Theoretical Quantum Matter

[Seminar] Vladimir Pudalov
2019-10-07 | NORDITA // South (Meeting Room 112.006B), Roslagstullsbacken 23, Stockholm, 106 91, Sweden
Condensed Matter Seminar Series
Speaker: Vladimir Pudalov, Lebedev Physical Institute
Title:“Thermodynamics of the correlated 2D electron system: Entropy per electron and spin magnetization per electron”

Abstract: In contrast to ubiquitous transport and optical measurements, interpretation of thermodynamic data is straightforward and is often model independent. However, due to the additive character of thermodynamic properties, their measurements for 2D electron systems is rather challenging. I shall report recent advancement in thermodynamic measurements of the correlated 2D electron system. Particularly, I focus on the derivatives of the chemical potential with respect to magnetic field and temperature. The former enables to determine spin magnetization per electron and the latter – entropy per electron.
I shall briefly describe recent results obtained with these technique and their impact on the current understanding of the still debatable problem of the ground state(s) of strongly correlated two-dimensional electron systems.

[CMT Seminar] Adrian Kantian/ Gunnar Bollmark - 1D electrons coupled to reservoirs: towards new systems of correlated quantum matter and novel devices
2019-10-02 10:30 - 12:00 (Europe/Stockholm) | NORDITA // South (Meeting Room 112.006B), Roslagstullsbacken 23, Stockholm, 106 91, Sweden
Speaker: Adrian Kantian and Gunnar Bollmark (Uppsala University)

Title: 1-D electrons coupled to reservoirs: towards new systems of correlated quantum matter and novel devices.

Time: Wednesday, 2nd of October, Fika 10:30-11:00 , Seminar 11:00-12:00,

Place: Nordita South, room 112:028 .

Abstract:
Correlated electrons are at the forefront of condensed matter theory. Interacting quasi-1D electrons have seen vast progress in analytical and numerical theory, and thus in fundamental understanding and quantitative prediction. Yet, in the 1D limit fluctuations preclude important technological use, particularly of superconductors. In contrast, high-Tc superconductors in 2D/3D are not precluded by fluctuations, but lack a fundamental theory, making prediction and engineering of their properties very difficult. In this talk we will present work in progress on combining the advantages of 1D systems with those of higher-dimensional ones, by advancing the theory of quasi-1D electrons coupled to an electron bath. We will focus on two aspects in particular: (1) How to enable a new state of matter, 1D electrons with true superconducting order, by using fluctuations from a coherent electronic reservoir. (2) A new approach for the deliberate engineering of superconducting systems with properties computable, and thus optimisable, ab-initio, i.e. from their microscopic properties. The use of hard-core bosons at this first stage allows us to rigorously validate our approach, which yields insight into the physics of dimensional crossover in strongly correlated many-body systems.

[Group Meeting] Sahar Pakdel
2019-10-01 15:00 - 17:00
Sahar Pakdel, Aarhus University (Denmark)
Title: Exploring 2D-materials in synergy with experimentalists!”.

Abstract: I will be talking about my experience studying some 2D systems with experimentalists. Specifically I will stress what I consider as the need for flexibility in the theory side while working on systems that are measured at the same time, in comparison with exploring well-presented data.

[Group Meeting] Johan Hellsvik - Impressions from the NNSP-SwedNess Neutron School 2019, Tartu
2019-09-24 15:00 - 16:00 | NORDITA // South (Meeting Room 112.006B), Roslagstullsbacken 23, Stockholm, 106 91, Sweden
Talk by Johan Hellsvik.
Title: Impressions from the NNSP-SwedNess Neutron School 2019, Tartu
Abstract: The European Spallation Source (ESS) is under development and construction in Lund. The Swedish and international neutron community are running a number of activities and projects in connection to ESS. One such activity is the SwedNess graduate school in neutron scattering. I will give impressions from the NNSP-SwedNess Neutron Summer School 2019, Tartu, and highlight some of the ESS neutron instruments that will be used for condensed matter physics.

Masters Thesis Defence - Roberto Díaz Pérez
2019-09-19 10:00 - 12:00

[CMT Seminar] Adrien Bouhon - Weyl nodes with non-Abelian charges
2019-09-17 10:30 - 12:00
Speaker: Adrien Bouhon from Nordita

Title: Weyl nodes with non-Abelian charges

Time: Tuesday
17th of September, Fika 10:30-11:00, Seminar 11:00-12:00,

Place: Nordita South, room 112:028 .

Abstract:
Weyl points trapped within a C_2T-symmetric plane (C2 = 180 degrees rotation, and T = time reversal) possess non-Abelian topological charges on top of their chirality. E.g. three-level systems realize the quaternion group. This picture requires to go beyond the modeling of a band structure as a Grassmannian (where a single spectral gap is specified). The non-Abelian nature of Weyl points implies new types of obstruction, where, for instance, two Weyl points with opposite chiralities may not annihilate. Also, the non-Abelian charges can be converted through the braiding of Weyl points in momentum space.

I will review three different yet equivalent ways of computing the non-Abelian charges: (i) as a non-cyclic phase defined from the parallel transport of the Hamiltonian along a base loop, (ii) as the Euler class of a two-band subspace over a patch bounded by the base loop, and (iii) as the winding number of the Pfaffian of the Wilsonnian Hamiltonian – of the two-band subspace – as the base loop flows over the patch.

[Remark: 50% of the material I will present is new as compared to the talk I gave at the Nordita workshop in beginning of August.]

[Group Meeting]
2019-09-17 08:00 - 17:00

[Group Meeting] Presentation by Roberto Díaz Pérez
2019-09-10 15:00 - 16:00 | NORDITA // South (Meeting Room 112.006B), Roslagstullsbacken 23, Stockholm, 106 91, Sweden
Title: Machine Learning in Matter at Different Scales

[Talk] Manuel Simonato - On the Spectral Properties of Inhomogeneous Luttinger Liquids
2019-09-03 12:00 - 13:00 | NORDITA // South (Meeting Room 112.006B), Roslagstullsbacken 23, Stockholm, 106 91, Sweden
Speaker: Manuel Simonato
Title: On the spectral properties of inhomogeneous Luttinger liquids
Abstract: The physics of one-dimensional systems of interacting fermions strongly differs from the picture offeredby Fermi liquid theory. The low-energy properties of metallic, paramagnetic 1D Fermi systems arewell described by the Luttinger liquid (LL) theory. Among other properties, the homogeneous LL isknown to exhibit power-law behaviour for various correlation functions. For spin rotational invariantmodels, the exponents can be expressed in terms of a single parameterKρ, which is known to benon-universal.The LL concept emerged in the study of homogeneous systems with periodic boundary condition(PBC). In these models thelocal spectral functionshows a power-law suppression at energies asymp-totically close to the chemical potentialμ. Theoretically, this is very well understood and the exactLL parameterKρis known for a set of integrable models. In the last decades however, the interestshifted towards the investigation of LLs with PBC including impurities. The fact that these systemshave been shown to scale to chains with open ends led to the study of several models where openboundary condition (OBC) are introduced. As the translational invariance is broken, the exponent ofthe power-law suppression close to the boundary differs from the one in the bulk.The present work is devoted to the study of the local spectral function of two lattice models of LLswith open boundaries, namely the spinless fermions model with nearest neighbour interaction and the1D Hubbard model, with the methods of perturbation theory. It is demonstrated that many aspectsof the spectral properties can be understood within the HF theory. Despite what one usually readsabout one-dimensional interacting Fermi systems,in the presence of the boundaryperturbation theoryis already capable of providing meaningful results. Concurrently, this method also allows us to developan understanding of the dip appearing in the local spectral function at an energy different from thechemical potential.

[CMT SEMINAR] Ipsita Mandal - Some aspects of correlated electrons
2019-09-03 11:00 - 12:00
Speaker: Ipsita Mandal

Affiliation: University of Stavanger,

Title: Some aspects of correlated electrons,

Time: Fika 10:30-11:00 , Seminar 11:00-12:00,
Place: Nordita South, room 112:028 .

Abstract:
In this talk, I am going to present exciting results from two of my ongoing/recent projects.
First part:
Spin-orbit coupling (SOC) assisted (spin j = 1/2) Mott insulators exhibiting bond-directional exchange interactions are expected to exhibit unconventional quantum magnetic phases like spin liquids [1, 2] predicted by the Kitaev model, dubbed as Kitaev spin liquid (KSL). The feature of a Kitaev material is that the Kitaev coupling is the dominant exchange coupling. Compounds like honeycomb iridates and α-RuCl_3 have been identified as candidate Kitaev materials. In the first part of my talk, I will address the unresolved question whether α-RuCl_3 is actually a Kitaev material. If yes, what is the value of the Kitaev coupling strength? If not, what are the dominant terms in the effective spin Hamiltonian that can describe the system? We focus on the results from resonant torsion magnetometry experiments by Modic et al, which they can measured the magnetotropic coefficient k, when a magnetic field B is applied making angle θ with the c-axis of the crystal. Using a simple Hamiltonian with a dominant paramagnetic term, we will show that we can fit the data obtained in the experiments.

Second part:
In sufficiently clean metals, it is possible for electrons to collectively flow as a viscous fluid at finite temperature. These viscous effects have been predicted to give a notable magneto resistance, but whether the magnetoresistance is positive or negative has been debated. We argue that regardless of the strength of inhomogeneity, bulk magnetoresistance is always positive in the hydrodynamic regime.

[Talk] Shota Kanasugi - Spin-orbit-coupled ferroelectric superconductivityand multiorbital effects in SrTiO3
2019-09-03 10:00 - 11:00 | NORDITA // South (Meeting Room 112.006B), Roslagstullsbacken 23, Stockholm, 106 91, Sweden
Speaker: Shota Kanasugi
Affiliation: Kyoto University
Title: Spin-orbit-coupled ferroelectric superconductivityand multiorbital effects in SrTiO3
Abstract: A lot of exotic superconducting statesemerge close to other ordered phases, such as anisotropic superconductivity in cuprates. Although previous studies have mainly focused on the superconductivityclose to magnetic instability, the ferroelectric (FE) counterpart is recently receiving increased attention since such superconductivity was reported in doped SrTiO3(STO). STO is a unique compound which exhibits both quantum paraelectricity and superconductivity. The superconducting transition temperature is enhanced in the vicinity of the FE quantum critical point [1-4]. Moreover, recent experiments suggest the coexistence ofsuperconductivity and ferroelectricity in doped STO [2]. These results imply a cooperative correlation between superconductivity and ferroelectricity against a long-held belief that they should be incompatible.In this talk, we present that aFE-like polar inversion symmetry breaking order can be controlled by the superconductivityin spin-orbit coupled systems. We introduce a model of electron-lattice coupled two-dimensional superconductor,in which the ferroelectricity-induced Rashba spin-orbit coupling [5]is included. It is shown that the FE superconductivity,which is characterized by a FE structural transition in the superconducting state,is realized through two different mechanisms [6]. First, the FE superconducting state is stabilized by the Lifshitz transition of the Rashba split bandsin lowcarrier density regimes.Second, the FE superconductingstate is stabilized under a Zeeman magnetic field, thanksto the suppressionof the Pauli depairingeffect in the FE phase. Furthermore, we show that the FE superconductivity in bulk STO is strongly affected by the multiorbital effectoft2gelectrons[7]. Then, we predict a topologicalWeyl superconducting state in the FE superconducting phase of bulk STO.
References
[1] A. Stucky et al., Sci. Rep. 6,37582 (2016).
[2] C. W. Rischau et al., Nat. Phys. 13, 643 (2017).
[3] Y. Tomioka et al., Nat. Commun. 10, 738 (2019).
[4] K. Ahadi et al., Sci. Adv. 5, eaaw0120 (2019).
[5] G. Khalsa, B. Lee, and A. H. MacDonald, Phys. Rev. B 88, 041302 (2013).
[6] S. Kanasugi and Y. Yanase, Phys. Rev. B 98, 024521 (2018).
[7] S. Kanasugi and Y. Yanase, arXiv: 1904.11113 (2019)

Odd-Frequency Meeting in Uppsala
2019-09-02 | Uppsala University, Uppsala, Sweden

Seminar - Markus Müller (PSI)
2019-08-30 08:00 - 17:00
Speaker: Markus Müller
Affiliation: Paul Scherrer Institut (PSI) in Switzerland

Title: Coherent oscillators, spectral hole-burning and emerging q-bits in rare earth magnets

Abstract: Experiments in rare earth compounds, such as LiHoYF4 and GGG (Gd-Ga-garnet), have demonstrated very long lived, coherent quantum degrees of freedom with sharply defined excitation energies, many orders of magnitude below the scale of typical magnetic interactions (exchange, dipolar or hyperfine interactions), and with life times on the order of seconds.
The nature of these coherent entities in the midst of a solid state environment has remained a mystery for a long time. I will discuss their likely origin and argue that they arise from an intricate interplay between several nuclear and electronic spins, and exist only in long lived states out of equilibrium. The sharpness of the excitations is protected by Anderson localization of composite objects, with an unusual type of power law hopping. The emerging, well isolated degrees of freedom may be promising candidates for magnetic qubits.
Time permitting I will discuss ideas of how to realise quantum memories and gates based on electro-nuclear qubits in rare earth systems.

Group Meeting
2019-02-12 15:00 - 16:15 | NORDITA // South (Meeting Room 112.006B), Roslagstullsbacken 23, Stockholm, 106 91, Sweden
Presentation by Valera Kozin, a visiting PhD student (profile here)

Title: “Multiply-connected nanostructures in the regime of strong light-matter coupling. Self-organizing maps as a method for detecting phase transitions”.
Abstract:
It is demonstrated that the dressing field drastically modifies all electronic characteristics of the quantum rings, including spin-orbit coupling, effective electron mass, and optical response.
The Chern insulator phase is shown to emerge in two-dimensional arrays of polariton rings where the time-reversal symmetry is broken due to the application of an out-of-plane magnetic field.
We employ a specific unsupervised machine learning technique—self-organizing maps—to create a low-dimensional representation of microscopic states, relevant for macroscopic phase identification and detecting phase transitions. We explore the properties of spin Hamiltonians of two archetype model systems: a two-dimensional Heisenberg ferromagnet and a three-dimensional crystal, Fe in the body-centered-cubic structure. The method of self-organizing maps, which is known to conserve connectivity of the initial dataset, is compared to the cumulant method theory and is shown to be as accurate while being computationally more efficient in determining a phase transition temperature.

Slot for APS March Meeting still available. Contact David for booking.

Group Meeting
2019-02-05 15:00 - 16:15 | NORDITA // South (Meeting Room 112.006B), Roslagstullsbacken 23, Stockholm, 106 91, Sweden
