Harvard University Department of Physics

The Morris Loeb Lecture in Physics: Jun Ye

Three lectures by Jun Ye: "Quantum matter and atomic clocks (Oct. 22) "; "Frequency Comb Spectroscopy – from mid-IR to XUV" (Oct. 23); "Cold molecules – a new playground for quantum and chemical physics" (Oct. 24).

New Book by David Morin

This book is a collection of 57 very challenging math problems with detailed solutions. It is written for anyone who enjoys pondering difficult problems for great lengths of time. The problems are mostly classics...

Physical Interpretation of the Partition Function for Colloidal Clusters

Colloidal clusters consist of small numbers of colloidal particles bound by weak short-range attractions. The equilibrium probability of observing a cluster in a particular geometry is well described by a statistical mechanical model originally developed for molecules. To explain why this model fits experimental data so well...

UPCOMING EVENTS

Today

CMP Special Seminar: Philip Moll (Ecole Polytechnique Federale de Lausanne), hosted by Philip Kim
Start: 01:30pm - End: 03:00pm
- Title: Designing landscapes of correlations and topology: a strain-gradient approach
  
  Abstract: The phase diagrams of strongly correlated materials are typically rich and complex, reflecting the competition of various ordered phases for states at the Fermi level. We currently explore new approaches to fabricate single crystal devices in which multiple of such phases coexist in different locations of a device. A spatial modulation of the correlation strength can be achieved by controlling a strong strain gradient in the sample. Using Focused-Ion-Beam machining, we fabricate micrometer-sized single crystal devices from quantum materials and subject them both to passive strain fields via thermal contraction as well as actively driving them in a MEMS setting. This allows the generation of strong yet well-controlled strain gradients. Two concrete recent examples will be discussed: In the heavy-fermion superconductor CeIrIn5, the hybridization strength of the 4f1-electron with the conduction band can be tuned in real space through strain gradient fields. In the Dirac semi-metal Cd3As2, strain gradients move (and potentially merge) the nodes, leading to the emergence of “pseudo-magnetic” fields which influence the quasiparticles akin to Landau quantization in real magnetic fields. I will present the physical questions that become tangible when strong strain gradients can be controlled, our experimental and technical approaches to generate them, as well as our recent results.
- 01:30pm
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MATH: Shamil Shakirov (Harvard), “Rational integrable systems associated with higher genus surfaces”
Start: 04:00pm - End: 05:00pm
- We construct some systems of commuting difference operators associated with higher genus surfaces.
- 04:00pm
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CUA Seminar/Loeb Lecture: Prof. Jun Ye (JILA)
Start: 04:30pm - End: 06:00pm
- 04:30pm
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Tomorrow

Loeb Lecture - Jun Ye (JILA)
Start: 04:00pm - End: 05:30pm
- 04:00pm
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Prof. Luyi Sun - UCONN
Start: 06:00pm - End: 07:30pm
- 06:00pm
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Thursday, October 25th

CMP Seminar: Vladimir Narovlansky (Weizmann Institute), hosted by Subir Sachdev
Start: 12:00pm - End: 01:00pm
- Title: The renormalization group of quenched disorder
  
  Abstract: A systematic analysis of classical and quantum theories with disorder is performed using renormalization group flow. For the quantum case, this can be done despite the appearance of non-locality in time in the replica theory, leading to several interesting results. The dynamical scaling exponent is shown to be related to a flow of a particular coupling constant, and has a universal form at weak disorder. The renormalization group flow also leads to the identification of new critical exponents for quantum disorder.
- 12:00pm
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ITAMP Lunch Seminar
Start: 12:00pm - End: 01:00pm
- Speaker: Valentin Walther
- 12:00pm
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Friday, October 26th

CMP Special Seminar: Dario Poletti (Singapore University of Technology and Design), hosted by Jamir Marino
Start: 12:00pm - End: 01:00pm
- Title: Current Rectification in Strongly Interacting Open Quantum Systems
  
  Abstract: We study the rectification of spin current in XXZ chains segmented in two parts, each with a different anisotropy parameter. Using exact diagonalization and a matrix product states algorithm we find that a large rectification (of the order of 10^4) is attainable even using a short chain of N=8 spins, when one half of the chain is gapless while the other has large enough anisotropy. We present evidence of diffusive transport when the current is driven in one direction and of a transition to an insulating behavior of the system when driven in the opposite direction, leading to a perfect diode in the thermodynamic limit. The above results are explained in terms of matching of spectrum of magnon excitations between the two halves of the chain. When considering heat baths, we also show a behavior for the heat current with strong similarities to that of the spin current, emphasizing the robustness of the rectification mechanism.
- 12:00pm
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Monday, October 29th

CMP Special Seminar: Alexander Zibrov (UCSB), hosted by Philip Kim
Start: 11:00am - End: 12:00pm
- Title: Even-denominator fractional quantum Hall (FQH) states in mono- and bilayer graphene.
  
  Abstract: In two dimensions, electrons subjected to strong magnetic fields from flat, massively degenerate bands called Landau levels. If a Landau level is partially filled to a rational fraction ν=p/q, interactions between electrons can form a fractional quantum Hall (FQH) insulator - a class of incompressible fluids with quasiparticle excitations carrying fractional charge and exchange statistics. The overwhelming majority of observed FQH states appear at odd-denominator fillings, while the even-denominator states remain enigmatic — some of which potentially hosting non-abelian excitations.In this talk I describe magneto-capacitance measurements of high quality mono- and bilayer graphene devices encapsulated in boron nitride and few-layer graphite gates, and show how graphene’s unique degrees of freedom lead to novel FQH states. In monolayer graphene we observe insulating even-denominator ν=±½ and ±¼ states, which appear only for a narrow range of magnetic fields accompanied by a series of phase transitions and crossovers in the neighboring odd-denominator states, a result of the interplay between spin and valley/sublattice degrees of freedom.
  Meanwhile, in bilayer graphene we observe a different class of even denominator states at 𝜈=−5/2, -1/2, 3/2 and 7/2 which we interpret as non-abelian, Pfaffian ground states. Non-abelian quasiparticles are predicted to behave differently under exchange than fermions or bosons: interchanging or “braiding” quasiparticles results in a different ground state, resulting in a ground state degeneracy which is not present for Abelian quasiparticles. The braiding properties of non-Abelian quasiparticles could be used for topological quantum computing. I will conclude this talk by describing our current efforts aimed at probing the ground state degeneracy of the even-denominator states in bilayer graphene.
- 11:00am
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Professor Robert Kleinberg, Columbia University, Boston University
Start: 04:15pm - End: 05:15pm
- m K to km: How Millikelvin Physics is Reused to Explore the Earth Kilometers Below the Surface
  
  Investigations of the super fluid phases of liquid helium-3 would seem to have little application to the study of rock formations thousands of meters below the surface of the earth. However, the physicist’s tool box is versatile, and the techniques used in one field of study can be reused, with appropriate adaptation, in very different circumstances. The temperature of liquid helium-3 in the millikelvin range can be measured using an unbalanced-secondary mutual inductance coil set designed to monitor the magnetic susceptibility of a paramagnetic salt. The loss signal is discarded by phase sensitive detection.
  Now consider the task of measuring the electrical conductivity, at centimeter scale, of the earth surrounding a borehole. Turn the mutual inductance coil set inside out, with secondary coils arranged to be unbalanced with respect to the rock wall. Instead of discarding the loss signal, use it to measure conductivity. A sensor based on this principle has been implemented in a widely deployed borehole geophysical instrument, used to estimate the prevailing direction of the wind millions of years ago, or to decide where to drill the next well in an oil field. Nuclear magnetic resonance may seem a very improbable measurement of the rock surrounding a borehole. Conventionally, we place the sample (which might be a human being) inside the NMR apparatus. In borehole deployment, the instrument is placed inside the sample, the temperature is as high as 175 C, pressure ranges to 140 MPa, and measurements must be made while moving at 10 cm/s. Apparatus with these specification have been deployed worldwide, and are used to measure a number of rock properties, including the distribution of the sizes of pores in sedimentary rock, and the viscosity of oil found therein. They have also been used for geological and oceanographic studies in northern Alaska and at the sea floor offshore Monterey, California.
- 04:15pm
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