Seminar

We show by standard quantum principles that two circuits, a small tunnel junction and a small metal loop with an electron, are related by a gauge transformation.  We show further that this same transform prevents momentum eigenfunctions from having gauge invariant de Broglie wave lengths around a ring. Thus persistent current on a metal ring and the Coulomb blockade on a small tunnel junction seem to be the same dynamical theory based on discontinuous Bloch waves on the perimeter of a circle. This is historically an area of simple quantum circuits where the principle of gauge invariance...

Recent progress in combining density functional theory and related methods with the Boltzmann transport equation are enabling spectacular advances in computing electron dynamics in materials from first principles. The interaction between electrons and lattice vibrations (phonons) plays a central role as it governs carrier dynamics near room temperature and at low energy. I will present our recently developed methods to compute electron-phonon scattering processes from first principles, and show how these advances enable calculations of electron dynamics in materials, including: 1) First-...

TBA

Research statement: Our research is concerned with the development of rigorous and practical methods for simulations of quantum processes in complex systems as well as with applications studies of photochemical processes in proteins, semiconductor materials, and systems of environmental interest. We have recently made significant progress toward the establishment of rigorous quantum mechanical approaches for describing equilibrium and dynamical properties of complex quantum systems. We are currently investigating how to extend these calculations to investigate quantum mechanical processes...

Molecular Biophysics Seminar

Abstract: High temperature superconductivity remains one of the major open problems in physics.  It also is connected to another open question:  what is the right way to think about “strange metals”, for which our standard fermi liquid model of metals doesn’t seem to apply?  Atomically precise, epitaxial tunnel junctions are a means to examine these materials in a nominally simple configuration that minimizes disorder and surface effects.  We use c-axis epitaxial junctions to examine tunneling between La(2-x)Sr(x)CuO4 source and drain electrodes separated by a 2 nm barrier...

The physical properties of matter change dramatically as atoms assemble into extended solids. Low dimensional crystals could be used to reveal the intricate evolution of material properties across extremes of scale. However, overcoming profound challenges to progress will require methods for systematic and precise control over the size, shape, and structure of these crystals. To this end, we have developed strategies for controlled crystallization of low-dimensional materials and have identified that even subtle tuning of their dimensionality and morphology yields substantial property...

The possibilities of manipulating magnetization without applied magnetic fields have attracted growing attention over the last two decades. The low-power manipulation of magnetization, preferably at ultra-short time scales, has become a fundamental challenge with implications for future magnetic information storage and memory technologies. I will discuss recent experiments on the optical manipulation of the magnetization of engineered materials and devices using 50-5000 fs optical pulses. We demonstrate that all optical switching can be observed in a broad range of materials and not...

Ordinary classical fluids have a single type of sound waves consisting of longitudinal compressional oscillations of the density. Solids, on the other hand, have also transverse or shear sound waves because of their non-zero restoring force to shear deformations. In this talk, I will illustrate that quantum fermi liquids can radically deviate from this paradigm by developing a sharp collective transverse or shear sound wave when their interactions increase beyond a threshold, even though their ground state remains in a liquid state without any form of proper static crystalline order. I...

Mathias Scheurer is a postdoctoral fellow working on theoretical condensed matter in Prof. Subir Sachdev's group at Harvard University.

Abstract: Two-dimensional (2D) systems have become a very active field of research due to their particularly rich physics. As we know from classical statistical mechanics, 2D systems are special as they are situated right at the lower critical dimension and, as such, just incapable of spontaneously breaking a continuous symmetry at finite temperature. Nonetheless, finite-temperature phase transitions are possible; these are, however, not...