Past Events

Event Dates Location
TOPNES Advisory Board and EQM Workshop 7-10 June 2016 St Andrews
ICSCE8 Conference 25-29 April 2016 Edinburgh
TOPNES Advisory Board Meeting 20 August 2015 St Andrews
TOPNES Winter Workshop 12-13 January 2015 Abingdon
Materials for Quantum Technologies Workshop 27-29 August 2014 St Andrews
TOPNES Advisory Board and Mid-term Review 2-3 July 2014 St Andrews
TOPNES Spring Workshop 20-21 March 2014 St Andrews
Higgs Centre Workshop - Topological quantum matter 2nd and 3rd September 2013 Edinburgh, Merchant's Hall
PDRA meeting July 16th 2013 St Andrews
Advisory Board & Scientific Meeting  27th May, 2013 St Andrews
Visiting Academic - Dr Michael Nicklas 18 - 21 November 2012 St Andrews
Visiting PhD Student - Alexander Steppke 18 September 2012 St Andrews
Visiting Academic - Professor Andrey Chubukov 19 - 31 August 2012 St Andrews
Visiting Academic - Professor Natalia Perkins 19 - 31 August 2012 St Andrews
Visiting Academic - Dr Jiannis Pachos 30 - 31 August 2012 St Andrews
Advisory Board & Scientific Meeting  9 - 11 July 2012 St Andrews
TOPNES Condensed Matter Physics Seminar 15 May 2012 St Andrews
Royal Holloway - UCL AWGPWave 16 - 17 March 2012 RHUL
TOPNES Management Board Meeting. Focus Strontium Ruthenate 8 - 9 December 2011 St Andrews
TOPNES Inaugural Scientific Meeting 22- 23 November 2011 St Andrews

## TOPNES Advisory Board and Engineering Quantum Matter Workshop

#### Venue: St Andrews

Following the annual Advisory Board Meeting on 7th June, the second major workshop for the TOPNES programme was held on 8-10 June. The Engineering Quantum Matter (EQM) workshop brought attracted over 100 delegates and brought together leaders in the field to focus on the challenge of controlling quantum states in order to design and exploit them for future technologies and establish a consensus of what the future milestones should be. There was a busy programme for the two and a half day event, including a public lecture by Prof J C Seamus Davis on 'Visualising Quantum Matter', a poster session and a Taste of Scotland dinner in one of the historic University buildings, as well as a wide range of invited and contributed talks.

Funding from the Engineering and Physical Sciences Research Council, the Institute for Complex Adaptive Matter and the International Max Planck Partnership enabled world-leading scientists to be invited to speak at the workshop, and bursaries to be offered to a number of junior scientists from the US and Europe to assist with travel costs.

The workshop was organised by several members of the TOPNES team - Dr Peter Wahl, Dr Jonathan Keeling, Dr Phil King, Prof Steve Simon, Dr Jean-Philippe-Reid and Sarah Webster. Video clips of invited talks are available here.

## ICSCE8 Conference

#### Venue: Edinburgh

TOPNES supported the eighth International Conference on Spontaneous Coherence in Excitonic Systems (ICSCE8), held in Edinburgh from 25th-29th April 2016.

TOPNES Co-Investigator Jonathan Keeling was a member of the organising committee and the conference aimed to bring together the community of researchers studying quantum collective phenomena in various kinds of electronic excitations in solids, and related phenomena in other physical systems.

#### Venue: St Andrews

The annual meeting of the TOPNES Advisory Board was held in St Andrews on 20th August. Several Co-Investigators, PDRA Dr Ana Maldonado and PhD student Thomas Scaffidi presented their latest research and the Board members were given a tour of the new Ultra Low Vibration Lab.

A poster session gave the Board members the opportunity to meet the TOPNES PDRAs and some of the PhD students working on TOPNES projects. Principal Investigator Dr Peter Wahl presented a report on how the recommendations of the Advisory Board and external reviewers given last year have been addressed and the PDRAs and PhD students met separately to discuss ideas for future collaborations.

## TOPNES Winter Workshop

#### Venue: Abingdon

TOPNES staff and a number of PhD students from St Andrews, Edinburgh, Oxford and University College London (UCL) attended a two day workshop in Abingdon in mid-January. Several staff and students presented their latest research and external talks were given by Frank Kruger from UCL, Peter Leek from Oxford and Elena Hassinger from the Max Planck Institute in Dresden. The emphasis was on discussion, with the theory and experimental groups meeting separately and together to discuss ideas and identify potential collaborations.

## Materials for Quantum Technologies Workshop

#### Venue: St Andrews

The Materials for Quantum Technologies (MQT) workshop brought together around 60 delegates to discuss the potential for exploiting condensed matter systems for future quantum technologies. There was a busy programme for the two and a half day event, including tutorials to introduce researchers to the field, a poster session with whisky tasting and an organ recital in St Salvator’s Chapel, as well as a wide range of invited and contributed talks.

Funding from the International Max Planck Partnership and the TOPNES programme enabled world-leading scientists to be invited to present at the workshop, including Prof Jochen Mannhart from the Max Planck Institute for Solid State Research, who came straight to St Andrews after collecting his EPS CMD Europhysics Prize in Paris.

The workshop was organised by several members of the TOPNES team - Dr Peter Wahl, Dr Phil King, Prof Andy Mackenzie, Dr Jean-Philippe-Reid and Sarah Webster - and the team hopes to be able to hold a similar event next summer, pending the availability of funding.

## TOPNES Advisory Board and Mid-term Review

#### Venue: St Andrews

St Andrews hosted the annual meeting of the Advisory Board for the TOPNES programme on 2nd July. The focus of this year's meeting was final preparations for the mid-term review taking place the following day and looking at how the Board's recommendations had been addressed over the past year.

The mid-term review by a panel of external reviewers was held on 3rd July and included a presentation by Principal Investigator Dr Peter Wahl and Co-Investigators Prof Steve Simon and Dr Phil King. A poster session was used to demonstrate the wide range of research covered by the TOPNES programme and gave the reviewers the opportunity to meet the TOPNES PDRAs and PhD students working on TOPNES projects.

The reviewers' report stated that "the panel were very impressed with the high quality of the research, evidenced by the high impact publications that have been produced so far. The group has a high profile with members of exceptional standing producing excellent science." The report made a number of recommendations which will be implemented during the next phase of the programme.

## TOPNES Spring Workshop

#### Venue: St Andrews

St Andrews hosted a workshop on 20th and 21st March, bringing together the grant investigators and post doctoral research fellows to discuss their work and prepare for a ‘mid-term review’ of the programme at the beginning of July.

TOPNES is a collaborative, multi-site research programme with headquarters in St Andrews, involving almost 20 staff from St Andrews, Oxford, Edinburgh, UCL and Cornell University in the US. This workshop was an opportunity for everyone to meet face to face, rather than by teleconference or Google hangouts – and provided a platform for exciting and inspiring discussions around emergent entanglement, topological protection, novel materials and non-equilibrium systems.

It was also a chance to reflect on successes and potential adjustments of the direction the research should take for the second half of this six-year programme. The workshop also encouraged the postdoctoral research fellows to develop their own vision, as well as discussing ideas for publicising their research to a wider audience.

A highlight of the two-day event was a lecture by visiting Professor Jim Sauls from Northwestern University on ‘Topological edge and surface states of superfluid Helium-3’.

## Topological quantum matter

#### Slides from the meeting (available via an https:// link)

Abstract:

The role of topology in condensed matter physics was first recognised several decades ago in the context of the Integer Quantum Hall Effect and in aspects of superfluid helium research, but recent work on topological insulators has given the field a huge boost. As is appropriate for a Higgs Centre meeting, there are overlaps with concepts from particle physics. Additionally there is a dynamic interplay between theory and experiment.

The aim of the meeting is to improve understanding of topological states in condensed matter. The meeting would act as a primer for those from other disciplines interested in the subject, provide a forum for establishing future collaboration between local theorists and leading international theorists and provide guidance for local experimental research by reviewing the state of the art in current experimental work and identifying the most pertinent experimental challenges.

Key speakers are:

• Steve Barnett (Strathclyde)
• Piers Coleman (Rutgers)
• Steve Girvin (Yale)
• David Goldhaber-Gordon (Stanford)
• Duncan Haldane (Princeton)
• Aharon Kapitulnik (Stanford)
• Stefan Kuhr (Strathclyde)
• Charlie Marcus (Copenhagen)
• Joel Moore (Berkeley)
• Xiao-Gang Wen (Perimeter Institute)

## PDRA Meeting

### Dr Michael Nicklas

Dr Michael Nicklas, Max Planck Institute, Dresden will be visiting St Andrews and will be delivering a talk on Tuesday 20 November at the Condensed Matter Physics Discussion Group entitled "'Hydrostatic and chemical pressure tuning of the iron-pnictides material CeFeAs1 xPxO".

Abstract:

The discovery of superconductivity in LaFeAsO1−xFx and the highest Tc’s up to 55 K observed in F-doped SmFeAsO have sparked tremendous interest among the scientific community. In most of the iron-pnictide materials, the application of hydrostatic pressure or chemical substitution (i) introduces superconductivity by suppressing the Fe spin-density wave (SDW) ordering in the non-superconducting parent compound and (ii) induces systematic changes in Tc in pnictide superconductors. Here, we present a combined pressure and substitution study on the iron-pnictide materials EuFe2As2 and CeFeAsO. Our investigation highlights the delicate interplay between iron and rare-earth magnetism, their sensitivity to structural properties, and, last but not least, their subtle connection to superconductivity.

## Visiting PhD Student

### Alexander Steppke

Alexander Steppke, Max Planck Institute, Dresden will be visiting St Andrews and will be delivering a talk on Tuesday 18 August at the Condensed Matter Physics Discussion Group entitled "Ferromagnetic Quantum Critical Point in YbNi4(P1−xAsx)2".

Abstract: Unconvential superconductivity and other novel phases of matter are known to exist in the vicinity of a quantum critical point (QCP): a continuous phase change of matter at zero temperature. QCPs have been identified in antiferromagnetic materials with magnetic instabilities at T = 0, whereas in ferromagnets there is still no clear evidence for a QCP. Intensive theoretical and experimental investigations have shown that metallic ferromagnets are inherently unstable and tend towards either a discontinuous quantum phase transition, superconducting or inhomogeneous magnetic phases, and thus do not exhibit ferromagnetic QCPs. Here, we show that the Gru ¨neisen ratio of the quasi one-dimensional ferromagnet YbNi4(P0.92As0.08)2 diverges upon cooling to T = 0. This demonstrates the existence of a ferromagnetic QCP  .

### Professor Andrey Chubukov and Professor Natalia Perkins

Professor Andrey Chubukov and Professor Natalia Perkins, University of Wisconsin, will be visiting St Andrews and will be delivering a series of seminars to the TOPNES group

### Professor Andrey Chubukov

seminars will include:

Tuesday 21 August 2012, 1:00pm, Room 307 - "Pairing mechanism and gap symmetry in Fe-based superconductors with only electron or only hole pockets ".

Abstract : The pairing in moderately doped Fe-pnictides and Fe-chalcogenides is generally understood as being due to magnetically enhanced interaction between hole and electron pockets. Recently, however, superconductivity has been observed in AFe2Se2 (A = K, Rb, Cs), which contain only electron pockets, and in KFe2As2, which contains only hole pockets. In the talk, I review different (and sometimes conflicting) scenarios for the pairing in these systems and propose my own. I argue that the pairing condensate in systems with only electron pockets necessary contains not only a conventional intra-pocket component, but also inter-pocket component, made of two fermions belonging to different electron pockets. I analyze the interplay between intra-pocket and inter-pocket pairing depending on the ellipticity of electron pockets and the strength of their hybridization and show that with increasing hybridization the system undergoes a transition from a d-wave state to an s+- state, in which the gap changes sign between hybridized pockets. This s+- state has the full gap and at the same time supports spin resonance, in agreement with the data. Near the boundary between d and s+- states the system develops s+id state which breaks time-reversal symmetry. For systems with only hole pockets, I argue for s+- state in which the gap changes sign between hole pockets. I show that this state is qualitatively different from s+- state when both hole and electron pockets are present. I further show that the transition from one s-wave state to the other involves highly unusual s+is state which again breaks time reversal symmetry.

Thursday 23 August, 3:00pm, Room 301 - "Superconductivity and magnetism in doped graphene".

Abstract : I discuss possible realization, in doped graphene monolayer, of a chiral superconductivity, which breaks time-reversal symmetry, and exotic spin-density-wave (SDW) order which preserves full Fermi surface. A unique situation arises in graphene at the critical doping where the Fermi surface is nested and the density of states is singular. In this regime, d-wave superconductivity and SDW order emerge from repulsive electron-electron interactions. Using a renormalization group method, I show that superconductivity wins at the critical doping, but SDW wins at slightly smaller or slightly larger doping. Superconductivity develops simultaneously in two degenerate d-wave pairing channels and is of chiral type, with the phase of the superconducting order parameter winding by 4 Π around the Fermi surface. Such a state breaks the time reversal symmetry and exhibit many other fascinating properties. SDW state is uniaxial, with order parameter breaking O(3) * Z4 symmetry. I show that this SDW state is a half-metal – excitations in one spin branch are gapped, but excitations in the other spin branch remain gapless and preserve full original Fermi surface. Such a state is highly desirable for nano-science as it allows for electrical control of spin currents.

Friday 24 August, 11.00am, Room 301 - "Superconductivity from repulsive interactions". (General Colloquium)

Abstract :The theory of superconductivity, for which Nobel Prize was given in 1972, named electron-phonon interaction as a glue that overcomes Coulomb repulsion and binds fermions into pairs which then condense and super-conduct. I review recent and not so recent works aiming to understand whether a nominally repulsive Coulomb interaction can by itself give rise to a superconductivity. I first discuss a generic scenario of the pairing by electron-electron interaction, put forward by Kohn and Luttinger back in 1965 in their attempt to explain superfluidity in 3He, and then move to discuss modern studies of the electronic mechanisms of superconductivity in the cuprates, Fe-pnictides, and in doped graphene.

### Professor Natalia Perkins

seminars will include:

Wednesday 22 August, 11:00am, Room 301 - "Critical Properties of the Kitaev-Heisenberg Model".

Abstract : We study the critical properties of the Kitaev-Heisenberg (KH) model on the honeycomb lattice at finite temperatures that might describe the physics of the quasi two-dimensional compounds, Na$_2$IrO$_3$ and Li$_2$IrO$_3$. The model undergoes two phase transitions as a function of temperature. At low temperature, thermal fluctuations induce magnetic long-range order by the order-by-disorder mechanism. This magnetically ordered state with a spontaneously broken $Z_6$ symmetry persists up to a certain critical temperature. We find that there is an intermediate phase between the low-temperature, ordered phase and the high-temperature, disordered phase. Finite-sized scaling analysis suggests that the intermediate phase is a critical Kosterlitz-Thouless phase with continuously variable exponents. We argue that the intermediate phase has been observed above the low-temperature, magnetically ordered phase in Na$_2$IrO$_3$, and also likely exists in Li$_2$IrO$_3$.

Tuesday 28 August, 1.00pm, Room 307 - "Magnetic order in FeTe".

Abstract: In my talk I will discuss the magnetic order in Fe-chalcogenide Fe$_{1+y}$Te -- the parent compound of high-temperature superconductor Fe$_{1+y}$Te$_{1-x}$Se$_x$. Experiments show that magnetic order in this materialcontains components with momentum $Q_1=(\pi/2, \pi/2)$ and $Q_2 =(\pi/2, -\pi/2)$ in Fe-only Brillouin zone. The actual spin order depends on the interplay between these two components. Previous works argued that the ordered state has a single-$Q$ (either $Q_1$ or $Q_2$). In such a state, spins form double stripes along one of diagonals breaking the rotational $C_4$ symmetry. We show that quantum fluctuations actually select another order -- a double $Q$ plaquette state with equal weight of $Q_1$ and $Q_2$ components, which preserves $C_4$ symmetry but breaks $Z_4$ translational symmetry. We argue that the order in Fe$_{1+y}$Te is determined by the competition between quantum fluctuations and magnetoelastic coupling.

### Dr Jiannis Pachos

Dr Jiannis Pachos, University of Leeds, will be visiting St Andrews and will be delivering a talk on Friday 31 August at 11.30am (Rm 301) entitled "Seeing topological order".

## TOPNES Advisory Board & Scientific Meeting

### 9 - 11 July 2012

The Management Group of the TOPNES Programme Grant held a three day Scientific Meeting alongside its annual Advisory Board Meeting. To celebrate its first anniversary, leading researchers in condensed matter physics were invited to speak at the event to enhance the collaborative research that is already well under way.

Professor Joe Orenstein, Professor at UC Berkley and Senior Scientist at the Lawrence Berkley National Laboratory is a world leader in non-equilibrium studies of metals and superconductors and gave the opening lecture at the meeting. Joe will soon be celebrating a special birthday and a dinner in honour of Joe was held on the first evening of the event to celebrate his outstanding research career.

Speakers
Name Lecture Title
Professor Joe Orenstein  "Time resolved optical spectroscopy of electron-doped NCCO"
Professor Louis Taillefer
Professor Dmitri Basov
Professor Andrea Cavalleri
Professor Joel Moore
Professor Nai Phuan Ong  "Transport experiments on Topological Insulators: Chasing the >Dirac Point"
Professor Nuh Gedik  "Ultrafast probing of dynamical spin-charge coupling in topological insulators"
Professor Steve Kivelson
Professor Seamus J C Davis

Celebrating Joe's outstanding research career on the Swilcan Bridge, Old Course, St Andrews.

## TOPNES Condensed Matter Physics Seminar

### May 2012

The TOPNES Management Group have invited Dr Ronny Thomale to give a lecture to the Condensed Matter Physics Discussion Group. Dr Thomale will deliver a lecture entitled 'A Kohn Luttinger perspective on topological superconductivity'.

### Abstract:

We argue that electron-driven pairing fluctuations, from the viewpoint of Fermi surface instabilities, generically provide a propensity towards topological superconductivity when the irreducible lattice representation associated with the Cooper pairs is multi-dimensional. For the square lattice, the px and py-type degenerate Cooper pairs yield p+ip triplet superconductivity, where we link its topological universality class to non-Abelian chiral spin liquid states recently introduced by us. In particular, in the case of water-intercalated cobaltates as well as graphene doped to van Hove filling, we illustrate this generation recipe for d+id topological singlet superconductivity on the triangular and honeycomb lattice. For the cobaltates, we can reconcile both the measured Knight shift data and excitation profile in the superconducting phase via an anisotropic d+id gap. For graphene, we predict a subtle competition between a spin density wave as well as d+id-wave and f-wave superconducting phases as a function of tight binding parameters, doping, and range of Coulomb interactions.

## Royal Holloway - University of London Advanced Working Group on P Wave

### 16 - 17 March 2012

This two day event was held at RHUL for invited participants only. Discussions were held between theorists and experimentalists regarding p-wave states of matter and the related quest for non-Abelian quasiparticles. More information regarding this event can be found at the RHUL website.

## TOPNES Management Board Meeting - Focus Strontium Ruthenate

### St Andrews

In addition to the Management Board Meeting, a series of short presentations were delivered to the TOPNES Management Group , Oxide Physics Group and CM-DTC .

### Presentations:

Name  Title
Professor Andrew Green   'Hawking radiation and Out-of-Equilibrium Current Noise in Quantum Critical Systems'
Jan Bruin  'Study of an electronic nematic with precise control of the applied magnetic field vector'
Dr Andreas Rost   'Quasiparticle Interference in LiFeAs'

The presentation given by Andreas Rost has since been published in Science 4 May 2012: Vol. 336 no. 6081 pp. 563-567. A short abstract is given below.

### Abstract:

If strong electron-electron interactions between neighboring Fe atoms mediate the Cooper pairing in iron-pnictide superconductors, then specific and distinct anisotropic superconducting energy gaps should appear on the different electronic bands. In this talk I will discuss how one can use intraband Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of the anisotropic gap structure in such materials, focusing on LiFeAs. We identify the three hole-like dispersions and we determine the anisotropy, magnitude, and relative orientations of their gap structure.

## TOPNES Inaugural Scientific Meeting

### St Andrews

Leading researchers from around the world visited St Andrews to deliver a series of lectures at this opening event for the TOPNES research group. Guests included the TOPNES collaborators as well as students funded by the Condensed Matter - Doctoral Training Centre (CM-DTC).

Professor Kirill Shtengel (University of California) delivered a lecture entitled 'Probing non-Albelian statistics with quantum Hall interferometry'. More information regarding Professor Shtengel's research can be found here.

Professor Joe Bhaseen (University of Cambridge) delivered a lecture entitled 'Holographic Approaches to Non-Equilibrium Dynamics'. More information regarding Professor Bhaseen's research can be found here.