures, more detailed than a “broad continuum” arguments, that evince the unique aspects of spin liquid states of magnetic matter.

In this talk I focus is on the prominent metal-like magnetic insulators – U(1) quantum spin liquids with spinon Fermi surface – excitations of which are represented by neutral spin-1/2 fermions (spinons) and emergent gauge fields. The gauge field mediates strong interactions between spinons. We argue that the full effect of this interaction becomes apparent when the spin liquid is partially magnetized by the Zeeman magnetic field. Under this condition, the spectrum of the spin liquid acquires a new transverse collective spin-1 mode, distinct from incoherent particle-hole excitations of the spinon continuum. Despite being located outside the spinon continuum, this novel collective excitation interacts with emergent gauge fluctuations which are responsible for partially damping it.

I present a tentative theory of this collective mode, including its dispersion, lifetime and other spectral characteristics, and identify conditions needed for its experimental observation. Collective properties of Dirac spin liquids, in which spinon bands form relativistic cone dispersion, will be described as well.

Friday, March 15th 2019

3:35 pm:

Spin-currents generated by thermal gradients are efficiently converted into charge-currents by the inverse spin-Hall effect in films of metals presenting strong spin-orbit coupling. The nature of the thermal induced effects depends on the relative orientation among the directions of the spin-current, the applied magnetic field (H), the thermal gradient and the electrical contacts in the metallic film. Mixings in the currents generated by different effects are expected to occur. In this work, the H-dependent anti-symmetric spin-Seebeck effect (SSE) was generated altogether with the symmetric planar Nernst effect in a NiO(100 nm)/Pt(6 nm) nanostructure grown on a 0.5 mm thick Si substrate. A sample holder adapted to a PPMS was used for measuring the voltage in the Pt-film for H in the range ±85 kOe and for temperatures (T) varying from 100 to 300 K. A simple procedure developed for separating the SSE from the planar Nernst effect yielded magnitudes for the SSE in the range ±30 pAcm/K for a temperature different of 10 K across the sample at 300 K. The magnitude of the SSE signal was found to vary with H and T in good agreement with a drift-diffusion magnonic theory. Work supported by CNPq, FACEPE, CAPES and FINEP (Brazilian Agencies).

Wednesday, March 20th 2019

1:25 pm:

Wednesday, March 27th 2019

1:25 pm:

Studying quantum entanglement over the past 1--2 decades has allowed us to make remarkable theoretical progress in understanding correlated many-body quantum systems. However electrons in real materials experience random heterogeneities ("dirt") whose theoretical treatment, including strong correlations, has been a challenge. I will describe how synthesizing ideas from quantum information theory, statistical mechanics, and quantum field theory gives us new insights into the role of randomness in 2D correlated quantum spin systems. First I will outline our results on weak bond-randomness in two theoretically controlled cases (valence-bond-solids and classical dimer models) and apply them to random quantum magnets to show that topological defects with free spins necessarily nucleate and control the low energy physics. Second I will describe how the results lead us to conjectures in 2D, and a proved theorem in 1D, of Lieb-Schultz-Mattis-type constraints on all possible low-energy fates of quantum magnets, that hold even with randomness. Third I will describe how the theory predicts a scaling collapse of the temperature and magnetic-field dependence of thermodynamic quantities that is consistent with experimental observations from multiple materials, suggesting that these materials exhibit randomness-driven long range entanglement.

Wednesday, April 3rd 2019

1:25 pm:

Condensed matter systems provide an exciting laboratory for observing new states of quantum matter via emergence, where the collective behavior of electrons results in quasi-particles with fractional statistics, spin-charge separation, magnetic monopoles and Majorana fermions (particles that are their own anti-particles). I will describe how we design and synthesize new quantum materials that can host these exotic new states of matter and then use a variety of experimental techniques including muon spin relaxation and neutron scattering to probe their properties.

Wednesday, April 10th 2019

1:25 pm:

Majorana fermions can be realized as quasiparticle excitations in a topological superconductor, whose non-Abelian statistics provide a route to developing robust qubits. In this context, there has been a recent surge of interest in the iron-based superconductor, FeSe0.5Te0.5. Theoretical calculations have shown that FeSe0.5Te0.5 may have an inverted band structure which may lead to topological surface states, which can in turn host Majorana modes under certain conditions in the superconducting phase. Furthermore, recent STM studies have demonstrated the existence of zero-bias bound states inside vortex cores which have been interpreted as signatures of Majorana modes. While most recent studies have focused on Majorana bound states, more generally, akin to elementary particles, Majorana fermions can propagate and display linear dispersion. These excitations have not yet been directly observed, and can also be used for quantum information processing. This talk is focused on our recent work in realizing dispersing Majorana modes. I will describe the conditions under which such states can be realized in condensed matter systems and what their signatures are. Finally, I will describe our scanning tunneling experiments of domain walls in the superconductor FeSe0.45Te0.55, which might potentially be first realization of dispersing Majorana states in 1D.

Wednesday, April 17th 2019

1:25 pm:

Topological materials are a new class of quantum materials with remarkable properties, which are rooted in the topology of the ground state wave function. Our understanding of topological electronic phases, in particular free fermion phases, relies on one of the central paradigms of modern electron band theory: the notion of a band inversion. In this talk I will first review what a band inversion is, and then describe our attempt to construct a many-body generalization of the band inversion transition, providing a new perspective for interacting topological phases. In particular, I will introduce a special class of band inversions in two dimensions for which interactions are expected to determine the fate of the transition and present evidence that these provide promising venues for a strongly correlated fractionalized fluid of electrons and holes. I will describe possible routes to material realizations and will discuss connections to new types of topological semimetals in three dimensions as well as superconductors.

Friday, April 19th 2019

11:00 am:

Magnetic monopoles are hypothetical elementary particles exhibiting quantized magnetic charge m_0=±(h⁄(μ_0 e)) and quantized magnetic flux ϕ_0=±h/e. A classic proposal for detecting such magnetic charges is to measure the quantized jump in magnetic flux Φ threading the loop of a superconducting quantum interference device (SQUID) when a monopole passes through it. Naturally, with the theoretical discovery that a plasma containing equal numbers of emergent magnetic charges 〖±m〗_** should exist in several lanthanide-pyrochlore magnetic insulators including Dy_2Ti_2O_7, this SQUID technique was proposed for their direct detection. Experimentally, this has proven extremely challenging because of the high number density, and the generation-recombination (GR) fluctuations, of the monopole plasma. Recently, however, theoretical advances have allowed the¬ spectral density of spontaneously generated magnetic-flux noise S_Φ (ω,T) due to a thermally generated plasma of magnetic monopoles 〖±m〗_**to be predicted for Dy2Ti2O7. I will describe development of a high-sensitivity, SQUID based flux-noise spectrometer, and consequent measurements of the frequency and temperature dependence of S_Φ (ω,T) for Dy2Ti2O7 samples. Virtually all the elements of S_Φ (ω,T) predicted for a magnetic monopole plasma, including the existence of intense magnetization noise and its characteristic frequency and temperature dependence, are detected directly. Moreover, measured correlation functions C_Φ (t) of the magnetic-flux noise Φ(t) reveal that the motion of magnetic charges is correlated. A final striking observation is that, since the GR time constants τ(T) are in the millisecond range for Dy2Ti2O7, magnetic monopole flux noise amplified by the SQUID is audible to human perception.

Wednesday, April 24th 2019

1:25 pm:

Electron correlation effects give rise to a variety of emergent phenomena in quantum materials—high temperature superconductivity, electronic nematicity, Mott insulating phase, magnetism. The family of Fe(Se,Te) superconductors plays a remarkable host to all of these phenomena in different parameter regimes. In this talk, I will present angle-resolved photoemission results on two aspects of electron correlation effects in this material family—i) orbital-selective Mott insulating behaviors towards the FeTe end of the phase diagram, and ii) electronic nematicity in completely detwinned FeSe. Both examples showcase the phenomenal way that correlation effects rewrite the low energy electronic states of a material system, and reveal the exceptional role the orbital degree of freedom plays in composing the fundamental physics in iron chalcogenide superconductors.

Wednesday, May 1st 2019

1:25 pm:

A collection of coupled linear oscillators is widely regarded as a trivial physical system. Nevertheless, in recent years it has become evident that weak loss (or gain) in these systems can result in a variety of qualitative surprises - even in the purely linear regime. Effects that have attracted considerable attention include: PT symmetry breaking, exceptional points, non reciprocity, and topological control. I will describe a simple framework that unites these "non-Hermitian" effects and explains why topology emerges generically in damped coupled linear oscillators. I will discuss the application of these concepts in classical systems and in quantum systems. Lastly, I will describe an optomechanical experiment that offers a natural way to realize generic non-Hermiticity.

Monday, May 13th 2019

10:00 am:

The spin Hall effect (SHE) was predicted nearly half a century ago [1, 2]. Following the proposal in [3], the (inverse) SHE was experimentally observed for the first time in [4], without arousing much interest. The first experimental observations of the (direct) SHE were reported [5, 6] more than 30 years after the original prediction, causing great excitement and many theoretical and experimental studies of this phenomenon.

The spin current density is described by a tensor qij, where the first index corresponds to the direction of flow, and the second one - to the component of the spin that is flowing.

The spin-orbit interaction provides coupling between the spin and charge currents, so that a charge current in the z direction produces the xy component of the spin current, resulting in spin accumulation at the lateral surfaces of the sample (direct spin Hall effect). In turn, the xy component of the spin current induces the z component of the charge current resulting in the change of the sample resistance (inverse spin Hall effect).

Related phenomena are the spin Hall magnetoresistance [7, 8] and the effect of swapping spin currents, which was predicted theoretically [9] but so far not yet observed experimentally.

Tuesday, May 14th 2019

1:00 pm:

The state of a classical computer at a given moment is described by a sequence (↑↓↑↑↓...), where ↑ and ↓ represent bits of information – realized as the on and off states of individual transistors. The computation process consists in switching some transistors between their ↑ and ↓ states according to a prescribed program.

In quantum computing one replaces the classical two-state element by a quantum element with two basic states, called the quantum bit, or qubit. The simplest object of this kind is the electron spin, which can have only two possible projections on any axis: +ћ/2 or −ћ/2. For some chosen axis, we can again denote the two basic quantum states of the spin as ↑ and ↓.

However an arbitrary spin state is described by the wave function ψ = a↑ + b↓, where a and b are complex numbers, satisfying the condition |a|2 + |b|2 = 1. In contrast to the classical bit that can be only in one of the two states, ↑ or ↓, the qubit can be in a continuum of states defined by the quantum amplitudes a and b. The qubit is a continuous object.

With N qubits, there are 2N basic states of the type (↑↓↓↑↑↓↑↓...). Accordingly, the general state of a system with N qubits is described by 2N complex parameters restricted by the normalization condition only. So, while the state of the classical computer with N bits at any given moment coincides with one of its 2N possible discreet states, the state of a quantum computer with N qubits is defined by the values of 2N continuous variables, that we should be able to control.

Thus, basic quantum mechanics tells us that the hypothetical quantum computer is an analog machine whose state at any given moment is described by a very large number of continuous parameters. Note that for a toy quantum computer with only 300 qubits this number greatly exceeds the number of particles in the observable Universe!

An important issue is related to the energies of the ↑ and ↓ states. While the notion of energy is of primordial importance in all domains of physics, both classical and quantum, it is not in the vocabulary of QC theorists. They implicitly assume that the energies of all 2N states of an ensemble of qubits are exactly equal. Otherwise, the existence of an energy difference ∆E leads to oscillations of the quantum amplitudes with a frequency Ω = ∆E/ ћ, where ћ is the Planck constant, and this again is a basic fact of Quantum Mechanics. (For example, one of the popular candidates for a qubit, the electron spin, will make a precession around the direction of the Earth's magnetic field with a frequency ~ 1 MHz).

On the basis of these elementary facts, the obvious answer to the question in title is - NO!

Friday, May 24th 2019

12:20 pm:

The initial theoretical proposal for the realization of Majorana bounds states in a condensed matter setup requires three simple ingredients: superconductivity, spin-orbit coupling and magnetic field. While this proposal is simple, the experiments are not, as they involve material science, fabrication steps, cooling, electrostatic control and actual measurements. The results of experiments are not unambiguous and allow for multiple interpretation by simple theoretical models. In order to bridge the gap one has to include peculiarities of experimental setup and engineer the modelling of systems supporting Majorana zero modes. In this talk I will show how the next generation of numerical models captures the effects of electric fields, disorder, orbital effects and how it can feedback and guide the ongoing experimental effort in the field.

Tuesday, May 28th 2019

11:00 am:

A preponderance of evidence suggests that the ground state of the nearest-neighbor S=1/2 antiferromagnetic Heisenberg model on the kagome lattice is a gapless spin liquid. Many candidate materials for the realization of this model possess in addition a Dzyaloshinskii-Moriya (DM) interaction. We study this system by tensor-network methods and deduce that a weak but finite DM interaction is required to destabilize the gapless spin-liquid state. The critical magnitude, Dc/J≃0.012(2), lies well below the DM strength proposed in the kagome material herbertsmithite, indicating a need to reassess the apparent spin-liquid behavior reported in this system.

Wednesday, July 10th 2019

1:25 pm:

While signatures of Majorana bound states have been observed in one-dimensional systems, there is an ongoing effort to find alternative platforms that can be easily scalable. In this talk, I will present a novel experimental approach based on two-dimensional materials. Using a Josephson junction made of HgTe quantum well coupled to thin-film aluminum, we can tune between a trivial and a topological superconducting state by applying an in-plane magnetic field and controlling the phase difference ϕ across the junction. First, I will recapitulate our supercurrent measurement in the context of spatially varying order parameter of the induced superconductor. Then, I will delve into the observation of its topological transition by measuring the tunneling conductance at the edge of the junction. At low fields, we observe a minimum in the tunneling spectra near zero bias, consistent with a trivial superconductor. However, as the magnetic field increases, the tunneling conductance develops a zero-bias peak which persists over a range of ϕ that expands systematically with increasing magnetic fields. Our observations are consistent with theoretical predictions for this system and with full quantum mechanical numerical simulations. This work provides a new platform for probing topological superconducting phases which can be generalized to other two-dimensional systems with spin-orbit coupling.

References

H. Ren, F. Pientka, S. Hart, A. T. Pierce, M. Kosowsky, L. Lunczer, R. Schlereth, B. Scharf, E. M. Hankiewicz, L. W. Molenkamp, B. I. Halperin, and A. Yacoby, “Topological superconductivity in a phase-controlled Josephson junction”, Nature (London) 569, 93 (2019). https://doi.org/10.1038/s41586-019-1148-9

S. Hart, H. Ren, M. Kosowsky, G. Ben-Shach, P. Leubner, C. Brüne, H. Buhmann, L. W. Molenkamp, B. I. Halperin, and A. Yacoby, “Controlled finite momentum pairing and spatially varying order parameter in proximitized HgTe quantum wells”, Nature Physics 13, 87 (2017). https://doi.org/10.1038/nphys3877

Wednesday, September 25th 2019

1:25 pm:

Wednesday, November 13th 2019

1:25 pm:

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