Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session Q24: Entropy Measurements in Mesoscopic and Nanoscopic SystemsInvited
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Sponsoring Units: DCMP Chair: Klaus Ensslin, ETH Zurich Room: Room 237 |
Wednesday, March 8, 2023 3:00PM - 3:36PM |
Q24.00001: Measuring Entropy of Strongly Correlated Systems Invited Speaker: Yigal Meir In recent years many candidate setups have been proposed to support exotic quasi-particles, such as Majorana fermions (MFs), which may be relevant for quantum computing, but whether these particles have been observed experimentally is currently a topic of a vivid debate. Entropy measurements can unambiguously separate these quasi-particles from other, simpler excitations. The entropy of a MFs is, for example, log2/2 (in units of the Boltzman constant), a fractional value that cannot be attributed to a localzed excitation. However, standard entropy measurements applicable to bulk systems cannot be utilized in measuring the additional entropy of a mesoscopic device, which may be due to less than a single electron in the device. In this talk I will describe recent theoretical and experimental progress in performing such measurements, either using thermopower and/or using the Maxwell relations [1,2]. Particular examples will be single and double quantum dots in the Coulomb blockade regime. Lastly I will show how the formalism has been generalized to deduce the entropy from conductance measurements, and, applying it to a setup where two and three-channel Kondo physics have been observed, yields the fractional entropy of a single MF and a single Fibonacci anyon [3]. |
Wednesday, March 8, 2023 3:36PM - 4:12PM |
Q24.00002: The Pomeranchuk Effect in Magic Angle Graphene Revealed by Electronic Entropy Measurements Invited Speaker: shahal ilani In the 1950's, Pomeranchuk predicted that, counterintuitively, liquid 3He may solidify upon heating, due to a high excess spin entropy in the solid phase. In this talk, I will review our electronic entropy and compressibility measurements, which demonstrate an analogous electronic effect in magic angle twisted bilayer graphene. Our experiments reveal a giant magnetic entropy (~1kB per moiré site) near a filling of one electron per moiré site. This entropy drives a Pomeranchuk-like transition from a rather convention metal to a correlated state with nearly-free magnetic moments. However, while in 3He it is easy to understand why the spins of localized atoms in the solid are practically free, it is very surprising to observe nearly-free moments in a metallic, compressible state in magic angle graphene. The nature of this newly observed correlated metallic state is thus still highly puzzling. |
Wednesday, March 8, 2023 4:12PM - 4:48PM |
Q24.00003: Direct entropy measurement of coupled quantum systems Invited Speaker: Joshua Folk Recent experiments have demonstrated the power of using Maxwell relations to measure entropy in nanoscale systems, from bulk electronic phases in magic angle graphene to isolated spins in semiconductor quantum dots. This talk will present an overview of this approach as it is applied to non-trivial mesoscopic quantum circuits, focusing on the challenges and opportunities created when the thermal "system" and "reservoir" become hard to disentangle. We will focus on two examples of entropy measurements in a quantum dot, when that dot is strongly coupled either to a second quantum dot or to a reservoir of conduction electrons in the lead. Then, we will give an outlook on the promise of this approach for addressing questions in mesoscopic physics. |
Wednesday, March 8, 2023 4:48PM - 5:24PM |
Q24.00004: Controlling the Entropy of a Single-Molecule Junction Invited Speaker: Jan Mol The statistical interpretation of entropy first given by Boltzmann connects entropy as a thermodynamic state function to the number of microstates available to the system. Yet, this connection is generally not quantifiable for macroscopic systems, containing Avogadro’s numbers (~1023)of molecules, which have too many configurations to be tractable. For mesopic and nanoscopic few-electron systems with tractable electron and spin configurations this connection is genrally not quantifiable either, as standard experimental ways of measuring entropy through heat capacity cannot be applied as they require the measurement of immeasurable small amounts of heat. To overcome this conundrum and reveal the relation between entropy and correlations in few-electron quantum systems, we have developed a self-consistent thermodynamic framework which connects macroscopic observable quantities, including electrical current and charge, to microscopic configuration entropy in the form of spin-degeneracy. We apply this thermodynamic framework and use thermoelectric conductance spectroscopy to directly measure the entropy of a single nitronyl nitroxide free radical single-molecule quantum device. Our entropy measurements indicate that the neutral molecule holds an unpaired electron and is therefore in a doublet ground state, while the ground state of the reduced molecule is a singlet with all electrons paired. By applying a magnetic field, a low-lying triplet excited state is revealed which was not observed in conventional charge transport measurements. The ability to directly measure entropy of individual molecules without the need of a priori knowledge of its electronic structure has the potential to provide insights into their nontrivial quantum states, such as high-spin ground states, Kondo states, Majorana modes, and nonabelian anyons, and shed light on the discriminating role of entropy in driving electron transfer rates. |
Wednesday, March 8, 2023 5:24PM - 6:00PM |
Q24.00005: Identifying dissipative phase transitions from entropy measurements Invited Speaker: Eran Sela Dissipative phase transitions (DPTs) occur when a small quantum system interacts with a bath of harmonic oscillators. They are expected to be accompanied by an entropy change, signaling the decoherence of the system. I will review recent experimental progress showing that charge sensors of quantum dot systems allow measurement of entropy through Maxwell relations, as well as theoretical predictions on exotic states that can be detected using this method. Typically, a weak coupling to the charge sensor is assumed. Here I will argue theoretically that more generally these experiments realize DPTs due to the decoherence of the system by the charge sensor. This may allow for a first observation of the equilibrium Kosterlitz-Thouless transition in the spin-boson model in a mesoscopic system. |
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