Theoretical Quantum Matter

Group Meeting
2021-02-09 15:30 - 16:30 (Europe/Stockholm), 2021-01-26 15:30 - 16:30 (Europe/Paris)
Internal Group Meeting

Seminar - Jason T. Haraldson - University of North Florida
2021-02-03 17:00 - 18:00 (Europe/Paris)
Examining multi-band effects on the electronic compressibility of a 2D electron gas: Insights into the negative compressibility of LAO/STO
In this study, we investigate the consequences of two electronic bands at the negative electronic compressibility (NEC) cross-over point for a two-dimensional electron gas (2DEG) using a simple homogeneous model with Coulombic interactions and first-order multi-band coupling. We examine the role of effective mass and relative permittivity in relation to the critical carrier density at the cross-over point through a comparison of one- and two-band models. In general, it is shown that the population of a second band, along with the presence of inter-band coupling, can dramatically change the cross-over carrier density. Given the difficulty in determining and confirming multi-band electronic systems, this model provides a potential method for identifying multi-band electronic systems using precise bulk electronic properties measurements. To help illustrate this method, we apply our results to the observed NEC in the 2D electron gas at the interface of LaAlO3/SrTiO3 (LAO/STO) and determine that, for the known parameters of LAO/STO, the system is likely a realization of a two-band 2D electron gas. Furthermore, we provide general limits on the inter-band coupling with respect to the electronic band population.

Zoom Link: https://kth-se.zoom.us/j/67773156040
Zoom ID: 627 5910 9592

Date: Wednesday, Feb 03

Time: 5pm Stockholm time

Conference on Dynamic Quantum Matter and Materials
2020-12-14 - 2020-12-16
DQMM 2020 – Virtual Conference
from Jacksonville, Florida, USA
Hosted by UNF, UCONN, and NORDITA

Dynamic Quantum Matter and Materials

December 14th – 16th, 2020

Quantum matter and materials, like topological states, 2D materials, and Dirac and Weyl materials has grown to be an active area of modern condensed matter. Fascinating properties of quantum materials might lead to technological applications such as spintronics, quantum technologies and quantum sensors. Combination of new materials discoveries and development of new probes of quantum matter has helped shape these topics into an exciting area. Recent dynamic and pumped probe experiments reveal a strong promise of Dynamic Quantum Matter as a new research direction. We strive to measure, understand and predict transient correlations and coherences in quantum materials upon different driving conditions. Therefore, we introduce it as a new topic of this year’s quantum matter conference. We seek to have an active discussion on hidden, entangled and dynamic orders that emerge in quantum materials and matter.
Main focus for this upcoming conference will be on the modeling and experimental observations of Dynamic Quantum Matter and Materials (DQMM). Overall, the goal of this workshop is to bring together researchers to discuss and highlight emerging topics and develop ideas for future research.

Peter Litttlewood - Dynamical phase transitions at many body exceptional point
2020-12-08 16:00 - 17:00 UTC
Seminar – Peter Litttlewood – Dynamical phase transitions at many body exceptional point

Zoom Link: https://kth-se.zoom.us/j/67773156040
Zoom ID: 677 7315 6040

Date: Tuesday Dec 8

Time: 4pm Stockholm time
Abstract:
Michel Fruchart, Ryo Hanai, Peter Littlewood, and Vincenzo Vitelli
Spontaneous synchronization is at the core of many natural phenomena. Your heartbeat is maintained because cells contract in a synchronous wave; some bird species synchronize their motion into flocks; quantum synchronization is responsible for laser action and superconductivity. The transition to synchrony, or between states of different patterns of synchrony, is a dynamical phase transition that has much in common with conventional phase transitions of state – for example solid to liquid, or magnetism – but the striking feature of driven dynamical systems is that the components are “active”. Consequently quantum systems with dissipation and decay are described by non-Hermitian Hamiltonians, and active matter can abandon Newton’s third law and have non-reciprocal interactions. This substantially changes the character of many-degree-of-freedom dynamical phase transitions and the critical phenomena in their vicinity, since the critical point is an “exceptional point” where eigenvalues become degenerate and eigenvectors coalesce. We will illustrate this in several different systems – a Bose-Einstein condensate of polaritons, models of multicomponent active matter such as flocks of birds, generalised Kuramoto models, and others. We argue that there is a systematic theory, generalized phase diagram, and corresponding universality classes for these dynamical systems.

Fruchart et al., arXiv:2003.13176
Hanai et al., 10.1103/PhysRevLett.122.185301

Sang Wook Cheong (Rutgers) - Trompe L'oeil Ferromagnetism
2020-11-30 15:30 - 17:00 UTC
Sang Wook Cheong (Rutgers) – Trompe L’oeil Ferromagnetism

Zoom Link: kth-se.zoom.us/j/61226950287
Zoom ID: 612 2695 0287
Date: Monday Nov 30
Time: 17.30 Stockholm time
Title: Trompe L’oeil Ferromagnetism
Abstract: The characteristics of ferro-(ferri)magnetism with non-zero magnetization include magnetic attraction, magnetic circular dichroism, and magneto-optical Kerr (MOKE), Faraday, and various anomalous Hall-type (Hall, Ettingshausen, Nernst, and thermal Hall) effects. Non-magnetic or antiferromagnetic materials in external electric fields or other environments (called specimen constituents) can share symmetry operational similarity (SOS) with magnetization in relation to broken symmetries. These specimen constituents can be associated with non-zero magnetization and/or show ferromagnetism-like behaviors, so we say that they exhibit Trompe L’oeil Ferromagnetism. Examples include linear magnetoelectric materials such as Cr2O3 under electric fields, Faraday effect in chiral materials such as tellurium with current flow, magnetic field induced by the motion of Neel- or Bloch-type ferroelectric walls, and magneto-optical Kerr (MOKE), Faraday effect, and/or anomalous Hall-type effects in certain antiferromagnets such as Cr2O3, MnPSe3, Mn4(Nb,Ta)2O9, and Mn3(Sn,Ge,Ga). A large number of new specimen constitutes having SOS with Magnetization will be discussed, and require future experimental verification of their ferromagnetism-like behaviors, and also theoretical understanding of possible microscopic mechanisms

Asia Pacific-Nordita Meeting on Quantum Matter
2020-08-17 - 2020-08-21 | NORDITA, 11 Roslagstullsbacken, Stockholm, Sweden
Recent exciting developments in topological matter, newly-discovered AdS and condensed matter connections, advances in quantum materials and machine learning applications to quantum matter open opportunities for a joint research in these directions. We will host a APCTP-Nordita one week workshop with the focus on Quantum Matter.

Dates: 17-21 August 2020
Location: Nordita, Roslagstullsbacken 11, Sweden
The aim of the meeting is to bring together researchers working on different aspects of entanglement and dynamics in quantum matter. We aim in a collaborative effort to distill the key aspects and develop common approaches to these concepts. We expect to have few speakers from each of these fields, with plenty of discussions in a small scale meeting with an open and interactive atmosphere.
While the workshop will focus on theoretical aspects we will seek connections with the experiments on new probes of matter, most prominently, MAX iV and ESS, that are capable of revealing a new and exciting behavior of quantum matter at the short time scale with high spatial resolution.
Quantum materials hold a promise as an important class to expand the class of materials for sensors used in a search for Dark matter. We will have lectures dedicated to the impact QM might have on a search for Dark Matter.
Topics
The workshop will cover a range of topics:

Entangled and competing orders
Odd-frequency and hidden orders
Transient dynamics and correlations
AdS/CFT and condensed matter connections, strange metals
Machine learning and quantum matter.

Speakers
Confirmed invited speakers:

Y. Tanaka ( U. Nagoya)
K. Schalm(U. Leiden)
J. Zaanen( U. Leiden)
T. Deveraux (U. Stanford)
Wei Ku (U. Shanghai)

Programme
Final schedule to be announced soon.
Organizers

A. Balatsky (Nordita/Uconn)
Y. K. Bang (APCTP, Postech)
M. Eschrig (Univ Greifswald)
J.Hancock (Uconn)
Sang-Jin Sin (Hanyang Univ)
A. Krikun (Nordita).

[Group Meeting]
2020-07-28 15:30 - 16:30 UTC
Details to be sent out internally.

Demitry Farfurnik (University of Maryland) - Coherence of Quantum Dots
2020-07-14 04:00 - 17:00 UTC
Speaker: Demitry Farfurnik (University of Maryland)
(https://scholar.google.com/citations?user=ebOgrf0AAAAJ&hl=en)

Location and time: Online @ 4pm Stockholm time

Zoom Link: https://kth-se.zoom.us/j/61809414846
Meeting ID: 618 0941 4846
Abstract:

Self-assembled Quantum Dots (QD) exhibit some of the best single photon emission properties, including nearly ideal efficiency and indistinguishability. As such, and considering their compatibility with nanofabrication techniques, on-chip integration of QDs as single photon emitters and non-linear components plays a key role in integrated photonic-based information processing and may pave the way toward the creation of quantum networks. Manipulating and storing quantum information utilizing QD spins, however, is limited by their short coherence times originating from interactions with a nuclear bath. In this talk, I will describe our approaches for addressing this challenge: First, the application of dynamical decoupling (DD) pulse sequences prolongs the coherence times by decoupling the QD spins from the environment. While the performance of such protocols is often limited by the accumulation of pulse imperfections, arbitrary spin control enabling composite and concatenated sequencing could further enhance the achievable spin control fidelities [1]. Such a sequencing is implementable by driving a Lambda system of the QD utilizing an optical signal arbitrarily modulated by a temperature-stabilized electro optical modulator (EOM). I will present a recent experimental demonstration of such a control [2], our analysis for optimizing the EOM working point for enhancing the achievable optical rotation Rabi frequencies, as well as the efficiency of the protocol for QDs strongly coupled to L3 photonic crystal cavities. Second, the resulting coherence properties may be further enhanced by utilizing molecules of coupled QDs (QDM), which offer a singlet-triplet ground state decoherence-free subspace [3]. Beyond the promising combination of such a subspace with the application of DD sequences, leveraging the isolated optical transitions of the QDM may offer single-shot spin readout capabilities. I will present our approach for implementing such a spin readout incorporating microwave pi-pulses, its experimental feasibility utilizing optimally designed transmission lines, and the expected readout fidelities based on the achievable microwave Rabi frequencies [4].
[1] D. Farfurnik et al., “Optimizing a dynamical decoupling protocol for solid-state electronic spin ensembles in diamond’’, Phys. Rev. B 92, 060301(R) (2015)
[2] J.H. Bodey et al., “Optical spin locking of a solid-state qubit’’, npj Quantum Information 5, 95 (2019)
[3] D. Kim et al., “Ultrafast optical control of entanglement between two quantum-dot spins’’, Nat. Phys. 7, 223–229 (2011)
[4] D. Farfurnik et al., “Experimental realization of time-dependent phase-modulated continuous dynamical decoupling’’, Phys. Rev. A 96, 013850 (2017)

Michele Ceriotti - Symmetry, locality and long-range interactions in atomistic machine learning
2020-05-12 16:00 - 17:00 (Europe/Stockholm)
Symmetry, locality and long-range interactions in atomistic machine learning
=================================================
Machine learning models are proving to be extremely effective in
predicting the properties of atomistic configurations of matter,
circumventing the need for time consuming electronic structure
calculations. The most succesful schemes achieve transferability
by means of a local representation of structures, in which the
problem of predicting a property is broken down into the prediction
of local, atom-centred contributions, and fundamental symmetries
are incorporated exactly. This approach is however not efficient
in describing long-range interatomic forces, such as those arising
due to electrostatics.
I will present a possible solution to this conundrum based on the
long-distance equivariant (LODE) framework, that combines a local
description of matter with the appropriate, long-range asymptotic
behaviour of interactions.
=================================================

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Paul McClarty(Max Planck Institute for the Physics of Complex Systems) - Topological Magnons.
2020-04-28 16:00 - 17:00 (Europe/Stockholm)
Date: Tuesday 28th April
Time: 4pm

Online via Zoom – stockholmuniversity.zoom.us/j/67021163168 (Meeting ID: 670 2116 3168)

Title: Topological Magnons

Abstract: I give an overview of the insights we and other people have had into the band topology of magnons and discuss in some detail three main topics from our work: (i) Neutron scattering intensity around magnon touching points, (ii) the non-Hermitian topology of spontaneous magnon decay and (iii) connections to experiment.

Preprint – Dirac magnons. Paper “Non-Hermitian topology of spontaneous magnon decay”.
1904.02160 https://arxiv.org/abs/1904.02160

[Group Meeting]
2020-04-28 15:30 - 16:00 (Europe/Stockholm)

[AlbaNova Colloquium] Ian Affleck - TBA
2020-04-23 17:00 - 18:00 (Europe/Stockholm)
Details TBA

Group Meeting
2020-04-14 15:30 - 17:00 (Europe/Stockholm)

Group Meeting
2020-04-07 15:30 - 17:00 (Europe/Stockholm)
Zoom details:
Topic: Group Meeting
Time: Apr 7, 2020 03:30 PM StockholmJoin Zoom Meeting
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Meeting ID: 318 117 311
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Schedule:

Admin matters
David Carvalho – An Update/Discussion about the group website
Sumantha Bandyopadhyay – Informal research talk on non-Hermitian quantum systems.

Group Meeting
2020-03-31 15:30 - 17:00 (Europe/Stockholm)
Via Zoom meeting.
Topic: Group Meeting
Time: This is a recurring meeting Meet anytime
Join Zoom Meeting
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Meeting ID: 646 429 220

HERO Retreat & Meeting '20
2020-02-12 08:00 - 2020-02-16 17:00 (Europe/Stockholm)
Location: TBC, Switzerland
Date: 12-16th February
Organizers: Alexander Balatsky,
Schedule TBA

[Group Meeting] Long Liang
2020-02-11 15:00 - 16:30 (Europe/Stockholm)
Talk by Long Liang (Nordita). Details TBH

Long Liang - Weyl semimetals in torsional spacetime
2020-02-10 16:00 - 16:30 (Europe/Stockholm)
Title: Weyl semimetals in torsional spacetime

Abstract: Weyl semimetals necessarily break the time-reversal or/and inversion symmetry. In the presence of spatially varying time-reversal or inversion symmetry breaking fields, we show that the Weyl fermions live in emergent torsional spacetime [1,2]. This allows us to fabricate interface between type I and type II Weyl semimetals [1], which is analogous to the black hole horizon. Torsion also contributes to the chiral anomaly [3], and this leads to a thermal magnetotorsional effect, which means that magnetization gradients perpendicular to the Weyl node separation give rise to temperature gradients depending only on the local positions of the Weyl nodes [2]. This effect could be observed in magnetic Weyl semimetal EuCd2As2 [4].

References:
[1] Long Liang and Teemu Ojanen, Phys. Rev. Research 1, 032006(R)(2019).
[2] Long Liang and Teemu Ojanen, arXiv: 1912.07732.
[3] J. Nissinen and G.E. Volovik, arXiv: 1909.08936; Ze-Min Huang, Bo Han, and Michael Stone, arXiv: 1911.00174.
[4] Lin-Lin Wang et al., Phys. Rev. B 99, 245147 (2019); J.-Z. Ma et al., Sci. Adv. 5 , 4718 (2019); J.-R. Soh et al., Phys. Rev. B 100, 201102(R) (2019).

Andrea Amoretti - A hydrodynamical description for transport in the strange metal phase of cuprates
2020-02-10 13:00 - 17:00 (Europe/Stockholm) | NORDITA, 11 Roslagstullsbacken, Stockholm, Sweden
Title: A hydrodynamical description for transport in the strange metal phase of cuprates
Speaker: Andrea Amoretti (Genova)

The talk is on Monday 10 February 2020, 13:15, room 132:028 (Nordita East).

Abstract: High temperature superconductors are strongly coupled systems which present a complicated phase diagram with many coexisting phases. This makes it difficult to understand the mechanism which generates their singular transport properties. Hydrodynamics, which mostly relies on the symmetries of the system without referring to any specific microscopic mechanism, constitutes a promising framework to analyze these materials. In this talk I will show that in the strange metal phase of the cuprates, a whole set of transport coefficients are described by a universal hydrodynamic framework. We corroborate our theoretical prediction by measuring the DC transport properties of Bi-2201 close to optimal doping, proving the validity of our approach. Our argument can be used as a consistency check to understand the universality class governing the behavior of high temperature cuprate superconductors

Danny Thonig - Ultrafast magnetism from ab-initio atomistic magnetisation dynamics
2020-02-04 11:00 - 12:00 (Europe/Stockholm)
Speaker: Danny Thonig (Örebro Universitet)
Title: Ultrafast magnetism from ab-initio atomistic magnetisation dynamics
Time: Fika 10:30-11:00 , Seminar 11:00-12:00,
Place: Nordita South, room 112:028 .
Abstract:
Writing and retrieving information in a magnetic material at high speed and with lower energy consumption has become an intense research field in solid state physics, since there is growing benefits for our society and within technical applications. For instance, by using all-optical control of magnetism, local magnetic moments can achieve remarkably fast switching times. The pioneering work by Beaurepaire et al. [1] demonstrated dynamics on a pico second time-scale. The experiment of Ref. [1] used a high-intensity laser pulse that pumped the sample, and after a time-delay, a second laser pulse was used to probe its magnetic state, via the magneto-optical effect. All hitherto proposed mechanisms of ultrafast magnetism (e.g. Refs. [2] and [3]) have been criticized, for not being consistent with observations and/or for not being physically relevant (reviewed e.g. in Ref.[4]). The most pressing, open questions connected to these experiments concerns the transfer of angular momentum.
To describe ultrafast magnetism from first-principles, we used atomistic magnetisation dynamics combined with density functional theory. The latter serves to calculate fundamental magnetic properties, magnetic interaction, and energy dissipation contributions.
We applied this machinery naturally to a wide range of materials, as recently also to Heusler and inverse Heusler materials. We studied demagnetisation and remagnetisation behaviour for different structural phases of Heusler’s and compared our results to experimental measurements with qualitative good agreement.
[1] E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996).
[2] B. Koopmans et al., J. Magn. and Magn. Mater. 286, 271 (2005).
[3] M. Battiato et al., Phys. Rev. Lett. 105, 027203 (2010).
[4] O. Eriksson, A.Bergman, L.Bergqvist and J.Hellsvik, Atomistic Spin-Dynamics, fundamentals and applications (Oxford university press, 2017).

[CMT] Danny Thonig ( Örebro University)
2020-02-04 10:30 - 12:00 (Europe/Stockholm)
Abstract TBA

[Talk] Hui-Ke Jin - Efficient tensor network representation for Gutzwiller projected states of paired fermions
2020-01-14 15:00 - 16:30 (Europe/Stockholm)
Title: Efficient tensor network representation for Gutzwiller projected states of paired fermions

Abstract: Recent work by Y.H. Wu {em et al}, [arXiv:1910.11011] proposed a numerical method, so called MPO-MPS method, by which several types of quantum many-body wave functions, in particular, the projected Fermi sea state, can be efficiently represented as a tensor network. In this paper, we generalize the MPO-MPS method to study Gutzwiller projected paired states of fermions, where the maximally localized Wannier orbitals for Bogoliubov quasiholes/quasiparticles have been adapted to improve the computational performance. The study of $SO(3)$ spin-1 chains reveals that this new method has better performance than variational Monte Carlo for gapped states and similar performance for gapless states. Moreover, we demonstrate that dynamic correlation functions can be easily evaluated by this method cooperating with other MPS-based accurate approaches, such as Chebyshev MPS method.

[Talk] Bastiaan J. Braams - Machine learning of equivariant functions inspired by atomistic modelling and three-dimensional image processing.
2019-12-10 14:00 - 15:00 (Europe/Stockholm) | NORDITA // West (Meeting Room 122:026)
Title: Machine learning of equivariant functions inspired by atomistic modelling and three-dimensional image processing.

Speaker: Bastiaan J. Braams, Centrum Wiskunde & Informatica (CWI), Amsterdam, The Netherlands.

Abstract:
Over the past several years big data methods, including but not limited to use of deep convolutional neural networks, have been very successful in computer science applications and there is increasing effort to apply big data or machine learning methods to problems in physical science and engineering. Conversely we are seeing that problems from physical science are influencing machine learning research done in computer science environments. A very important application of big data methods in physical science where we see this mutual influence is the construction of effective interatomic potentials and force fields for atomistic modelling of molecular and condensed phase systems (e.g. [1]). This application shares features with certain applications in three-dimensional image processing in having data associated with point clouds and in seeking to represent functions that are invariant or covariant with respect to a permutation group (applied to the labelling of points in the cloud) and with respect to spatial groups of translations, rotations and inversion. Some by now almost classical big data approaches to the atomistic problem include use of Gaussian process approximation (kernel ridge regression) [2] and use of spherical wavelet expansions [3]. In addition deep neural networks are being applied (e.g. [4], [5]) and here we see the closest link to machine learning research with key words such as Point Cloud Convolutional Networks, Deep Sets, Spherical CNNs, Tensor Field Networks and Gauge Equivariant Neural Networks [6]. The presentation will provide a survey of these machine learning developments in the context of the application in physical science.

References:
[1] Ceriotti, Michele. “Atomistic machine learning between predictions and understanding.” arXiv preprint arXiv:1902.05158 (2019).
[2] Bartók, Albert P., and Gábor Csányi. “Gaussian approximation potentials: A brief tutorial introduction.” International Journal of Quantum Chemistry 115, no. 16 (2015): 1051-1057.
[3] Eickenberg, Michael, Georgios Exarchakis, Matthew Hirn, Stéphane Mallat, and Louis Thiry. “Solid harmonic wavelet scattering for predictions of molecule properties.” The Journal of chemical physics 148, no. 24 (2018): 241732.
[4] Schütt, Kristof T., Huziel E. Sauceda, P-J. Kindermans, Alexandre Tkatchenko, and K-R. Müller. “SchNet–A deep learning architecture for molecules and materials.” The Journal of Chemical Physics 148, no. 24 (2018): 241722.
[5] Zhang, Linfeng, Jiequn Han, Han Wang, Wissam Saidi, Roberto Car, and Weinan E. “End-to-end symmetry preserving inter-atomic potential energy model for finite and extended systems.” In Advances in Neural Information Processing Systems, pp. 4441-4451. 2018.
[6] Extensive further references to be provided in the presentation.

HERO Meeting
2019-12-06 10:30 - 12:00 (Europe/Stockholm)

Kazuki Ikeda (Osaka) - Mathematics and Physics of Hofstadter Butterfly, and Quantum Computation
2019-11-22 11:00 - 17:00 (Europe/Stockholm) | NORDITA // South (Meeting Room 112.006B), Roslagstullsbacken 23, Stockholm, 106 91, Sweden
Speaker: Kazuki Ikeda from Osaka university.
Title: Mathematics and Physics of Hofstadter Butterfly,
and Quantum Computation

Abstract:
Quantum Hall effect (QHE) is one of the most important phenomena in condensed matter physics, high energy physics, and mathematical physics. Especially QHE is the crucial for the recent advances in symmetry protected topological phase and topological quantum computation. In my talk, I’ll explain the mathematics and physics of QHE by the following plan.

In part 1, I explain mathematical foundation of quantum Hall effect and Hofstadter butterfly from a viewpoint of the Langlands program, which is the “grand unification theory” of mathematics. In part 2, I address the Hofstadter problem on a lattice with defects. I show the energy spectra associated with the localised current around the defect appear in the energy gaps of fractal butterfly. In part 3, I describe application of adiabatic quantum computation to the material search.

Open Acess Publishing - Peter Mitchell (IntechOpen)
2019-11-20 11:00 - 12:00 (Europe/Stockholm)
Open Access books presentation / Q&A Session

Peter Mitchell (IntechOpen, London)
20 November

You are invited to join a short session with Peter Mitchell from IntechOpen, a start-up publisher specialising in Open Access books and progressive funding models.
The session will cover:

Disseminating your work with impact: the benefits of non-journal Open Access publishing
Funding Open Access, including a new “no-author-fee fund” for physicists, engineers, and computer scientists
Q&A: ask anything about Open Access text and reference, ideas for new works, how to submit a proposal, etc.

[Colloquium] A. Balatsky - Dynamic Quantum Matter
2019-11-14 15:00 - 16:00 (Europe/Stockholm)
Speaker: Alexander Balatsky from NORDITA.

Title: Dynamic Quantum Matter
Time and place: Thursday November 14 at 15.15 (coffee and tea from 15.00) in the Oskar Klein auditorium FR4.

Group Meeting
2019-11-12 15:00 - 16:00 (Europe/Stockholm) | NORDITA // South (Meeting Room 112.006B), Roslagstullsbacken 23, Stockholm, 106 91, Sweden
Schedule:
Presentation by Razmik.

Di Peng - Antiferromagnetic order and possible quantum spin liquid in sodium doped 2,2′-bipyridine
2019-11-07 16:45 - 17:15 (Europe/Stockholm)
Antiferromagnetic order and possible quantum spin liquid in sodium doped 2,2′-bipyridine

Di Peng
Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China

Organic molecular solids with correlated electronics based on π-electron networks provide fertile ground for the search for peculiar materials, including antiferromagnetic and quantum magnetism. We report a development method for doping alkali metals into organic materials and forming a homogeneous phase together with the experimental discoveries of two antiferromagnetic phase in sodium doped 2,2′-bipyridine at different molar ratios. An antiferromagnetic phase of 160 K is found in sodium-doped 2,2′-bipyridine with a molar ratio of less than 1:1. Another antiferromagnetic phase with Neel temperature of 25 K is observed in samples with molar ratios of 1:1 and 1:2. Applying magnetic field can slowly suppress the 160 K antiferromagnetic phase, while the 25 K antiferromagnetic phase is insensitive to the applied field rather than showing an increase of magnetization below 6 K. This behavior is consistent with the characteristics of quantum spin liquid, in which strong quantum fluctuations lead to the disorder at low temperatures. We will discuss more about the experimental details about whether a strong spin frustrated antiferromagnetic phase is formed in sodium-doped 2,2′-bipyridine and whether it enters a quantum spin liquid state at low temperatures.
