Vivek Aji

Transport spectroscopy of symmetry-broken insulating states in bilayer graphene

Bilayer graphene is an attractive platform for studying new two-dimensional electron physics, because its flat energy bands are sensitive to out-of-plane electric fields and these bands magnify electron-electron interaction effects. Theory predicts a variety of interesting broken symmetry states when the electron density is at the carrier neutrality point, and some of these states are characterized by spontaneous mass gaps, which lead to insulating behaviour. These proposed gaps are analogous to the masses generated by broken symmetries in particle physics, and they give rise to large Berry phase effects accompanied by spontaneous quantum Hall effects. Although recent experiments have provided evidence for strong electronic correlations near the charge neutrality point, the presence of gaps remains controversial. Here, we report transport measurements in ultraclean double-gated bilayer graphene and use source-drain bias as a spectroscopic tool to resolve a gap of ∼2 meV at the charge neutrality point. The gap can be closed by a perpendicular electric field of strength ∼15 mV nm(-1), but it increases monotonically with magnetic field, with an apparent particle-hole asymmetry above the gap. These data represent the first spectroscopic mapping of the ground states in bilayer graphene in the presence of both electric and magnetic fields.

Adler-Bell-Jackiw anomaly in Weyl semi-metals: Application to Pyrochlore Iridates

Weyl semimetals are three dimensional analogs of graphene where the energy of the excitations are a linear function of their momentum. Pyrochlore Iridates

Theory of the helical spin crystal: a candidate for the partially ordered state of MnSi.

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Tunable Line Node Semimetals

with A =yttrium or lanthanide element) are conjectured to be examples of such a system, with the low energy physics described by twenty four Weyl nodes. An intriguing possibility is that these materials provide a physical realization of the Adler-Bell-Jackiw anomaly. In this letter we investigate the properties of pyrochlore iridates in an applied magnetic field. We find that the dispersion of the lowest landau level depends on the direction of the applied magnetic field. As a consequence the magneto-conductivity in an electric field, applied parallel to the magnetic field is highly anisotropic, providing a detectable signature of the semi-metallic state.

Effect of contacts on spin lifetime measurements in Graphene

MnSi is an itinerant magnet which at low temperatures develops a helical spin-density wave. Under pressure it undergoes a transition into an unusual partially ordered state whose nature is debated. Here we propose that the helical spin crystal (the magnetic analog of a solid) is a useful starting point to understand partial order in MnSi. We consider different helical spin crystals and determine conditions under which they may be energetically favored. The most promising candidate has bcc structure and is reminiscent of the blue phase of liquid crystals in that it has line nodes of magnetization protected by symmetry. We introduce a Landau theory to study the properties of these states, in particular, the effect of crystal anisotropy, magnetic field, and disorder. These results compare favorably with existing data on MnSi from neutron scattering and magnetic field studies. Future experiments to test this scenario are also proposed.

Fluctuations in finite critical and turbulent systems.

Weyl semimetals are examples of a new class of topological states of matter which are gapless in the bulk with protected surface states. Their low energy sector is characterized by massless chiral fermions which are robust against translationally invariant perturbations. A variant of these systems have two non-degenerate bands touching along lines rather than points. A proposal to realize such a phase involves alternating layers of topological insulators and magnetic insulators, where the magnetization lies perpendicular to the symmetry axis of the heterostructure. The shape, size and the dispersion in the vicinity of the nodal lines varies with the strength of the magnetization, offering a new knob to control the properties of the system. In this paper we map out the evolution of the nodal lines and the dependence of the conductivity on magnetization and identify signatures of the low energy sector in quantum oscillation measurements.

Odd-parity superconductivity in Weyl semimetals

Injection, transmission, and detection of spins in a conducting channel are the basic ingredients of spintronic devices. Long spin lifetimes during transit are an important ingredient in realizing this technology. An attractive platform for this purpose is graphene, which has high mobilities and low spin-orbit coupling. Unfortunately, measured spin lifetimes are orders of magnitude smaller than theoretically expected. A source of spin loss is the resistance mismatch between the ferromagnetic electrodes and graphene. While this has been studied numerically, here we provide a closed form expression for Hanle spin precession which is the standard method of measuring spin lifetimes. This allows for a detailed characterization of the nonlocal spin valve device.

Theory of the quantum critical fluctuations in cuprate superconductors.

We show that hyperscaling and finite-size scaling imply that the probability distribution of the order parameter in finite-size critical systems exhibit data collapse. We consider the examples of equilibrium critical systems, and a statistical model of ecology. We propose an explanation for recent observations that the probability distribution of turbulent power fluctuations in closed flows is the same as that of the harmonic 2DXY model.

Field-effect mobility enhanced by tuning the Fermi level into the band gap of Bi2Se3

Unconventional superconducting states of matter are realized in the presence of strong spin orbit coupling. In particular, non degenerate bands can support odd parity superconductivity with rich topological content. Here we study whether this is the case for Weyl semimetals. These are systems whose low energy sector, in the absence of interactions, is described by linearly dispersing chiral fermions in three dimensions. The energy spectrum has nodes at an even number of points in the Brillouin zone. Consequently both intranodal finite momentum pairing and internodal BCS superconductivity are allowed. For local attractive interaction the finite momentum pairing state with chiral p-wave symmetry is found to be most favorable at finite chemical potential. The state is an analog of the superfluid

Universality of Single-Particle Spectra of Cuprate Superconductors

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