# Publications

## Real-world data of high-grade lymphoma patients treated with CD19 CAR-T in the UK

BRITISH JOURNAL OF HAEMATOLOGY 189 (2020) 30-31

A Kuhnl, C Roddie, E Tholouli, T Menne, K Linton, S Lugthart, S Chaganti, R Sanderson, M O'Reilly, J Norman, W Osborne, J Radford, C Besley, R Malladi, P Patten, M Marzolini, N Martinez-Cibrian, G Shenton, A Bloor, S Robinson, C Rowntree, D Irvine, C Burton, B Uttenthal, S Iyengar, O Stewart, W Townsend, K Cwynarski, K Ardeshna, A Ardavan, K Robinson, T Pagliuca, K Bowles, G Collins, R Johson, A McMillan

## Mott polaritons in cavity-coupled quantum materials

New Journal of Physics IOP Publishing 21 (2019) 073066

M Kiffner, J Coulthard, F Schlawin, A Ardavan, D Jaksch

We show that strong electron-electron interactions in quantum materials can give rise to electronic transitions that couple strongly to cavity fields, and collective enhancement of these interactions can result in ultrastrong effective coupling strengths. As a paradigmatic example we consider a Fermi-Hubbard model coupled to a single-mode cavity and find that resonant electron-cavity interactions result in the formation of a quasi-continuum of polariton branches. The vacuum Rabi splitting of the two outermost branches is collectively enhanced and scales with USD g_{\text{eff}}\propto\sqrt{2L} USD, where USD L USD is the number of electronic sites, and the maximal achievable value for USD g_{\text{eff}} USD is determined by the volume of the unit cell of the crystal. We find that USD g_{\text{eff}} USD for existing quantum materials can by far exceed the width of the first excited Hubbard band. This effect can be experimentally observed via measurements of the optical conductivity and does not require ultrastrong coupling on the single-electron level. Quantum correlations in the electronic ground state as well as the microscopic nature of the light-matter interaction enhance the collective light-matter interaction compared to an ensemble of independent two-level atoms interacting with a cavity mode.

## Coherent spin manipulation of individual atoms on a surface

Science American Association for the Advancement of Science 366 (2019) 509-512

K Yang, W Paul, S-H Phark, P Willke, Y Bae, T Choi, T Esat, A Ardavan, A Heinrich, C Lutz

Achieving time-domain control of quantum states with atomic-scale spatial resolution in nanostructures is a long-term goal in quantum nanoscience and spintronics. Here, we demonstrate coherent spin rotations of individual atoms on a surface at the nanosecond time scale, using an all-electric scheme in a scanning tunneling microscope (STM). By modulating the atomically confined magnetic interaction between the STM tip and surface atoms, we drive quantum Rabi oscillations between spin-up and spin-down states in as little as ~20 nanoseconds. Ramsey fringes and spin echo signals allow us to understand and improve quantum coherence. We further demonstrate coherent operations on engineered atomic dimers. The coherent control of spins arranged with atomic precision provides a solid-state platform for quantum-state engineering and simulation of many-body systems.

## Unconventional field-induced spin gap in an S=1/2 Chiral staggered chain

Physical Review Letters American Physical Society 122 (2019) 057207-

J Liu, S Kittaka, R Johnson, T Lancaster, J Singleton, T Sakakibara, Y Kohama, J Van Tol, A Ardavan, BH Williams, SJ Blundell, ZE Manson, JL Manson, PA Goddard

We investigate the low-temperature magnetic properties of the molecule-based chiral spin chain ½CuðpymÞðH2OÞ4SiF6 · H2O (pym ¼ pyrimidine). Electron-spin resonance, magnetometry and heat capacity measurements reveal the presence of staggered g tensors, a rich low-temperature excitation spectrum, a staggered susceptibility, and a spin gap that opens on the application of a magnetic field. These phenomena are reminiscent of those previously observed in nonchiral staggered chains, which are explicable within the sine-Gordon quantum-field theory. In the present case, however, although the sineGordon model accounts well for the form of the temperature dependence of the heat capacity, the size of the gap and its measured linear field dependence do not fit with the sine-Gordon theory as it stands. We propose that the differences arise due to additional terms in the Hamiltonian resulting from the chiral structure of ½CuðpymÞðH2OÞ4SiF6 · H2O, particularly a uniform Dzyaloshinskii-Moriya coupling and a fourfold periodic staggered field.

## Manipulating quantum materials with quantum light (vol 99, 085116, 2019)

Physical Review B (2019)

MARTIN Kiffner, F Schlawin, A Ardavan, DIETER Jaksch

© 2019 American Physical Society. The interaction Hamiltonian (Formula Presented) Eq. (14) describing the interaction between the cavity and the electronic system was obtained by expanding the Peierls Hamiltonian in Eq. (A4) up to first order in the small parameter (Formula Presented) All results presented in the paper are consistent with this appro imate interaction Hamiltonian, leading to an effective Hamiltonian that depends quadratically on. However, it turns out that a straightforward improvement of the parameters entering the effective Hamiltonian in Eq. (26) can be obtained by including the second-order term in the Peierls Hamiltonian in Eq. (A4). This term gives rise to modifications of our results that are also of order through a renormalization of the nearest-neighbor hopping amplitude (Formula Presented) The authors would like to thank M. A. Sentef for bringing the importance of the second-order term in Eq. (A4) to our attention.

## Manipulating quantum materials with quantum light

Physical Review B American Physical Society 99 (2019) 085116-

M Kiffner, J Coulthard, F Schlawin, A Ardavan, D Jaksch

We show that the macroscopic magnetic and electronic properties of strongly correlated electron systems can be manipulated by coupling them to a cavity mode. As a paradigmatic example we consider the Fermi-Hubbard model and find that the electron-cavity coupling enhances the magnetic interaction between the electron spins in the ground-state manifold. At half filling this effect can be observed by a change in the magnetic susceptibility. At less than half filling, the cavity introduces a next-nearest-neighbor hopping and mediates a long-range electron-electron interaction between distant sites. We study the ground-state properties with tensor network methods and find that the cavity coupling can induce a phase characterized by a momentum-space pairing effect for electrons.

## Hyperfine interaction of individual atoms on a surface

Science American Association for the Advancement of Science 362 (2018) 336-339

P Willke, Y Bae, K Yang, JL Lado, A Ferron, T Choi, A Ardavan, J Fernández-Rossier, AJ Heinrich, CP Lutz

Taking advantage of nuclear spins for electronic structure analysis, magnetic resonance imaging, and quantum devices hinges on knowledge and control of the surrounding atomic-scale environment. We measured and manipulated the hyperfine interaction of individual iron and titanium atoms placed on a magnesium oxide surface by using spin-polarized scanning tunneling microscopy in combination with single-atom electron spin resonance. Using atom manipulation to move single atoms, we found that the hyperfine interaction strongly depended on the binding configuration of the atom. We could extract atom- and position-dependent information about the electronic ground state, the state mixing with neighboring atoms, and properties of the nuclear spin. Thus, the hyperfine spectrum becomes a powerful probe of the chemical environment of individual atoms and nanostructures.

## Molecular electronic spin qubits from a spin-frustrated trinuclear copper complex

Chemical Communications Royal Society of Chemistry 54 (2018) 12934-12937

B Kintzel, M Bohme, J Liu, A Burkhardt, J Mrozek, A Buchholz, A Ardavan, W Plass

The trinuclear copper(II) complex [Cu3(saltag)(py)6]ClO4 (H5saltag = tris(2-hydroxybenzylidene)triaminoguanidine) was synthesized and characterized by experimental as well as theoretical methods. This complex exhibits a strong antiferromagnetic coupling (J = −298 cm−1) between the copper(II) ions, mediated by the N–N diazine bridges of the tritopic ligand, leading to a spin-frustrated system. This compound shows a T2 coherence time of 340 ns in frozen pyridine solution, which extends to 591 ns by changing the solvent to pyridine-d5. Hence, the presented compound is a promising candidate as a building block for molecular spintronics.

## Publisher Correction: Magnetic edge states and coherent manipulation of graphene nanoribbons.

Nature (2018)

M Slota, A Keerthi, WILLIAM Myers, E Tretyakov, M Baumgarten, ARZHANG Ardavan, H Sadeghi, CJ Lambert, A Narita, K Müllen, LAPO Bogani

In Fig. 1 of this Letter, there should have been two nitrogen (N) atoms at the 1,3-positions of all the blue chemical structures (next to the oxygen atoms), rather than one at the 2-position. The figure has been corrected online, and the original incorrect figure is shown as Supplementary Information to the accompanying Amendment.

## Author Correction: How to probe the spin contribution to momentum relaxation in topological insulators.

Nature communications (2018)

MOON-SUN Nam, B Williams, YULIN Chen, S Contera, S Yao, M Lu, YULIN Chen, GA Timco, CA Muryn, ARZHANG Ardavan

The original version of this Article contained an error in the spelling of the author Benjamin H. Williams, which was incorrectly given as Benjamin H. Willams. This has now been corrected in both the PDF and HTML versions of the Article.

## How to probe the spin contribution to momentum relaxation in topological insulators (vol 8, 2017)

NATURE COMMUNICATIONS 9 (2018) ARTN 729

M-S Nam, BH Willams, Y Chen, S Contera, S Yao, M Lu, Y-F Chen, GA Timco, CA Muryn, REP Winpenny, A Ardavan

## Author Correction: How to probe the spin contribution to momentum relaxation in topological insulators.

Nat Commun 9 (2018) 729-

M-S Nam, BH Williams, Y Chen, S Contera, S Yao, M Lu, Y-F Chen, GA Timco, CA Muryn, REP Winpenny, A Ardavan

The original version of this Article contained an error in the spelling of the author Benjamin H. Williams, which was incorrectly given as Benjamin H. Willams. This has now been corrected in both the PDF and HTML versions of the Article.

## Magnetic edge states and coherent manipulation of molecular graphene nanoribbons

Nature Springer Nature 557 (2018) 691-695

M Slota, A Keerthi, W Myers, E Tretyakov, M Baumgarten, A Ardavan, H Sadeghi, CJ Lambert, K Mullen, L Bogani

Graphene, a single-layer network of carbon atoms, shows outstanding electrical and mechanical properties, and graphene ribbons with nanometer-scale widths, should exhibit half-metallicity, quantum confinement and edge effects. Magnetic edges in graphene nanoribbons have undergone intense theoretical scrutiny, because their coherent manipulation would be a milestone for spintronic and quantum computing devices. Experimental investigations are however hampered by the fact that most nanoribbons do not have the required atomic control of the edges, and that the proposed graphene terminations are chemically unstable. Here we solve both of these problems, by using molecular graphene nanoribbons functionalized with stable spin-bearing radical groups. We observe the predicted delocalized magnetic edge states, and test present theoretical models about the spin dynamics and the spin-environment interactions. Comparison with a non graphitized reference material allows clear identification of fingerprint behaviours. We quantify the spin-orbit coupling parameters, define the interaction patterns, and unravel the spin decoherence channels. Even without any optimization, the spin coherence time is in the μs range at room temperature, and we perform quantum inversion operations between edge and radical spins. This new approach to problem of spins in well-defined electronic nanostructures offers a long awaited experimental testbed for the theory of magnetism in graphene nanoribbons. The observed coherence times open up encouraging perspectives for the use of magnetic nanoribbons in quantum spintronic devices.

## Electrically controlled nuclear polarization of individual atoms

Nature Nanotechnology Nature Publishing Group 13 (2018) 1120-1125

K Yang, P Willke, Y Bae, A Ferrón, JL Lado, A Ardavan, J Fernández-Rossier, AJ Heinrich, CP Lutz

Nuclear spins serve as sensitive probes in chemistry1 and materials science2 and are promising candidates for quantum information processing3,4,5,6. NMR, the resonant control of nuclear spins, is a powerful tool for probing local magnetic environments in condensed matter systems, which range from magnetic ordering in high-temperature superconductors7,8 and spin liquids9 to quantum magnetism in nanomagnets10,11. Increasing the sensitivity of NMR to the single-atom scale is challenging as it requires a strong polarization of nuclear spins, well in excess of the low polarizations obtained at thermal equilibrium, as well as driving and detecting them individually4,5,12. Strong nuclear spin polarization, known as hyperpolarization, can be achieved through hyperfine coupling with electron spins2. The fundamental mechanism is the conservation of angular momentum: an electron spin flips and a nuclear spin flops. The nuclear hyperpolarization enables applications such as in vivo magnetic resonance imaging using nanoparticles13, and is harnessed for spin-based quantum information processing in quantum dots14 and doped silicon15,16,17. Here we polarize the nuclear spins of individual copper atoms on a surface using a spin-polarized current in a scanning tunnelling microscope. By employing the electron–nuclear flip-flop hyperfine interaction, the spin angular momentum is transferred from tunnelling electrons to the nucleus of individual Cu atoms. The direction and magnitude of the nuclear polarization is controlled by the direction and amplitude of the current. The nuclear polarization permits the detection of the NMR of individual Cu atoms, which is used to sense the local magnetic environment of the Cu electron spin.

## Endohedral Metallofullerene as Molecular High Spin Qubit: Diverse Rabi Cycles in Gd2@C79N.

Journal of the American Chemical Society 140 (2018) 1123-1130

Z Hu, B-W Dong, Z Liu, J-J Liu, J Su, C Yu, J Xiong, D-E Shi, Y Wang, B-W Wang, A Ardavan, Z Shi, S-D Jiang, S Gao

An anisotropic high-spin qubit with long coherence time could scale the quantum system up. It has been proposed that Grover's algorithm can be implemented in such systems. Dimetallic aza[80]fullerenes M2@C79N (M = Y or Gd) possess an unpaired electron located between two metal ions, offering an opportunity to manipulate spin(s) protected in the cage for quantum information processing. Herein, we report the crystallographic determination of Gd2@C79N for the first time. This molecular magnet with a collective high-spin ground state (S = 15/2) generated by strong magnetic coupling (JGd-Rad = 350 ± 20 cm-1) has been unambiguously validated by magnetic susceptibility experiments. Gd2@C79N has quantum coherence and diverse Rabi cycles, allowing arbitrary superposition state manipulation between each adjacent level. The phase memory time reaches 5 μs at 5 K by dynamic decoupling. This molecule fulfills the requirements of Grover's searching algorithm proposed by Leuenberger and Loss.

## How to probe the spin contribution to momentum relaxation in topological insulators

Nature Communications Springer Nature 9 (2018) 56

M-S Nam, BH Williams, Y Chen, S Antoranz Contera, S Yao, M Lu, Y-F Chen, GA Timco, CA Muryn, Winpenny, A Ardavan

Topological insulators exhibit a metallic surface state in which the directions of the carriers’ momentum and spin are locked together. This characteristic property, which lies at the heart of proposed applications of topological insulators, protects carriers in the surface state from back-scattering unless the scattering centres are time-reversal symmetry breaking (i.e. magnetic). Here, we introduce a method of probing the effect of magnetic scattering by decorating the surface of topological insulators with molecules whose magnetic degrees of freedom can be engineered independently of their electrostatic structure. We show that this approach allows us to separate the effects of magnetic and non-magnetic scattering in the perturbative limit. We thereby confirm that the low-temperature conductivity of SmB6 is dominated by a surface state and that the momentum of quasiparticles in this state is particularly sensitive to magnetic scatterers, as expected in a topological insulator.

## Strong coupling of microwave photons to antiferromagnetic fluctuations in an organic magnet

Physical Review Letters American Physical Society 119 (2017) 147701-

M Mergenthaler, J Liu, J Le Roy, N Ares, A Thompson, L Bogani, F Luis, S Blundell, T Lancaster, A Ardavan, GAD Briggs, PJ Leek, E Laird

Coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium (DPPH) radicals and a superconducting microwave resonator is investigated in a circuit quantum electrodynamics (cQED) architecture. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multi-spin modes in organic crystals are suitable for cQED, offering a platform for their coherent manipulation. They also utilize the cQED architecture as a way to probe spin correlations at low temperature.

## The spin resonance clock transition of the endohedral fullerene $^{15}\mathrm{N@C}_{60}$

Physical Review Letters American Physical Society 119 (2017) 140801-

RT Harding, S Zhou, J Zhou, T Lindvall, WK Myers, A Ardavan, GAD Briggs, K Porfyrakis, EA Laird

The endohedral fullerene $^{15}\mathrm{N@C}_{60}$ has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock's projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.

## Quantum interference in graphene nanoconstrictions

Nano Letters American Chemical Society 16 (2016) 4210–4216-

P Gehring, H Sadeghi, S Sangtarash, CS Lau, J Liu, A Ardavan, JH Warner, CJ Lambert, GAD Briggs, JA Mol

We report quantum interference effects in the electrical conductance of chemical vapour deposited graphene nanoconstrictions fabricated using feedback controlled electroburning. The observed multi-mode Fabry-Pérot interferences can be attributed to reflections on potential steps inside the channel. Sharp anti-resonance features with a Fano line shape are observed. Theoretical modelling reveals that these Fano resonances are due to localised states inside the constriction, which couple to the delocalised states that also give rise to the Fabry-Pérot interference patterns. This study provides new insight into the interplay between two fundamental forms of quantum interference in graphene nanoconstrictions.

## Ordering gold nanoparticles with DNA origami nanoflowers

ACS Nano American Chemical Society 10 (2016) 7303–7306-

A Turberfield, R Schreiber, A Ardavan, I Santiago

Nanostructured materials, including plasmonic metamaterials made from gold and silver nanoparticles, provide access to new materials properties. The assembly of nanoparticles into extended arrays can be controlled through surface functionalization and the use of increasingly sophisticated linkers. We present a versatile way to control the bonding symmetry of gold nanoparticles by wrapping them in flower-shaped DNA origami structures. These ‘nanoflowers’ assemble into two-dimensonal gold nanoparticle lattices with symmetries that can be controlled through auxiliary DNA linker strands. Nanoflower lattices are true composites: interactions between the gold nanoparticles are mediated entirely by DNA, and the DNA origami will only fold into its designed form in the presence of the gold nanoparticles.