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Speakers: Theo Nieuwenhuizen (Amsterdam), Gerwin van Dalum (Utrecht), Jette van der Broeke (Utrecht), Tom Lubensky  (University of Pennsylvania & Leiden). Friday 4 May at 14:00-18:00, Leiden University, Instituut Lorentz, Oort 276 [Casimir room].

Event details of Quantum and Topological Matter meeting
Date 4 May 2018
Time 14:00 -18:00
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This event is part of a regular series of meetings on Quantum and Topological Matter, sponsored by Delta ITP. The objective is to bring together the theoretical physics communities in Amsterdam, Leiden and Utrecht. We encourage researchers from different areas in theoretical physics to participate!

Friday 4 May at 14:00-18:00
Location: Leiden University, Instituut Lorentz, Oort 276 [Casimir room]

Programmme

14:00 Tea & Coffee

14:30 Theo Nieuwenhuizen (Amsterdam)

Dynamics of quantum measurement and the measurement problem 

15:20 Gerwin van Dalum (Utrecht)

Thermal transport in a charge two-channel Kondo setup 

16:00 Coffee break

16:30 Jette van der Broeke (Utrecht)

Thermodynamic study of topological Kondo insulators

17:10 Tom Lubensky  (University of Pennsylvania & Leiden)

Topological Mechanics of Critically Coordinated Lattices

Abstracts

Theo Nieuwenhuizen

Dynamics of quantum measurement and the measurement problem 

Abstract: Quantum mechanics allows ideal measurements to be treated by postulates, which has led to various postulate-based interpretations of varying merit. In a laboratory a measurement is performed with a physical apparatus. Much can be learned from solving the Curie-Weiss model for quantum measurement, where the z-component of a spin 1/2 is measured with a Curie-Weiss magnet.
 
In this exactly solvable model three dynamical mechanisms have been identified:
1) truncation of the density matrix (disappearance of off-diagonal "Schrödinger cat" terms)
2) registration, where the macroscopic pointer benefits from a first order phase transition
3) subensemble relaxation inside the magnet after decoupling it from the spin
 
The which-basis question and the ready-state of the apparatus find obvious meanings. While 1) and 2) are concordant with the postulates, 3) is related to the quantum measurement problem: how can one describe individual events in the ensemble  approach? While this is trivial classically (a coin lies face up or down), it is believed to be unsolvable within quantum mechanics. Our approach allows to formulate minimal postulates to connect to the reality in laboratories.  Most "quantum probabilities" should be left without interpretation. The Born rule is connected to indications of the macroscopic magnet, while the state of the microscopic spin is inferred from it. The frequency interpretation emerges.

Gerwin van Dalum

Thermal transport in a charge two-channel Kondo setup 

Abstract: The original Kondo model describes the behaviour of a non-magnetic metal containing a single magnetic impurity. Motivated by the experimental realization of a remarkable quantum dot device [Iftikhar et al., Nature 526, 233 (2015)], we consider the more complicated charge two-channel Kondo (2CK) model. In particular, we discuss the thermal transport properties of this 2CK model due to a temperature gradient between the leads. We predict a violation of the Wiedemann-Franz law in the Fermi liquid regime, and propose that the heat conductance may provide a way to experimentally verify the Majorana character of the dot region at the critical non-Fermi liquid point.

Jette van der Broeke

Thermodynamic study of topological Kondo insulators
 
A combined theoretical and experimental thermodynamic study of the Kondo insulator SmB6 is pursued to elucidate the well-known anomalous low-temperature electronic-like specific heat contribution conjectured to arise from metallic surface states. A general thermodynamic description of topological Kondo insulators is developed using a mean-field slave-boson approximation and Hill thermodynamics to study the phase transitions with the critical exponents of the model, showing consistency with the Josephson hyper-scaling relation. Applying to SmB6, the separation of bulk and boundary contributions to the heat capacity reveals that, while the surface states contribute to an increase in the heat capacity upon lowering temperature, the effect is unmeasurably small. A systematic experimental study of the dependence of SmB6 heat capacity on surface to volume ratio confirms this to be true, and suggests an alternate explanation is required for the anomalous low-temperature contributions to specific heat in this material.

Tom Lubensky

Topological Mechanics of Critically Coordinated Lattices 

Abstract: Frames consisting of nodes connected pairwise by rigid rods or central-force springs, possibly with preferred relative angles controlled by bending forces, are useful models for systems as diverse as architectural structures, crystalline and amorphous solids, sphere packings and granular matter, networks of semi-flexible polymers, proteins, and origami. Particularly interesting today is the increasing number of 3D-printed micron-scale metamaterials. This talk will present an overview of the elastic and vibrational properties of versions of these frames, called Maxwell lattices, whose constraints match the translational degrees of freedom of their nodes. They include the square, kagome, and pyrocholore and lattices and their modifications with nearest-neighbor central-force springs as well as jammed packings of soft spheres. Like the Su-Schrieffer-Heeger model of Polyacetylene, topological insulators, and Weyl semi-metals, these lattices have a topological characterization that in their case determines the number and  nature of their zero-energy edge modes, the nature of their long-wavelength elasticity, and whether or not they have isolated topologically protected Weyl-like zero modes in the bulk. If time permits, the talk will present a mechanical model whose vibrational spectrum reproduces the electronic spectrum of graphene with different hopping for each of the three bond directions.

Location

Leiden University, Instituut Lorentz, Oort 276 [Casimir room] Niels Bohrweg 2, Leiden (>>GoogleMaps)

Organisers

Vladimir Gritsev, Amsterdam

Vadim Cheianov, Leiden

Lars Fritz, Utrecht