All Visitors

Understanding Molecular and Hybrid Crystals from First Principles

Talk time: 
Thursday, January 17, 2019 - 2:30pm
Talk location: 
Chevron 150
Abstract: 

Molecular crystals are crystalline solids composed of molecules bound together by relatively weak intermolecular interactions, typically consisting of van der Waals interactions and/or hydrogen bonds. Hybrid crystals combine molecular units and covalent/ionic networks.

Both classes of crystals play an important role in many areas of science and engineering, ranging from biology and medicine to mechanics and electronics. Therefore, much effort has been dedicated to understanding their structure and properties.

Predicting the behavior of such materials from first principles is highly desired for understanding their unique properties and for allowing rational design of novel materials and structures. Preferably, we would like to obtain such understanding from density functional theory (DFT), because the relative computational simplicity afforded by DFT allows us to attack realistic, experimentally accessible problems. Unfortunately, despite many other successes, DFT has traditionally struggled with useful prediction of properties of crystals that contain weakly-bound units.

Here, I will show how state-of-the-art DFT approaches allow us to overcome these limitations, quantitatively. I will focus on our recent progress in explaining and even predicting important classes of collective effects, i.e., phenomena that the individual units comprising the crystal do not exhibit, but arise through their interaction. Specifically, I will address unique structural, mechanical, electrical, and optical properties of both biogenic and synthetic crystals, with an emphasis on constructive interaction between theory and experiment.

Synthesizing quantum matter with electrons and microwave photons

Talk time: 
Thursday, January 24, 2019 - 4:00am
Talk location: 
321 Allen Hall
Abstract: 

Experimental research at the nanoscale continues to challenge our ability to predict the behavior of quantum systems. Advances with lithographically patterned solid-state electronic devices have enabled multiple platforms for the simulation of quantum matter. In particular, semiconductor quantum dots and superconducting qubits provide tools for studying the wealth of physics induced by nonlinearities at the single electron and single microwave-photon level, respectively, and have been separately pursued as enabling technologies for qubits. In recent years, hybrid devices that combine such historically distinct lines of research have received greater attention, whether to enable novel sensing or measurement applications, or to couple small systems of qubits together at long range (e.g. quantum transduction). I will showcase the rich behaviors and phases of quantum matter that coupled quantum dots can exhibit, including a surprising transport mechanism called cotunneling drag [PRL 117, 066602 (2016)], signatures of Kondo physics with emergent symmetry [Nature Physics 10, 145 (2013)], and non-Fermi liquid states [Nature 536, 237–240 (2015)]. I will also discuss my work towards fabricating superconducting qubits on silicon-on-insulator substrates for hybrid device applications [Appl. Phys. Lett. 111, 042603 (2017)]. The integration of quantum dots and superconducting resonators promises to yield new probes for studying quantum matter, and superconducting qubits are coming of age in their own right for the implementation of many-body spin models.

mK to km: How Millikelvin Physics is Reused to Explore the Earth Kilometers Below the Surface

Talk time: 
Monday, January 28, 2019 - 4:00pm
Talk location: 
102 Thaw Hall
Abstract: 

It is a common, but still surprising observation that many physics students have never met a physicist outside of an academic setting.  Thus many undergraduate and graduate students know of few sources of information to help them understand what opportunities may exist beyond university environments.  The purpose of the APS Distinguished Lecturer program is to show how some physicists have navigated the transition to the “real world”.

Investigations of the superfluid 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 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 oilfield.

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 specifications 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 seafloor offshore Monterey, California.

From Platinum to Planck: The biggest revolution in metrology since the French Revolution

Talk time: 
Thursday, January 31, 2019 - 4:00pm
Talk location: 
104 Thaw Hall
Abstract: 

The International System of Units (SI) is the basis for measurements worldwide, and NIST, as the premier National Metrology Institute, has both shaped the development of the SI and led its implementation.  On November 16, 2018 the Member States of the Treaty of the Meter voted to revise the International System of Units (SI), changing the world's definition of the kilogram, the ampere, the kelvin and the mole.   This decision occurred at the 26th meeting of the General Conference on Weights and Measures (CGPM) in Versailles, France, means that all SI units will now be defined in terms of constants that describe the natural world. This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.  The actual revision official will come into force on World Metrology Day, 20 May 2019.  In the reformed SI, all of the base units will be defined by reference to unchanging constants of nature, finally eliminating any connection between units and special artifacts.  This talk will describe why such a radical change was needed; how it is being achieved, in large part driven by work done at NIST; and how new technologies are transforming how we realize and disseminate the SI.

Chemical and Physical Considerations in the Production of a Cup of Coffee

Talk time: 
Monday, February 11, 2019 - 4:30pm
Talk location: 
Thaw 102
Abstract: 

Despite coffee’s ubiquity and tremendous economic value (~1.5% of the USA GDP), there remains very little research in the field. Yet, numerous physical and chemical processes play a determining role in cup quality, ranging from agricultural practices, to roasting and brewing. This talk canvases the landscape of coffee research to date, detailing areas that require further study, as well as discussing our early efforts to better understand the key factors that determine cup quality and reproducibility.

Past Visitors

Coupling a Superconducting Qubit to a Metamaterial Resonator

Talk time: 
Tuesday, November 27, 2018 - 4:00pm
Talk location: 
Allen 321
Abstract: 

Superconducting metamaterials formed from arrays of thin-film lumped circuit elements provide a route for implementing novel dispersion relations and band structure in a circuit QED environment. We have implemented metamaterial resonators from left-handed transmission lines and characterized their dense spectrum of modes through a combination of microwave transmission measurements and laser scanning microscopy imaging of the standing-wave structure on the various resonances. By appending a segment of a conventional transmission line on one end of our metamaterial, we have coupled a flux-tunable transmon qubit to the structure and observed the interaction of the qubit with the metamaterial by tuning its transition frequency through resonance with each of the modes. Through time-domain qubit measurements at different flux bias points, we are able to explore the variation in the qubit relaxation time as a function of frequency, with significant dips in lifetime as the qubit transition approaches each mode and a recovery of the lifetime in between resonances. In addition, we have measured the Stark shift of the qubit transition as we drive the metamaterial with different powers on each of its resonances that are coupled to the qubit.

Reciprocal and nonreciprocal amplification at the quantum level

Talk time: 
Wednesday, November 28, 2018 - 2:00pm
Talk location: 
321 Allen Hall
Abstract: 

Preserving the quantum coherence of signals is of paramount importance for components utilized in quantum information processing, quantum computation and quantum measurement setups. In recent years a tremendous progress has been made in the development of quantum-limited components, such as reciprocal and nonreciprocal amplifiers, circulators and isolators. A promising way to design these devices is based on parametric modulation of coupled modes, where the required mode-mixing processes are realized by utilizing Josphson junction-based tunable couplers or via coupling to mechanical degrees of freedom. All designs come with their unique challenges, such as higher-order nonlinearities – limiting the dynamical range, or high thermal occupations – leading to noise contamination of the signal. In addition, standard cavity-based parametric amplifiers suffer from a fixed gain-bandwidth product.

In this talk we present possible ways to face these challenges, complemented with an introduction of the basic concept on how to engineer nonreciprocal interactions and devices based on balancing a coherent interaction with the corresponding dissipative interaction. Furthermore, we present possible implementations in superconducting circuit and optomechanical architectures and discuss routes for optimizing the design of microwave circuits enabling nonreciprocal and reciprocal amplification at the quantum limit.

Room-temperature quantum fluids of light

Talk time: 
Monday, October 22, 2018 - 4:00pm
Talk location: 
Thaw 11
Abstract: 

Light-matter interaction is at the heart of most optical processes we are familiar with such as absorption, emission and scattering. These are normally treated by assuming that the incident light does not significantly modify the underlying electronic states of the material it interacts with. The strong coupling regime consists of the extreme case where light-matter interaction is so strong that it must be treated non-pertubatively. Polaritons, the resulting mixed light-matter particles, can be the source of many unique phenomena. We will describe how these quasiparticles can be exploited to enhance the photoluminescence yield of molecular emitters, engineer solar cells and photodetectors, modify photochemistry, and create room-temperature analogs to Bose-Einstein condensates1,2 and superfluid He.3

We will first focus on our work which uses organic microcavities that allow for room-temperature operation and then describe recent experiments using atomic monolayers of WS2 as the active medium, which allow for ballistic polariton flow over macroscopic >100 mm distances and the first room-temperature measurements of exciton-exciton interactions in 2D semiconductors. We will conclude with an outlook on the various material sets for room-temperature polaritonics and requirements for practical applications.

References

[1]   Kéna-Cohen S., Forrest S.R., “Room-temperature polariton lasing in an organic single-crystal microcavity”, Nature Photonics, Vol. 4, (2010), p. 371.

[2]   Daskalakis K.D., Maier S.A., Murray R., Kéna-Cohen S. “Nonlinear interactions in an organic polariton condensate”, Nature Materials, Vol. 13, (2014), p. 271.

[3]   Lerario et al., “Room-temperature superfluidity in a polariton condensate”, Nature Physics, Vol 13, 837 (2017)

Orbital selective pairing in Fe-based superconductors

Talk time: 
Thursday, October 18, 2018 - 4:00pm
Talk location: 
321 Allen
Abstract: 

Iron-based superconductors are unconventional superconductors with relatively high Tc that derive from metallic parent compounds with several Fe d-states dominant at the Fermi level. This gives rise to a number of novel effects based on differentiated degree of correlation of the different orbital states. I discuss the influence on spin-fluctuation pairing theory of orbital selective strong correlation effects in Fe-based superconductors, particularly Fe chalcogenide systems. This paradigm yields remarkably good agreement with the experimentally observed anisotropic gap structures in both bulk and monolayer FeSe, as well as LiFeAs, indicating that orbital selective Cooper pairing plays a key role in the more strongly correlated iron-based superconductors.  Recently new experiments such as quasiparticle interference and inelastic neutron scattering, have observed similar effects in the normal state, which can also be understood in the framework of this theory.

Are we quantum computers, or merely clever robots?

Talk time: 
Monday, October 15, 2018 - 4:00pm
Talk location: 
Thaw 11
Abstract: 

Of course quantum information processing is not possible in the warm wet brain. There is, however, one \loophole" - oered by nuclear spins - that must be closed before acknowledging that we are merely clever robots. Putative neural quantum processing with nuclear spins seemingly requires fulllment of many unrealizable conditions: for example, a common biological element with a very isolated nuclear spin to serve as a qubit, a mechanism for quantum entangling qubits, a mechanism for quantum memory storage and processing, a quantum to biochemical transduction that modulates neuron ring rates, among others. My strategy, guided by these requirements, is one of reverse engineering seeking to identify the bio-chemical substrate and mechanisms hosting such putative quantum processing. Remarkably, a specic neural qubit and a unique collection of ions, molecules and enzymes is identied, illuminating an apparently single path towards nuclear spin quantum processing in the brain.

Building a quantum computer using silicon quantum dots

Talk time: 
Thursday, October 11, 2018 - 4:00pm
Talk location: 
Allen 321
Abstract: 

In principle, quantum computers that exploit the nature of quantum physics can solve some problems much more efficiently than classical computers can.  Motivated by the tremendous scalability of classical silicon electronics, we are working to build a large-scale quantum computer using silicon technology similar to that used to build current classical computers.  This talk will discuss the fundamental physics and materials science challenges that arise, and how close coupling between theory and experiment has enabled substantial progress towards the goal of high fidelity qubits.  Prospects for further development will also be discussed.

Using Interfacial Electric Fields at Domain Walls to Stabilize Novel Ground States

Talk time: 
Thursday, September 27, 2018 - 4:00pm
Talk location: 
321 Allen Hall
Abstract: 

Interfaces between two distinct complex oxide materials can display ground states which diverge greatly from the parent compounds, making them a playground to establish emergent phenomena. Particularly intriguing are the so-called polar interfaces where a diverging electrostatic potential leads to charge transfer. The canonical polar interface between two insulating oxides, LaAlO3/SrTiO3, forms a two-dimensional electron liquid which superconductors at low-temperature and where the conductivity can be manipulated by changing the film surface. Here, I will demonstrate novel functionality at a very different type of polar interface – a charged domain wall in a ferroelectric. Similar to the polar heterointerfaces, the polarization mismatch causes local, diverging electrostatic potential also requiring charge compensation and hence a change in the electronic structure. Combining mesoscale transport, atomic-scale spectroscopy and theory, we demonstrate electric-field control of the transport at such ferroelectric domain walls. In a separate system, I will alternatively demonstrate how the electrostatic potential from the ferroelectric polarization can drive the system to assume a distinct ground state which can be manipulated with an electric field. Combined these systems present charged ferroelectric domain walls as a platform for stabilizing novel functionality.

Steve Hellberg Visit

Talk time: 
Wednesday, September 26, 2018 - 9:15am
Abstract: 

Visit

Statistical mechanics of the transition to turbulence

Talk time: 
Wednesday, September 26, 2018 - 10:00am
Talk location: 
321 Allen Hall
Abstract: 

How do fluids become turbulent as their flow velocity is increased? In recent years, careful experiments in pipes and Taylor-Couette systems have revealed that the lifetime of transient turbulent regions in a fluid appears to diverge with flow velocity just before the onset of turbulence, faster than any power law or exponential function. I show how this superexponential scaling of the turbulent lifetime in pipe flow is related to extreme value statistics, which I show is a manifestation of a mapping between transitional turbulence and the statistical mechanics model of directed percolation.  This mapping itself arises from a further surprising and remarkable connection: laminar and turbulent regions in a fluid behave as a predator-prey ecosystem. Such ecosystems are governed by individual fluctuations in the population and being naturally quantized, are solvable by path integral techniques from field theory. I explain the evidence for this mapping, and propose how a unified picture of the transition to turbulence emerges in systems ranging from turbulent convection to magnetohydrodynamics. 

Faster than fourier (pre)revisited: vorticulture, fractals, escape…

Talk time: 
Monday, September 24, 2018 - 4:30pm to 5:30pm
Talk location: 
Thaw Hall 11
Abstract: 

Band-limited functions can oscillate arbitrarily faster than their fastest Fourier component over arbitrarily long intervals: they can ‘superoscillate’. In physics, this counterintuitive mathematical phenomenon is associated with almost-destructive interference, and occurs near phase singularities in optics and on the world’s ocean tides; and it is associated with quantum weak measurements. Where superoscillations occur, functions are exponentially weak and vulnerable to noise. They are an unexpectedly compact way of representing fractals. Superoscillations in red light can escape as gamma radiation. 

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