Theoretical Quantum Matter

[Talk] Shota Kanasugi - Spin-orbit-coupled ferroelectric superconductivityand multiorbital effects in SrTiO3
2019-09-03 10:00 - 11:00 (Europe/Stockholm) | 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
