Submit Feedback

All Videos

The laws of thermodynamics are fundamental laws of nature that classify energy changes for macroscopic systems as work performed by external driving and heat exchanged with the environment. In the past decades, these principles have been successfully extended to the level of classical trajectories of microscopic systems to account for thermal fluctuations. In particular, experimentally tested generalizations of the second law, known as fluctuation theorems, quantify the occurrence of negative entropy production. The extension of thermodynamics to include quantum fluctuations faces unique challenges such as the proper identification of heat and work and clarification of the role of quantum coherence. I will present experiments that allow us to track heat and work along single quantum trajectories of a superconducting qubit evolving under continuous unitary evolution and measurement. We are able to verify the first law of thermodynamics in that the measured heat and work sum to the total energy change of the quantum system.  We then verify the second law of thermodynamics in the form of the Jarzynski equality by employing a novel quantum feedback loop that cancels the heat exchanged at each point in time with additional work.  Our results successfully generalize stochastic thermodynamics to the quantum regime, paving the way for future experimental and theoretical investigations of quantum information and thermodynamics.

Geometrical effects influencing the measured spin coherence and quantum phase coherence in mesoscopic structures were characterized by low-temperature spin-dependent quantum transport experiments.  The findings are of possible relevance for the design of devices for quantum technologies, and have foundational aspects as well.  The materials studied have strong spin-orbit interaction and are heterostructures of InSb, InAs, or InGaAs, and the semimetal Bi with its surface states. The materials were patterned into mesoscopic stadia, narrow channels or quantum interferometers, of typical size ~ 1 micron, comparable to the spin and quantum phase coherence lengths.  Aharonov-Bohm experiments, antilocalization, and universal conductance fluctuations were used to quantify the spin- and quantum phase coherence lengths.  Using geometrical constraints on the accumulation of quantum geometric phases, the work shows a correspondence, in a diffusive transport regime, between mesoscopic dephasing effects due to time-reversal symmetry breaking by magnetic fields, and spin decoherence due to spin-orbit interaction (Aharonov-Bohm / Aharonov-Casher correspondence).  The work also reveals device-geometrical influences on quantum phase coherence from coupling to the classical environment and geometrical effects of electron-electron interactions.

The Pittsburgh Quantum Institute mission is to help unify and promote quantum science and engineering in Pittsburgh. For more information, please visit our website:

Christopher White, Caltech IQIM

I will describe a method "DMT" for approximating density operators of 1D systems as low bond dimension matrix product operators that, when combined with a standard framework for time evolution (TEBD), makes possible simulation of the dynamics of strongly thermalizing systems to arbitrary times. The method performs well for both near-equilibrium initial states (Gibbs states with spatially varying temperatures) and far-from-equilibrium initial states, including quenches across phase transitions and pure states. I will also discuss ongoing work applying the method to the diffusive-subdiffusive transition in the ergodic phase of the random-field Heisenberg model.

Hot electron processes at metallic heterojunctions are central to optical-to-chemical or electrical energy transduction. Ultrafast nonlinear photoexcitation of graphite (Gr) has been shown to create hot thermalized electrons at temperatures corresponding to the solar photosphere in less than 25 fs. Plasmonic resonances in metallic nanoparticles are also known to efficiently generate hot electrons. Here we deposit Ag nanoclusters (NC) on Gr to study the ultrafast hot electron generation and dynamics in their plasmonic heterojunctions by means of time-resolved two-photon photoemission (2PP) spectroscopy. By tuning the wavelength of p-polarized femtosecond excitation pulses, we find an enhancement of 2PP yields by 2 orders of magnitude, which we attribute to excitation of a surface-normal Mie plasmon mode of Ag/Gr heterojunctions at 3.6 eV. The 2PP spectra include contributions from (i) coherent two-photon absorption of an occupied interface state (IFS) 0.2 eV below the Fermi level, which electronic structure calculations assign to chemisorption-induced charge transfer, and (ii) hot electrons in the π*-band of Gr, which are excited through the coherent screening response of the substrate. Ultrafast pump–probe measurements show that the IFS photoemission occurs via virtual intermediate states, whereas the characteristic lifetimes attribute the hot electrons to population of the π*-band of Gr via the plasmon dephasing. Our study directly probes the mechanisms for enhanced hot electron generation and decay in a model plasmonic heterojunction.

View on ACS Publications

Experimental methods for ultrafast microscopy are advancing rapidly. Promising methods combine ultrafast laser excitation with electron-based imaging or rely on super-resolution optical techniques to enable probing of matter on the nano–femto scale. Among several actively developed methods, ultrafast time-resolved photoemission electron microscopy provides several advantages, among which the foremost are that time resolution is limited only by the laser source and it is immediately capable of probing of coherent phenomena in solid-state materials and surfaces. Here we present recent progress in interference imaging of plasmonic phenomena in metal nanostructures enabled by combining a broadly tunable femtosecond laser excitation source with a low-energy electron microscope.

View on ACS Publications

Strontium titanate is a bulk insulator that becomes superconducting at remarkably low carrier densities. Even more enigmatic properties become apparent at the strontium titanate/lanthanum aluminate (STO/LAO) interface and it is important to disentangle the effects of reduced dimensionality from the poorly-understood pairing mechanism. Recent experiments measuring the surface photoemission spectrum[1] and bulk tunneling spectrum[1] have found a cross-over, as a function of carrier density, from a polaronic regime with substantial spectral weight associated with strongly coupled phonons, to a more conventional weakly coupled Fermi liquid. Interestingly, it is only the polaronic state that becomes superconducting at low temperatures, although the properties of the superconducting phase itself appear entirely conventional. We interpret these results in a simple analytical model that extends an Engelsberg-Schrieffer theory of electrons coupled to a single longitudinal optic phonon mode to include the response of the electron liquid, and in particular phonon-plasmon hybridization. We perform a Migdal-Eliashberg calculation within our model to obtain this material's unusual superconducting phase diagram.

[1] Z. Wang et al, Nat. Mater. (2016)
[2] A.G. Swartz et al, arXiv:1608.05621

In this talk, I am going to present some of the work that I have done during my PhD. In the first part I will mostly focus on building a low-temperature Andreev reflection spectroscopy probe (which is basically a simpler version of an STM). In the second part, I will briefly talk about the observation of a new phase of matter, tip-induced superconductivity (TISC), that emerges only under mesoscopic metallic point contacts on topologically non-trivial semimetals like a 3-D Dirac semimetal Cd3As2, and a Weyl semimetal, TaAs and comment on the possible mechanism that might lead to the emergence of such a surprising phase of matter. All these experiments were done using our home-built low-temperature probe.
If time permits, I will also talk about some experiments that we did using various scanning probe based microscopic techniques, e.g., piezo-response force microscopy (PFM) and ferroelectric lithography. I will show how certain “artifacts” can limit the application of PFM in the investigation of ferroelectric materials, and how, under certain circumstances, such “artifacts” can actually turn out to be useful.

[1] L. Aggarwal, A. Gaurav, G. S. Thakur, Z. Haque, A. K. Ganguli & G. Sheet, Unconventional Superconductivity at Mesoscopic Point-contacts on the 3-Dimensional Dirac Semi-metal Cd3As2.” Nature Materials 15, 32 (2016).
[2] L. Aggarwal, S. Gayen, S. Das, R. Kumar, V. Sϋß, C. Shekhar, C. Felser & G. Sheet, “Mesoscopic superconductivity and high spin-polarization coexisting at metallic point contacts on Weyl semimetal TaAs.” Nature Communications, 8, 13974 (2017).
[3] J. S. Sekhon, L. Aggarwal & G. Sheet, “Voltage induced local hysteretic phase switching in silicon.” Applied Physics Letters 104, 162908 (2014). 

The two-dimensional diffusive metal stabilized at the interface of SrTiO3 and the Mott Insulator perovskite LaTiO3[1-2] has challenged many notions related to the formation and electronic behavior of the two-dimensional electron gas (2DEG) at the well studies LaAlO3-SrTiO3 interface. Here we discuss specifically the stability of the superconducting phase[3] at LaTiO3 – SrTiO3 interface, the nature of the superconductor – normal metal quantum phase transition (T=0 limit) driven by magnetic field, significance of the field vis-à-vis the Chandrasekhar - Clogston limit for depairing, and how the transition is initiated when the extent of Coulomb interaction amongst charge carriers is modulated by electrostatic gating[4]. The nature of the superconducting condensate is highlighted in the light of the Ti - t2g orbital driven bands and their filling in the presence of a strong Rashba spin – orbit interaction (SOI). Towards the end of the talk, we will discuss the prominent effects of Rashba SOI on normal state quantum transport and how it renormalizes a Kondo-like electronic behavior in range of temperature Tc< T < 5K[5-7]. The prominence of the Ti 3d0 and Ti 3d1 correlated electron physics in these systems will be demonstrated further from our recent studies of 2DEG in ion irradiated SrTiO3 crystals[8,9].
1. Advanced Materials 22, 4448(2010). 2. Phys. Rev. B 86, 075127(2012).
3. Nature Communications, 1, 89(2010). 4. Nature Materials 12, 542(2013).
5. Phys. Rev. B (Rapid Communication) 90, 081107(2014). 6. Phys. Rev. B 90, 075133(2014). 7. Phys. Rev. B 94, 115165 (2016).
8. Phys. Rev. B 91, 205117(2015). 9. Phys. Rev. B 92, 235115 (2015).

Advancements in nanoscale engineering of oxide interfaces and heterostructures have led to discoveries of emergent phenomena and new artificial materials. Combining the strengths of reactive molecular-beam epitaxy and pulsed-laser deposition, we show that atomic layer-by-layer laser molecular-beam epitaxy (ALL-Laser MBE) significantly advances the state of the art in constructing oxide materials with atomic layer precision. Using Sr1+xTi1-xO3 as example, we demonstrate the effectiveness of the technique in producing oxide films with stoichiometric and crystalline perfection. With the growth of La5Ni4O13, a Ruddlesden-Popper phase with n = 4 that has never been reported in the literature, we demonstrate that ALL-Laser MBE allows us to push the equilibrium thermodynamic boundary further. By growing LaAl1+yO3 films of different stoichiometry on TiO2-terminated SrTiO3 substrate at high oxygen pressure, we show that the behavior of the two-dimensional electron gas at the LaAlO3/SrTiO3interface can be quantitatively explained by the electronic reconstruction mechanism. In LaNiO3 films on LaAlOsubstrate with LaAlObuffer layer, we observed the metal insulator transition in 1.5 unit cells, which is driven by oxygen vacancies in addition to epitaxial strain and reduced dimensionality.