Applied Physics / Physics Colloquium

Atomic Collapse in Graphene [AP/Physics Colloquium]

Topic: 
Atomic Collapse in Graphene
Abstract / Description: 

Since the discovery that electrons in graphene behave as massless Dirac fermions, the single-atom-thick material has become a fertile playground for testing exotic predictions of quantum electrodynamics, such as Klein tunneling and the fractional quantum Hall effect. Now add to that list atomic collapse, the spontaneous formation of electrons and positrons in the electrostatic field of a superheavy atomic nucleus. The atomic collapse was predicted to manifest itself in quasi-stationary states which have complex-valued energies and which decay rapidly. However, the atoms created artificially in laboratory have nuclear charge only up to Z = 118, which falls short of the predicted threshold for collapse. Interest in this problem has been revived with the advent of graphene, where, because of a large fine structure constant, the collapse is expected for Z of order unity. I will discuss the symmetry aspects of atomic collapse, in particular the anomalous breaking of scale invariance that underpins the collapse phenomenon. I will also describe recent experiments that use scanning tunneling microscopy (STM) to probe atomic collapse near STM-controlled artificial compound nuclei.

 

Presented by APPLIED PHYSICS/PHYSICS COLLOQUIUM
Professor Stanley S. Hanna Visiting Professorship Program

Date and Time: 
Tuesday, January 10, 2017 - 4:15pm to 5:30pm
Venue: 
Hewlett, Room 200

Applied Physics/Physics Colloquium: The Dawn of Gravitational-Wave Astrophysics

Topic: 
The Dawn of Gravitational-Wave Astrophysics
Abstract / Description: 

The very first observing run of the Advanced LIGO detectors led to the discovery of binary-black-hole inspiral and merger events, and overnight we got launched into the era of gravitational-wave astrophysics. In this talk I will review the key results from the recent LIGO data search, signal characterization, and measurements of source properties. I will discuss how this observation enables the first tests of general relativity in the strong-field regime and what this discovery implies for black-hole astrophysics.


 

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 200. Refreshments in the lobby of Varian Physics at 4:15 pm.

Autumn 2016/2017, Committee: R. Blandford (Chair), A, Kapitulnik, R. Laughlin, L. Senatore

Date and Time: 
Tuesday, November 29, 2016 - 4:30pm to 5:30pm
Venue: 
Hewlett 200

Applied Physics/Physics Colloquium: Spotting the Elusive Majorana Under the Microscope

Topic: 
Spotting the Elusive Majorana Under the Microscope
Abstract / Description: 

There has been considerable interest in creating condensed matter systems that host boundary Majorana fermions. This goal is motivated in part by Majorana fermions' potential use in topological qubits to perform fault-tolerant computation aided by their non-Abelian characteristics. Recently, we have proposed a new platform for the realization of Majorana fermions in condensed matter systems, based on chains of magnetic atoms on the surface of a superconductor. This platform lends itself to measurements with the scanning tunneling microscope (STM) that can be used to directly visualize the Majorana edge mode with both high energy and spatial resolution. Using unique STM instrumentation, we have succeeded in creating this platform and have observed the predicted signatures of localized Majorana zero mode. I will describe our Majorana platform, the experiments to date, and the outlook for further experiments on Majoranas fermions in condensed matter systems.


Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 200. Refreshments in the lobby of Varian Physics at 4:15 pm.

Autumn 2016/2017, Committee: R. Blandford (Chair), A, Kapitulnik, R. Laughlin, L. Senatore

Date and Time: 
Tuesday, November 15, 2016 - 4:30pm to 5:30pm
Venue: 
Hewlett 200

Applied Physics/Physics Colloquium

Topic: 
Emergent Behavior in Quantum Matter
Abstract / Description: 

We live in an emergent universe in which interactions between the basic building blocks of matter and their environment give rise to unpredicted and unexpected emergent behavior at every scale. As physicists we seek to identify the organizing principles responsible for that behavior, construct soluble models that incorporate these, and explain experiment. I illustrate this approach to understanding emergent behavior in quantum matter through three examples: collective modes and quasiparticles in electron and helium, liquids; the emergence of superconductivity in conventional and unconventional superconductors, nuclei, and neutron stars; and the emergence of heavy electrons and quantum critical behavior in Kondo lattice materials.


 

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 200. Refreshments in the lobby of Varian Physics at 4:15 pm.

Autumn 2016/2017, Committee: R. Blandford (Chair), A, Kapitulnik, R. Laughlin, L. Senatore

Date and Time: 
Tuesday, November 8, 2016 - 4:30pm to 5:30pm
Venue: 
Hewlett 200

Applied Physics/Physics Colloquium

Topic: 
Precession in the Dark: CASPEr and GNOME
Abstract / Description: 

The Cosmic Axion Spin-Precision experiment and the Global Network of Optical Magnetometers for Exotic physics searches will be discussed.


Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 200. Refreshments in the lobby of Varian Physics at 4:15 pm.

Autumn 2016/2017, Committee: R. Blandford (Chair), A, Kapitulnik, R. Laughlin, L. Senatore

Date and Time: 
Tuesday, November 1, 2016 - 4:30pm to 5:30pm
Venue: 
Hewlett 200

Applied Physics/Physics Colloquium

Topic: 
Discovery of the Majorana Fermion
Abstract / Description: 

Majorana fermion is a hypothetical fermionic particle which is its own anti-particle. Intense research efforts focus on its experimental observation as a fundamental particle in high energy physics and as a quasi-particle in condensed matter systems. I shall report the theoretical prediction and the experimental discovery of the chiral Majorana fermion in a topological state of quantum matter. In the hybrid system of a quantum anomalous Hall thin film coupled with a conventional superconductor, a series of topological phase transitions are controlled by the reversal of the magnetization, where the half-integer quantized conductance plateau (0.5e2/h) is observed as a compelling signature of the Majorana fermion.


 

 

Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 200. Refreshments in the lobby of Varian Physics at 4:15 pm.

Autumn 2016/2017, Committee: R. Blandford (Chair), A, Kapitulnik, R. Laughlin, L. Senatore

Date and Time: 
Tuesday, October 25, 2016 - 4:30pm to 5:30pm
Venue: 
Hewlett 200

Applied Physics/Physics Colloquium

Topic: 
Particle Physics Beyond Colliders
Abstract / Description: 

When we think about particle physics, the first thing that comes to mind is colliders and high energies. Recently there have been several proposals of low-energy precision experiments that can also look for new particles, new forces, and the Dark Matter of the Universe in a way that is complementary to collider searches. In this talk, I propose two different experiments that search for a type of Dark Matter naturally arising in String Theory. In String Theory, fundamental constants, such as the electron mass or charge, are determined by fields known as moduli. When these fields are the Dark Matter of our Universe, they cause the fundamental constants to oscillate with a frequency set by the Dark Matter mass. For frequencies smaller than 1 Hz, atomic clocks with their unprecedented sensitivity can pick up these oscillations. For higher frequencies above 1 kHz, Dark Matter can excite acoustic modes in resonant mass detectors originally designed to detect gravitational radiation from astrophysical sources. Both techniques extend searches for this type of Dark Matter by several orders of magnitude in the near future.


 

 Held Tuesdays at 4:30 pm in the William R. Hewlett Teaching Center, room 200. Refreshments in the lobby of Varian Physics at 4:15 pm.

Autumn 2016/2017, Committee: R. Blandford (Chair), A, Kapitulnik, R. Laughlin, L. Senatore

Date and Time: 
Tuesday, October 18, 2016 - 4:30pm to 5:30pm
Venue: 
Hewlett 200

Applied Physics/Physics Colloquium

Topic: 
Emergent Locality and Gravity from Quantum Error Correction
Abstract / Description: 

The Anti-de Sitter/Conformal Field Theory correspondence has given us a precise description of quantum gravity in asymptotically Anti-de Sitter space, as a quantum field theory (without gravity) with one fewer spacetime dimension. This proposal has passed many tests, but the emergence of the extra dimension on the gravitational side remains rather mysterious.


Daniel will present recent work explaining how this emergence can be naturally understood as a manifestation of quantum error correction, an idea originally introduced to solve the seemingly unrelated problem of protecting a quantum computer from decoherence. We will see that this gives precise meaning to speculations over the past decade relating boundary entanglement to the emergence of bulk geometry; to emphasize this precision I will illustrate the proposal using an exactly soluble model of the correspondence constructed via tensor networks.

Date and Time: 
Tuesday, October 11, 2016 - 4:30pm to 5:30pm
Venue: 
Hewlett 200

Applied Physics/Physics Colloquium

Topic: 
Single Cell Genomics
Abstract / Description: 

An exciting emerging area revolves around the use of microfluidic tools for single-cell genomic analysis. We have been using microfluidic devices for both gene expression analysis and for genome sequencing from single cells. In the case of gene expression analysis, it has become routine to analyze hundreds of genes per cell on hundreds to thousands of single cells per experiment. This has led to many new insights into the heterogeneity of cell populations in human tissues, especially in the areas of cancer and stem cell biology. These devices make it possible to perform "reverse tissue engineering" by dissecting complex tissues into their component cell populations, and they are also used to analyze rare cells such as circulating tumor cells or minor populations within a tissue.

We have also used single-cell genome sequencing to analyze the genetic properties of microbes that cannot be grown in culture - the largest component of biological diversity on the planet—as well as to study the recombination potential of humans by characterizing the diversity of novel genomes found in the sperm of an individual. We expect that single cell genome sequencing will become a valuable tool in understanding genetic diversity in many different contexts.

Date and Time: 
Tuesday, October 4, 2016 - 4:30pm to 5:30pm
Venue: 
Hewlett 200

Stanford Friends of Physics Special Lectures

Topic: 
Superconductivity and Quantum Mechanics at a Human Scale
Abstract / Description: 

Superconductivity is perhaps the most spectacular macroscopic quantum phenomenon. A "persistent current" in a ring of superconducting wire will continue to flow forever—a laboratory realization of perpetual motion. A voltage across a junction between two superconductors produces an oscillating current with a frequency that is determined exactly by the voltage and the fundamental constant of quantum mechanics, Planck's constant. Superconductivity is the quintessential example of an "emergent phenomenon" in physics, in which the collective behavior cannot be understood in terms of the properties of any finite collection of microscopic constituents (i.e., electrons). Notable physicists including Einstein, Heisenberg, and Feynman tried and failed for half a century to achieve the basic understanding of superconductivity that was only achieved in the mid 1950's and early 1960's. However, many fundamental issues remain to be resolved, including those related to the more recent discovery of unconventional "high temperature superconductivity" in a variety of synthetic metals, and the construction of coherent superconducting "Q-bits," which act as laboratory realizations of Schrodinger's cat.


 

Greetings, Friends of Physics! We hope some of you can attend the upcoming SITP mini-course by Steven Kivelson, the Prabhu Goel Family Professor of Physics. Professor Kivelson is a leading researcher in theoretical condensed matter physics at the Stanford Institute for Theoretical Physics (SITP).


SITP mini-courses feature SITP faculty members lecturing on their fields and research. The presentations are intended for audiences with an interest in learning about science more deeply than the level of popular media, and some scientific or mathematical background. You are welcome to attend the lectures in person or access them on the web when they are posted (see below).

Date and Time: 
Monday, May 16, 2016 - 7:00pm to 8:30pm
Venue: 
Hewlett 201

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