Condensed Matter Physics Seminars

Superconducting detectors represent the gold standard for high efficiency, low noise infrared single photon detection. In the 21^st century these devices underpin a host of scientific and technological applications. In this seminar I will survey current types of superconducting photon detectors and give case studies of emerging applications, including the search for dark matter, quantum communications and single photon LiDAR.

Photonic cavities can be high-finesse, confining photons for very long times, but due to the diffraction limit usually have large mode volumes. On other hand, nanoplasmonic cavities achieve extremely sub-wavelength mode volumes that allowed us to bring strong-coupling at room temperature, but are very lossy (both radiative and material losses). In this talk I will present our work on two very different nanophotonic cavities(i) a nanoplasmonic cavity formed by tightly-coupling two nanometallic structures and (ii) a high-finesse photonic crystal cavity that also achieves sub-wavelength mode volumes.
Plasmonic nanocavities have the ability to significantly confine and enhance light, while at the same time efficiently radiate energy to the far-field. Due to these properties, unprecedented light-matter interactions have been realised at room temperature, such as light-matter strong-coupling. In this talk, I will first explain how the strong-coupling regime is achieved in these systems, and explain the complexities on building a quantization scheme for such systems. I will show that multiple quantum emitters placed within plasmonic nanocavities can produce sub-radiant entangled states that are long-lived.

Fig. 1:(a) The nanoparticle-on-mirror configuration with Methylene Blue within the plasmonic nanocavity and (b) the photonic crystal waveguide cavity.

However, Rabi oscillations do not survive for long periods of time in plasmonic nanocavities, due to their high losses. A high-finesse cavity on the other hand, retains photons within the cavity for long times. I will present photonic crystal waveguide designs that are both high-finesse (Q=10⁷) and achieve sub-wavelength mode volumes (V~0.6 λ³). Our new designs operate at 780nm to couple with cold atoms, and easily reach deep within the strong coupling regime to form an ideal environment to realise quantum entanglement. Through the entropy dynamics of two emitters we demonstrate local multi-partite entanglement, which is very robust to atomic displacements. Such photonic cavity designs are easily scaled-up to construct large quantum networks with both local and global entanglement.

GaSb self-assembled quantum dots (QDs) and quantum rings (QRs) in GaAs are an unusual and interesting system with physics that is very different from type-I QDs because just one carrier, the hole, is confined [1], with the electron only bound by the Coulomb interaction, somewhat like a rather disordered atom. For example, we have previously reported redshifts and blueshifts of the emission with increasing excitation [2], Mott transitions [3] and competition between Coulomb binding and screening of ‘outer shell’ electrons [4]. GaSb QRs have very broad emission (>100 nm) with spontaneous emission times >1 ms [5], and are so strongly Type-II that quantum Monte-Carlo calculations of binding energies for exciton complexes have even approximated their electron-hole wavefunction overlap to zero [6]. They are thus a far from obvious candidate as the basis for light-emitting devices. However, their unusual physics justifies investigating this unconventional choice. Room-temperature emission is naturally at telecoms wavelengths, the 610 meV hole localisation energy [7] is advantageous for high-temperature operation and the broad emission is primarily a result of capacitive charging of the rings [8], not inhomogeneity, such that any QR is capable of emitting over a wide range of wavelengths. In this talk I will review the varied physics of GaSb QRs and motivate the thesis that their long spontaneous emission times can be taken advantage of in the development of novel cavity-enhanced devices such as vertical-cavity surface-emitting lasers (VCSELs) [9] and single-photon light-emitting diodes (SPLEDs) [10], even leading to the incorporation of a spinout, Photarix.

_{[1]Hayne et al., Appl. Phys. Lett. 82, 4355 (2003).
[2]Hayne et al., Phys. Rev. B 70, 081302(R) (2004).
[3]Bansal et al. Phys. Rev. B 77, 241304(R) (2008).
[4]Ahmad Kamarudin et al., Phys. Rev. B 83, 115311 (2011).
[5]Hodgson et al., J. Appl. Phys. 119, 044305 (2016).
[6]Thomas et al., Phys. Rev. B 99, 115306 (2019).
[7]Nowozin et al., Phys. Rev. B 86, 035305 (2012).
[8]Hodgson et al., J. Appl. Phys. 114, 073519 (2013).
[9]Jones et al., Proc. SPIE 12904, Vertical-Cavity Surface-Emitting Lasers XXVIII, (2024).
[10]Acar et al., Proc. SPIE 12906, Light-Emitting Devices, Materials, and Applications XXVIII (2024).}

Dynamic nuclear polarisation (DNP) is the transfer of the greater thermal spin polarisation of electrons to nuclear spins. This can dramatically increase the sensitivity of NMR experiments. DNP experiments today typically use stable organic radicals as the unpaired electron source, but spin-only Gd(III) ions can also give narrow EPR lines in symmetrical environments. I will show how the 17O DNP of Gd-doped CeO2 depends on the EPR properties, isotopic enrichment, and magic-angle spinning. When the nuclear hyperpolarisation is large enough, the system generates a spontaneous NMR signal, acting as a radiofrequency maser or “raser”. Finally, I will show an example of extending endogenous Gd DNP to 1H nuclei and the related challenges.
_{Refs: 10.1021/acs.jpclett.0c03457, 10.1021/acs.jpcc.1c04479, 10.1016/j.jmr.2023.107509}

The Ta dichalcogenides are well known as prototypical charge density waves materials. Their chemistry allows for many different crystal structures, known as “polytypes”, all with the nominal composition Ta(S,Se)₂ and tunable by subtle changes in the growth protocols. Here I will focus on the 4Hb polytype, which interleaves “T” and “H” -type layers, either of which can be the termination of the crystal in an ARPES measurement. With precise control of the spatial position, we resolve the different electronic structures on both types of termination. While resolving the electronic reconstructions on both, we find that the band filling at the surface differs from the bulk. The results constrain theoretical models for the currently active discussion of unusual superconducting signatures in 4Hb-TaS₂.
_{Date, …, Watson. arXiv:2508.16411v1 (2025)
Watson, …, Da Como. Communications Materials 6, 24 (2025)}

On the sub nanoscale, a material’s magnetic properties come from spin correlations of its septillions of interacting electrons.In many cases, this collective electron behaviour can be characterised in terms of atomic-scale, relatively long-lived magnetic moments. The moments determine the overall magnetisation and its resilience and response to magnetic fields, as well as affecting temperature dependent spin-polarised electronic structure. At high temperatures, the moments disorder so that there is no overall magnetisation in the paramagnetic state. As the temperature is lowered, they order into patterns describing magnetic phases whose structures and topologies depend on the nature of the local moment interactions. At T=0K, where magnetic order is complete, Density Functional Theory (DFT) calculations describe magnetic properties well and can be predictive. This talk will describe how the disordered local moment (DLM) theory [1,2] extends DFT to finite temperatures for magnetic materials and, together with experimental investigations, enables quantitative modelling of properties. It also produces input for further modelling and experimental analysis. Some aspects of earlier discussion on the electronic structure of ferromagnetic metals above their Curie temperatures will be given. The talk will then go on to discussa range of applications: magnetic order of the heavy rare earths [3], skyrmion lattices in Gd intermetallics [4], caloric effects in La(Fe_1-x Si_x)₁₃ [5] and the hard magnetic properties of the ubiquitous permanent magnet Nd₂Fe₁₄B [6].

_{[1] B L Gyorffy et al. J. Phys. F: Met. Phys. 15 1337 (1985); [2] J B Staunton et al., Phys. Rev. Lett. 93, 257204 (2004); [3] I D Hughes et al. Nature, 446, 650, (2007); [4] J Bouaziz et al. Phys. Rev. Lett. 128, 157206 (2022); [5] E Mendive Tapia et al., J. of Phys.: Energy 5, 034004 (2023); [6] J. Bouaziz et al. Phys. Rev. B 10, L020401 (2023).}

Diamond defects have demonstrated immense promise in applications such as quantum computing, biomedical probing, and diamond treatment identification. Many of these functional defects, such as the nitrogen vacancy centre, demonstrate a spectral signature in the visible region of the electromagnetic spectrum¹. However, many defects also exhibit vibrational resonances in the mid-infrared, although their energy structure and relaxation dynamics remain largely unexplored. Understanding both the energy structure of these defects and their interaction with the host crystalline environment is critical for finding new defect candidates for future diamond-enabled applications.

Ultrafast two-dimensional infrared (2DIR) spectroscopy is a powerful technique that examines the interplay between vibrating bonds in a molecular system, uniquely revealing its energy resonances, ultrafast mode-coupling, and interactions with the local environment^2,3. For materials with coexisting vibrational systems, such as in diamonds which host multiple defects, 2DIR can enable assignment of vibrational resonances to each vibrational system. The energy relaxation lifetime and pathways of these systems can also be examined using this technique, with temporal resolution down to femtosecond timescales.

In this talk, I will discuss our latest results on using 2DIR to investigate the H1a absorption band and the N_s:H-C defect in diamond. The H1a has a unique structure with two orthogonal C-N-C bonds thought to be energetically independent of each other⁴. By resolving the excitation spectrum of this defect via 2DIR, we have unravelled vibrational coupling that would otherwise be obscured in transient absorption measurements. Meanwhile, the N_s:H-C defect serves as a diamond analogue of the bond-centred hydrogen defect commonly found in semiconductors⁵. By combining 2DIR and ab-initio calculations, we demonstrate the formation of a transient hot ground state caused by the generation of localised phonons during the relaxation of the defect’s vibrational stretch mode. This highlights the role of non-equilibrium phonons in the optical manipulation of defects in diamond, despite the superior thermal conductivity of diamond.

<sub>1. M. N. R. Ashfold, J. P. Goss, B. L. Green, P. W. May, M. E. Newton, and C. V. Peaker, Chem. Rev. 120, 5745 (2020).
2. P. Hamm and M. Zanni, Concepts and Methods of 2D Infrared Spectroscopy (Cambridge University Press, 2011).
3. S. H. Shim and M. T. Zanni, Phys. Chem. Chem. Phys. 11, 748 (2009).
4. S. Liggins, M. E. Newton, J. P. Goss, P. R. Briddon, and D. Fisher, Phys. Rev. B 81, 085214 (2010).
5. S. Liggins, Identification of point defects in treated single crystal diamond, Ph.D. thesis, University of Warwick (2010).</sub>

Ultrafast transient spectroscopy is a vital tool to investigate the photophysical processes taking place in energy harvesting materials. In particular, the findings provide valuable insights into the efficiency limiting processes in solar cells. The focus of this talk will be on solution-processed metal-halide perovskite solar cells as they have received immense attention in the field of photovoltaic research due to their outstanding power conversion efficiency, which has surpassed 26% in a relatively short time. Understanding carrier losses at metal halide perovskite/charge transport layer interfaces is a pre-requisite to bring the efficiency closer to the Shockley-Queisser limit. Interfacial recombination can to some extent be accessed through time resolved techniques, however identifying this is often a challenge due to the complexity associated to the interpretation and modelling of the extracted charge carrier transients.

Our approach is to utilize complementary transient spectroscopic techniques, namely transient absorption spectroscopy and transient photoluminescence spectroscopy, not only to unravel the processes limiting the solar cell’s short circuit density and the open circuit voltage but also to evaluate different charge recombination channels and extraction. Herein, we focus on spectroscopic studies on the photoactive layer and the respective stacks containing the hole transport layers. Several different hole transport materials (PTAA, NiO_x,,and 4PACz) are adjacent to the photoactive layer. We report on challenges faced during performing complementary spectroscopic techniques and interpretation of the extracted transients. Specifically, differentiation between photophysical processes such as charge extraction and interfacial charge recombination.

In addition, charge generation in non-fullerene acceptor (NFA) based organic solar cells (OSCs) have gained prodigious attention, and with this the critical role of the energetics at the donor/acceptor interface. Ionization energy (IE), upon optical excitation, drives the Foerster energy transfer from donor to acceptor and subsequently hole transfer (from acceptor to donor), prerequisite that the low bandgap component is the emitter in the organic photoactive blend. In this regime, the correlation between the cells’ internal quantum efficiency (IQE) and the photoluminescence quenching efficiency (PLQE) with the donor/acceptor offset is critical for this charge generation channel.

Transport phenomena serve as a powerful tool for probing the quantum nature of solids, from single-particle behavior to many-body physics. In this talk, I will discuss how various transport measurements—such as electrical, thermal, and thermoelectric—are employed in our group to investigate diverse forms of quantum matter. Specifically, I will highlight our studies on anyonic quantum matter and twisted quantum systems based on two-dimensional materials.

The main research focus of the Quantum Transport Group @ IISc, Bangalore is on "Transport phenomena in emerging quantum nano-devices." The group utilizes advanced transport probes such as electrical conductance, thermal conductance, noise and thermopower to investigate new collective phases and quantum phenomena. Their studies are centered on van der Waals heterostructures of graphene, twisted bilayer graphene and other two-dimensional materials.

Recent advancement in terahertz technology has spurred an ongoing paradigm shift in the field from physics to engineering. Consequently, the field inches closer towards practical applications in sensing and communications. Soon, we can envisage deployment of terahertz systems for in-situ non-destructive evaluation, stand-off security screening, and 6G+ communications with terabit-per-second data rates. Here in this talk, I will provide an overview of latest research activities in my group, spanning broadly from devices and systems towards sensing and communications. We will delve into metasurfaces that break the limitations of natural materials in this frequency range. We will explore unorthodox antennas and our own proprietary integrated platform, specifically designed for terahertz waves. We will understand how terahertz waves can offer new sensing capabilities in security, defence, food, and agriculture. Our recent breakthroughs in communications over 300 GHz carriers will be discussed. Owing to the unique location of terahertz waves on the electromagnetic spectrum, techniques and tools from both the microwave and photonic domains have been adapted and enhanced by novel materials, fabrication, and design approaches. A forward-looking dimension will accompany all these research activities.