Math
Phys seminar

**Fyrirlesari: ****Jakob Björnberg****,
****Chalmers University of Technology**

**Titill: **Random permutations and Heisenberg models.

#### Ágrip:

We discuss probabilistic representations of
certain quantum spin systems, including the ferromagnetic Heisenberg model, in
terms of random permutations. Properties of the cycle structure of the random
permutations are connected with phase transitions in the spin-system. In particular, it is expected that the cycle
structure converges to a distribution known as Poisson–Dirichlet, in the limit
of large systems. This problem is open
but we present some partial progress.

Math Phys seminar

### Fyrirlesari: Daniel Fernández Moreno, Háskóli Íslands

### Titill: The philosophy of emergent spacetime

Staðsetning: HB5 (Háskólabíó)

Tími: Föstudag 18.oktúber kl. 11:40

Ágrip:

One of the most startling observations in recent theoretical physics is that certain phenomena are better described as resulting from a higher dimensional spacetime. The gauge-gravity correspondence projects them into a surface infinitely far away. The existence of such a duality between two fully consistent physical theories reduces the number of spacetime dimensions to a mere choice, one that can be more or less useful depending on the physics we want to describe.

This observation brought forth the idea that Spacetime should be understood as an emergent property from quantum field theory. This is usually presented in abstract grounds, disconnected from its consequences on our theoretical perspective of fundamental physics. Consequences which challenge the basic intuitions from classical physics that are otherwise vastly useful in most situations. For this reason, as opposed to most seminars in the topic, this talk will ignore the structure of the reasoning and the mathematical rigor. Instead, I will present to you the topic of emergent Spacetime focused on gaining an intuitive feeling about the connection of such a seemingly abstract concept with the real world.

Math colloquium

#### Fyrirlesari: Danny Brattan, University of Genoa

#### Titill: Hydrodynamical charge density wave description for transport in the strange metal phase of cuprates

Staðsetning: Naustið-Endurmenntun

Tími: Miðvikudag 9.oktúber kl. 11:00

**Ágrip:**

The mechanism controlling the exotic behavior of the transport properties in the strange metallic phase of high temperature superconductors is one of the main unresolved problems in condensed matter physics. I will discuss our recent paper (1909.07991) where we develop a framework for describing the hydrodynamics of charge density wave (CDW) order in a magnetic field (extending earlier theoretical developments) and where we determine the DC transport coefficients within this formalism. In this work we performed a complete characterization of the DC transport coefficients (including less common ones like transverse thermal conductivity and Nernst effect) of a single crystal of Bi-2201 close to optimal doping and we found complete self-consistent agreement of this data with the CDW model. This suggests CDW order may be sufficient to explain the unusual properties of the strange metal phase of the cuprates.

Math phys colloquium

**Fyrirlesari: **Emil Have, University of Edinburgh

**Titill: **Newton-Cartan Submanifolds and Biophysical (Fluid) Membranes

Staðsetning: L-201 Lögberg

Tími: Þriðjudag 8.oktúber kl. 11:00

#### Ágrip:

Originally developed to provide a geometric foundation for
Newtonian gravity, Newton-Cartan geometry and its torsionful generalization
have recently experienced a revival of interest, particularly in the contexts
of non-AdS holography and various condensed matter problems — notably the quantum
Hall effect. In this talk, I will describe a general theory of Newton-Cartan
submanifolds. A covariant description of non-relativistic fluids on surfaces is
an important open problem with a wide range of applications in for example
biophysics. Recasting ‘elastic’ models, such as the Canham-Helfrich bending
energy, in a Newton-Cartan setting allows for a covariant notion of
non-relativistic time and provides the ideal starting point for a treatment of
Galilean fluids on extremal submanifolds using the technology of hydrostatic
partition functions.

Math Phys seminar

**Fyrirlesari: Jesús Zavala Franco, Háskóli
Íslands og Javier Israel Reynoso Córdoba, Universidad de Guanajuato**

**Titill: **The Boltzmann equation for a rarefied fluid in linear perturbation
theory

Staðsetning: HB5 (Háskólabíó)

Tími: Föstudag 4.oktúber kl. 11:40

#### Ágrip:

Linear perturbation theory is the basis upon which we understand
the initial growth of density perturbations in the early Universe. This theory
has been developed and studied extensively in two extreme regimes: i) the fluid
regime and ii) the collisionless regime. The former is applicable to the
photon-baryon plasma, while the latter is commonly used to describe dark
matter. There is however, a relevant class of dark matter models, known
generically as self-interacting dark matter, where the Knudsen number is around
1, which lies in between these regimes. Linear perturbation theory in this
regime remains essentially unexplored, requiring a full treatment of the
Collisional Boltzmann equation. In this talk, we will present an overview of
the problem and present our preliminary progress towards describing this regime.

Math Phys seminar

**Fyrirlesari: Hólmfríður Sigríðar Hannesdóttir , Harvard University**

**Titill: I**nfrared Finite S-matrix elements and Cross Sections

Staðsetning: A-050

Tími: Þriðjudag 27.ágúst kl. 14:00

#### Ágrip:

Quantum field theory (QFT) works remarkably well for making theoretical predictions in collider scattering experiments. One of the fundamental objects in these calculations, the scattering matrix (S-matrix), is inspired by a well defined operator in non-relativistic quantum mechanics, but is plagued with both ultraviolet (UV) and infrared (IR) divergences in QFT. The UV divergences are now understood through the program of renormalization, but IR divergences remain an active area of research. Three approaches have been explored to define IR finite quantities, which will all be discussed in this talk: i) The cross section method, ii) modification of the S-matrix, and iii) the dressed state formalism. The minimal set of processes required for a finite cross section will be examined, along with the need for forward scattering and disconnected diagrams. We will furthermore explore how the usual assumptions about evolution of the scattering states at asymptotic times are broken in a theory with massless particles, and how the universality of the interactions leading to IR divergences in such theories can be exploited to define finite S-matrix elements.

PhD midway evaluation

**Fyrirlesari: Juan Fernando Angel Ramelli, HÍ**

**Titill: **Entanglement entropy of the quantum Lifshitz model

Staðsetning: A-050

Tími: Þriðjudag 27.ágúst kl. 09:00

#### Ágrip:

The quantum Lifshitz model (QLM) is a toy model used for studying scale invariant systems in which time and space scale on different footing. This type of behavior occurs, for example, in the context of critical phenomena. Entanglement entropy is a purely quantum measure of correlation between subsystems. It is a useful theoretical probe, as it encodes certain universal properties of critical models. I review the QLM as well as its generalization to higher dimensions, and show how to construct its excited states on compact manifolds. Through careful application of the replica method, I demonstrate that analytic results can be obtained for the entanglement entropy of both its ground state and its excited states.

Math Phys seminar

**Fyrirlesari: Mohab Abou Zeid, Imperial College London**

**Titill: **T-duality in (2,1) superspace and SKT geometry

Staðsetning: VR-II, V-156

Tími: Föstudag 23.ágúst kl. 14:00

#### Ágrip:

I will explain our recent derivation of the T-duality transformation rules for two-dimensional (2,1) supersymmetric sigma-models in (2,1) superspace. To this end I will first review the gauging of sigma-models in (2,1) superspace and present a new manifestly real and geometric expression for the gauged action. I will also discuss the obstructions to gauging (2,1) sigma-models and show that the obstructions to (2,1) T-duality are considerably weaker than the obstructions to gauging. Our complexified T-duality transformations are equivalent to the usual Buscher duality transformations (including an important refinement) together with diffeomorphisms. Time permitting, I will mention some possible applications of our results.

Math Phys seminar

**Fyrirlesari: Ro Jefferson, Albert Einstein Institute Potsdam**

**Titill: **Black hole interiors and modular inclusions

Staðsetning: VR-II, V-156

Tími: Fimmtudag 22.ágúst kl. 14:00

#### Ágrip:

We show how the traversable wormhole induced by a double-trace deformation of the thermofield double state can be understood as a modular inclusion of the algebras of exterior operators. The effect of this deformation is the creation of a new region of spacetime deep in the bulk, corresponding to a non-trivial center between the left and right algebras. This set-up provides a precise framework for investigating how black hole interiors are encoded in the CFT. In particular, we use modular theory to demonstrate that state dependence is an inevitable feature of any attempt to represent operators behind the horizon. Building on this geometrical structure, we propose that modular inclusions may provide a more precise means of investigating the nascent relationship between entanglement and geometry in the context of the emergent spacetime paradigm.

Math Phys seminar

**Fyrirlesari: Jorge Laraña Aragon,University of Stockholm**

**Titill: Quenched coupling, effective thermalization and evanescence in the free $O(N)$ vector model**

Staðsetning: VR-II, V-156

Tími: Fimmtudag 15.ágúst kl. 14:00

#### Ágrip:

In this project we study two coupled free $O(N)$ vector models implementing the coupling via a quench of mixing. Since the theory is integrable pure thermalization does not occur but the quantum modes thermalize independently. Generalized thermal behavior is obtained at late times in the limit of weak initial mixing. We investigate this from both the effective density matrix and the spectral density function. When computing the latter, we observe that late-time correlation functions of composite operators in the post-quench free QFT surprisingly share interesting properties with correlators in strongly coupled systems. We discuss a possible holographic interpretation in terms of the so called evanescent modes.