« fall 2018 - spring 2019 - summer 2019 »

This week | Next week | This semester | All future | Print view

This week | Next week | This semester | All future | Print view

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:

Wednesday, April 10th 2019

1:25 pm:

Wednesday, April 17th 2019

1:25 pm:

Wednesday, April 24th 2019

1:25 pm:

Wednesday, May 1st 2019

1:25 pm:

Wednesday, September 25th 2019

1:25 pm:

Wednesday, November 13th 2019

1:25 pm:

The weekly calendar is also available via subscription to the physics-announce mailing list, and by RSS feed.