The Dodd-Walls Centre is proud to present a series of seminars hosted by our themes on different topics and everyone is welcome to attend. Anyone can join from anywhere using a laptop, computer, mobile phone or other Zoom capable device with Zoom ID # 932-223-670.

##### PREVIOUS SEMINARS

**16.8.19****Title: Giant vortex clusters in a quantum fluid****Presenter:** Ashton Bradley, UoO**Abstract: **Atomic Bose-Einstein condensates (BECs) provide a uniquely controllable setting in which to study quantum fluid dynamics. In a stirred superfluid, quantized vortices typically proliferate, injecting linear and angular momentum into the fluid. In 1949, while studying the point-vortex model, Onsager predicted that confinement of quantum vortices can produce a surprising result: the possibility of vortices reaching negative temperatures. Negative temperature states contain significant energy, forming a collective storm of vortices circulating in the same direction: a giant vortex cluster. Vortex cluster states are the quantum analogue of the Great Red Spot, visible on the surface of Jupiter as a manifestation of classical fluid turbulence. I will describe our work on the theory of giant vortex clusters, and joint work with the BEC group at the University of Queensland to observe them for the first time in a quantum gas controlled by a digital micromirror device. Despite expectations that such high energy states should be unstable, we observe giant quantum vortex clusters with very long lifetimes. Our work confirms Onsager’s prediction after some 70 years, and opens the door to a new regime for quantum vortex matter at negative absolute temperatures, with implications for quantum turbulence, helium films, nonlinear optical materials, and fermi superfluids.

**9.8.19**

Title: Using vibrational spectroscopy as a photonic tool to extract actionable insight from complex materials**Presenter:** Dr. Michel Nieuwoudt, UoA**Abstract:** Much information lies hidden in complex materials about their molecular composition, structure and interactions. Because Raman and Infrared spectroscopy measure molecular vibrations, they can extract this information from complex materials like food, biological tissue and bone. In addition, they can do this both non-invasively and rapidly and so provide faster alternatives to chemical reference testing or biochemical assays. Ongoing advances in photonics and optical instrumentation enable the development of increasingly sophisticated light sources and smaller, portable spectrometers with more options to analyse complex materials in situ and, in the case of living tissue, in vivo, so that Raman and infrared spectroscopies are fast emerging as tools in medical diagnostics and food safety.

There is also significant interest in imaging technologies that combine spatial and/or morphological information with the chemical information inherent in vibrational spectra. Diverse fields such as food and forensic science, medical imaging technologies, the cosmetic and pharmaceutical industries are increasingly adopting chemical imaging as invaluable tools in their research. These include hyperspectral imaging using near infrared (NIR), mid infrared (MIR), Raman and fluorescence spectroscopy. Spectroscopic techniques generate large spectral datasets and require multivariate statistical tools to analyse the large amounts of data. Chemometrics can be used to extract relevant information from this data to find patterns or trends, optimize experiment design, monitor reactions, determine composition, detection limits, or predict future behaviour. In this seminar I will describe how vibrational spectroscopy with chemometrics can be used rapidly and non-invasively to gain meaningful insight from materials as diverse as milk, skin, cortical bone and iron passive film.

**17.7.19****Title: Polar-Core Spin Vortices of Spin 1 Bose-Einstein condensates****Presenter: **Andrew Underwood, MSc, UoO**Title: A simple model of supersolidity in an elongated dipolar condensate****Presenter:** Blair Blakie, UoO

**15.7.19Title: Use and misuse of artificial neural networks in Chemometrics**

**Prof. Federico Marini, University of Rome La Sapienza**

Presenter:

Presenter:

**Artificial neural networks (ANN) are mathematical-statistical models, which owe their name to the fact that they have been originally inspired by and (very naively) modeled on biological networks of neurons. However, despite the biological analogy that has, at least in the beginning, triggered their popularity, their strength resides in their unique computational characteristics. Indeed, ANNs can, at least in principle, model any functional relationships, no matters how complex they are, and they do this through interconnected mathematical nodes (which can be thought of as basis functions), which form a network. This characteristic makes ANN particularly suited to problems that involve non-linear interpolation in the presence of multiple input variables, especially when the nature of such functional relationship is not hypothesizable a priori. However, this large flexibility and adaptability is also the reason why many people, fascinated by ANNs and using them as black-boxes, without proper validation, may be misled by apparently good results which are just the effect of overfitting. Indeed, one of the most common mistakes is not to consider that the development of a neural network with good performance requires that there be an adequate quantity of experimental data available. Indeed, one should always keep in mind that the number of adjustable parameters in the network (weights) increases dramatically with the number of hidden neurons so that one may arrive to the point where the number of the connections to be fitted is larger than the number of the data pairs available for training (i.e., to the case where a solution is still calculated but it is mathematically undetermined).**

Abstract:

Abstract:

In the present communication, the most common neural network architectures (feed-forward networks, self-organizing maps, but also convolutional networks and Boltzmann machines) will be discussed and presented with some examples of application to different real world problems. At the same times, the critical issues for their correct training and validation will also be discussed.

**28.6.19**

Title: Probing Strong Coupling Between Ions and a Microwave Cavity with Raman Heterodyn

Title: Probing Strong Coupling Between Ions and a Microwave Cavity with Raman Heterodyn

**Presenter:**Gavin King, UoO

**Abstract:**Superconducting qubits have transitions in the microwave frequency regime but microwave frequency photons get lost in thermal noise at room temperature. Converting microwave photons into optical photons removes the sensitivity to thermal noise, and allows the transfer of quantum states between distant superconducting qubit based quantum computers[1, 2, 3]. One method to coherently convert the microwave photons to optical photons is to use a three-level system in a sum-frequency-generation arrangement[4], as schematically shown in the ﬁgure. The rare earth ions are well suited to this method, having narrow optical transitions and readily accessible microwave transitions through Zeeman splitting. Erbium in particular is ideal for this, with optical transitions in the lowest-loss window of silica ﬁbre. At 50ppm in yttrium orthosilicate (YSO) conversion eﬃciencies of 10−5 have been seen at 4K[5], limited by thermal population of the upper microwave state, parasitic reabsorption from isotopic impurities, and weak coupling between the resonator and ions. Recently we measured an isotopically pure 170Er3+ in YSO at around 100mK. We saw strong coupling between the ions and the cavity, conversion using Zeeman splitting in the excited state, which neatly avoids the problem of thermal population of the microwave transition, and have measured the spin-lattice relaxation time of the ions.

** ****21.6.19Title: Accurate Numerical Calculations for Strongly Correlated Fermi Gases with the Transcorrelated ApproachPresenter: **Dr Péter Jeszenszki, Massey University

**Abstract:**In the description of strongly correlated Fermi system, the exact diagonalization approach is frequently applied in order to achieve reliability and accuracy in theoretical calculations. In this approach, the energies and the wave functions are obtained by diagonalizing the Hamiltonian in a many-body Fock basis. As the size of Hilbert space combinatorially increases with the size of the system, most of the calculations are limited to few-body systems and/or intermediate interaction strength. Therefore, understanding and improving convergence properties is crucial in order to make the approach more widely applicable.

The rate of convergence of physical observables with increasing basis size is determined, for the most part, by the nature of the particle-particle interaction itself. In ultracold atoms, the interaction potential is usually modeled by a zero-range pseudopotential, which introduces a singularity in the wave function at the particle-particle coalescence point. This singularity causes painfully slow convergence in one spatial dimension whereas in two or three dimensions it can lead to pathologic behavior.

We apply the transcorrelated approach, where the wave function is considered as a product of a Jastrow-type two-particle function and a linear combination of Fock basis states [1, 2]. The Jastrow

factor, which contains the singularity of the wave function, is folded into the Hamiltonian by a similarity transformation. Thus the singularity is removed from the Fock-space expansion to leading order.

The transformation thus smoothes out the singularity of the original zero-range pseudopotential, which significantly improves the convergence rate of the transcorrelated eigenfunction in the Fock basis states[2,3].

We will present numerical examples for the one-dimensional Fermi gas at strong interactions and for fermions in three dimensions at unitarity. In one dimension the transcorrelated approach improves the convergence of the energy error from M^{−1} to M^{−3}, where M is the number of the single-particle basis functions [3]. In three dimensions due to the pathological zero-range pseudopotential, the exact diagonalization of the original Hamiltonian is not possible. The transcorrelated transformation eliminates the pathological nature of the Hamiltonian and yields a significant improvement compared to the standard renormalization approaches.

**14.6.19**

**Title: Ultrafast spectroscopy: techniques and applications****Presenter:** Dr. Kai Chen**Abstract: **Ultrafast optical spectroscopy is a powerful tool to provide the rich information of photophysics in optoelectronic materials for the rational designs of device structures and material synthesis. In the past few years, we had developed several ultrafast spectroscopy techniques to study various material systems. In this scheme seminar, I will focus on two methods: transient absorption and ultrafast photoluminescence spectroscopy.[1] The basic principle and unique features of our experimental setups will be described. Furthermore, I will address several recent technical improvements. By combining the information from both techniques, we can gain insights into photoexcitation dynamics in ultrafast timescales. I will demonstrate the application of our spectroscopic techniques based on our recent studies of new generation materials for organic solar cells.

**24.5.19Title: Coupling of ferrimagnetic modes via photons**

**Presenter:**Dr Nicholas Lambert, UoO

**Abstract:**

Magnetostatic modes in yttrium iron garnet (YIG) have been extensively explored as classical and quantum objects, both in isolation and when coupled to photons, qubits and phonons. The field is motivated in part by the ability of highly polished YIG spheres to also support optical frequency whispering gallery modes, which are coupled to the YIG magnetization via the Faraday effect.

In this talk I will describe strong coupling between such magnetostatic modes and a transmission line cavity. The coupling to non-uniform magnetostatic modes is large because of the significantly non-uniform magnetic component of the cavity modes. We identify the magnetostatic modes by measuring the strength of their coupling to the cavity. One of our primary measurement tools is the dispersive shift of the cavity frequency due to the magnon excitations and we also exploit this effect to couple two spatially separated magnets. We demonstrate that the interaction is due to the cavity, rather than dipole-dipole interactions.

**17.5.19Title: THERMALIZATION OF SPIN-ORBIT COUPLED BOSE-EINSTEIN CONDENSATES**

**Presenter:**Dylan Brown, UoA

**Abstract:**

Illuminating a Bose-Einstein condensate (BEC) with a pair of counterpropagating laser beams can couple together internal states of the atoms creating a Spin-Orbit coupled (SOC) BEC.Rapidly changing the coupling strength imparts a spin-dependent electric force on the pseudospin states, accelerating them in opposite directions where they oscillate in the harmonic potential, colliding together.

We measure the rate of thermalization of the oscillating pseudospin states in both the uncoupled and SOC regimes. We find the presence of SOC during thermalization greatly enhances the rate at which the system approaches thermal equilibrium and results in a larger condensate fraction remaining. We calculate the temperature of the final thermalized state and compare the temperature in different SOC environments with various spin polarizations.

**12.4.19Title: Is a Doubly Quantized Vortex Dynamically Unstable in Uniform Superfluids?**

**Presenter:**Dr Hiromitsu Takeuchi from Osaka City University

**Abstract:**We revisit the fundamental problem of the splitting instability of a doubly quantized vortex in uniform single-component superfluids at zero temperature. We analyze the system-size dependence of the excitation frequency of a doubly quantized vortex through large-scale simulations of the Bogoliubov–de Gennes equation, and find that the system remains dynamically unstable even in the infinite-system-size limit. Perturbation and semi-classical theories reveal that the splitting instability radiates a damped oscillatory phonon as an opposite counterpart of a quasi-normal mode. https://arxiv.org/abs/1710.10810, https://journals.jps.jp/doi/10.7566/JPSJ.87.023601

**12.4.19Title: Luminescence Spectroscopy and Temperature Sensing of Ln3+-Doped Fluoride Nanoparticles**

**Presenter:**Sangeetha Balabhadra, UoC

**Abstract:**

Luminescence arising from trivalent lanthanide ions (Ln3+) embedded in fluoride hosts has stirred interest in telecommunication, solar cells, thermometry, therapeutic agents, optical imaging and many other applications. However the widespread implementation of these materials remains limited by the low emission intensity, bio-dispersibility, and a relatively low-resolution sensing capability. To overcome such limitations, it is required to fully understand and optimize the physics behind the energy transfer processes. For this purpose, CaF2:Ln3+ (Yb3+/Ho3+/Eu3+) nanoparticles were taken as an example and their high-resolution, site-selective spectroscopy have been systematically investigated. Moreover, the potential applicability of the bright emitting nanoparticles (SrF2:Yb3+/Er3+) were tested for nanothermometry.

**15.3.19Title: Interplay of Feshbach Resonances with a Virtual State & the Effect on Bound-State Self-Interaction**

**Presenter:**Ryan Thomas, UoO

**Abstract:**In non-relativistic quantum scattering theory there are two broad classes of scattering resonances: Feshbach resonances and shape resonances. Shape resonances arise when scatterers become temporarily trapped by a feature in the open-channel potential energy, and Feshbach resonances occur when scatterers become temporarily trapped through a transition to an energetically closed channel. In both cases, the resonance can be thought of as arising from a short-lived bound state that decays into the continuum.

In this talk, I will describe a recent experiment at the University of Otago where we explore the interplay of a Feshbach resonance and a special case of a shape resonance – known as a virtual state – in the collisions of ultracold potassium-40 and rubidium-87. We find that the trajectory of the resonant energy as a function of magnetic field is highly non-linear which is indicative of a large bound-state self-interaction energy. We quantitatively compare our measurements with theoretical predictions from a coupled-channels model, and we find excellent agreement after a small correction to the long-range parts of the interaction potentials.

Finally, we will discuss future work involving d-wave Feshbach and shape resonances in rubidium-87.

**8.2.19Title:**

**SELECTED STUDIES OF CLUSTER-BASED MATERIALS: PAST EXPERIENCES* AND RECENT DEVELOPMENTS****

**Presenter:**Vladimir Golovko - University of Canterbury

**Abstract:**Selected studies of cluster-based materials: past experiences* and recent developments**

The major part of this presentation will cover topic of chemically synthesised ligand protected metal clusters. We start with fundamental studies correlating features observed in the far-IR spectra of pure chemically-synthesised clusters obtained at the synchrotron with those predicted based on DFT models. Since support-immobilized and activated clusters are of great technological importance, we move to XPS, MIES, HR STEM and STM-AFM studies of such species, trying to understand their unique electronic properties and behaviour on surfaces. Correlation of experimental results with calculations, again, helps to advance these studies. Selected examples of applications of such materials as catalysts and sensors will be briefly presented.

Final part of the talk will briefly outline selected aspirations of the collaborative project underway within our UC team focused on lanthanide-doped upconverting nanoparticles.

In collaboration with

* D.P. Anderson1, D. Ovoshchnikov1, B. Donoeva1, J.-Y. Ruzicka1, F. Abu Bakar1, R. Adnan1, G. Metha2, J. Alvino2, T. Bennett2, R. White2, T. Kee2, G. Andersson,3 H. Al Qahtani3, G. Krishnan3, K. Kimoto,4 T. Nakayama,4 W. Wlodarski5, M.Z. Ahmad5, J. Steven6, and A. Marshall6 and ** Jamin Martin,1 Sangeetha Balabhadra,1 Mike Reid1 and Jon-Paul Wells.1

1 School of Physical and Chemical Sciences, University of Canterbury, Christchurch, New Zealand

2 Flinders Centre for NanoScale Science and Technology, Flinders University, Adelaide Australia

3 Department of Chemistry, The University of Adelaide, Adelaide SA 5005, Australia

4 National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan

5 School of Electrical & Computer Engineering, RMIT University, Melbourne, Australia

6 CAPE, University of Canterbury, Christchurch, New Zealand

e-mail: vladimir.golovko@canterbury.ac.nz