Publications


Probing resonating valence bond states in artificial quantum magnets

Nature Communications Springer Nature 12 (2021) 993

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

Designing and characterizing the many-body behaviors of quantum materials represents a prominent challenge for understanding strongly correlated physics and quantum information processing. We constructed artificial quantum magnets on a surface by using spin-1/2 atoms in a scanning tunneling microscope (STM). These coupled spins feature strong quantum fluctuations due to antiferromagnetic exchange interactions between neighboring atoms. To characterize the resulting collective magnetic states and their energy levels, we performed electron spin resonance on individual atoms within each quantum magnet. This gives atomic-scale access to properties of the exotic quantum many-body states, such as a finite-size realization of a resonating valence bond state. The tunable atomic-scale magnetic field from the STM tip allows us to further characterize and engineer the quantum states. These results open a new avenue to designing and exploring quantum magnets at the atomic scale for applications in spintronics and quantum simulations.


Coherent electric field manipulation of Fe3+-spins in PbTiO3

Science Advances American Association for the Advancement of Science 7 (2021) eabf8103

J Liu, V Laguta, K Inzani, W Huang, S Das, R Chatterjee, E Sheridan, S Griffin, A Ardavan, R Ramesh

Magnetoelectrics, materials which exhibit coupling between magnetic and electric degrees of freedom, not only offer a rich environment for studying the fundamental materials physics of spin-charge coupling, but also present opportunities for future information technology paradigms. We present results of electric field manipulation of spins in a ferroelectric medium using dilute Fe3+-doped PbTiO3 as a model system. Combining first-principles calculations and electron paramagnetic resonance (EPR), we show that the Fe3+ spins are preferentially aligned perpendicular to the ferroelectric polar axis, which we can manipulate using an electric field. We also demonstrate coherent control of the phase of spin superpositions by applying electric field pulses during time-resolved EPR measurements. Our results suggest a new pathway towards the manipulation of spins for quantum and classical spintronics.


Electron spin as fingerprint for charge generation and transport in doped organic semiconductors

Journal of Materials Chemistry C Royal Society of Chemistry (2021)

A Privitera, P Warren, G Londi, P Kaienburg, J Liu, A Sperlich, AE Lauritzen, O Thimm, A Ardavan, D Beljonne, M Riede

We use the electron spin as a probe to gain insight into the mechanism of molecular doping in a p-doped zinc phthalocyanine host across a broad range of temperatures (80–280 K) and doping concentrations (0–5 wt% of F6-TCNNQ). Electron paramagnetic resonance (EPR) spectroscopy discloses the presence of two main paramagnetic species distinguished by two different g-tensors, which are assigned based on density functional theory calculations to the formation of a positive polaron on the host and a radical anion on the dopant. Close inspection of the EPR spectra shows that radical anions on the dopants couple in an antiferromagnetic manner at device-relevant doping concentrations, thereby suggesting the presence of dopant clustering, and that positive polarons on the molecular host move by polaron hopping with an activation energy of 5 meV. This activation energy is substantially smaller than that inferred from electrical conductivity measurements (∼233 meV), as the latter also includes a (major) contribution from charge-transfer state dissociation. It emerges from this study that probing the electron spin can provide rich information on the nature and dynamics of charge carriers generated upon doping molecular semiconductors, which could serve as a basis for the design of the next generation of dopant and host materials.


Severe Dirac Mass Gap Suppression in Sb<sub>2</sub>Te<sub>3</sub>-Based Quantum Anomalous Hall Materials.

Nano letters 20 (2020) 8001-8007

YX Chong, X Liu, R Sharma, A Kostin, G Gu, K Fujita, JCS Davis, PO Sprau

The quantum anomalous Hall (QAH) effect appears in ferromagnetic topological insulators (FMTIs) when a Dirac mass gap opens in the spectrum of the topological surface states (SSs). Unaccountably, although the mean mass gap can exceed 28 meV (or ∼320 K), the QAH effect is frequently only detectable at temperatures below 1 K. Using atomic-resolution Landau level spectroscopic imaging, we compare the electronic structure of the archetypal FMTI Cr<sub>0.08</sub>(Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>1.92</sub>Te<sub>3</sub> to that of its nonmagnetic parent (Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>2</sub>Te<sub>3</sub>, to explore the cause. In (Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>2</sub>Te<sub>3</sub>, we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr<sub>0.08</sub>(Bi<sub>0.1</sub>Sb<sub>0.9</sub>)<sub>1.92</sub>Te<sub>3</sub> with concurrent but uncorrelated Dirac mass gap disorder. These two classes of SS electronic disorder conspire to drastically suppress the minimum mass gap to below 100 μeV for nanoscale regions separated by <1 μm. This fundamentally limits the fully quantized anomalous Hall effect in Sb<sub>2</sub>Te<sub>3</sub>-based FMTI materials to very low temperatures.


Observation of a neutron spin resonance in the bilayered superconductor CsCa<sub>2</sub>Fe<sub>4</sub>As<sub>4</sub>F<sub>2</sub>.

Journal of physics. Condensed matter : an Institute of Physics journal 32 (2020) 435603-

DT Adroja, SJ Blundell, F Lang, H Luo, Z-C Wang, G-H Cao

We report inelastic neutron scattering (INS) investigations on the bilayer Fe-based superconductor CsCa<sub>2</sub>Fe<sub>4</sub>As<sub>4</sub>F<sub>2</sub> above and below its superconducting transition temperature T <sub>c</sub> ≈ 28.9 K to investigate the presence of a neutron spin resonance. This compound crystallises in a body-centred tetragonal lattice containing asymmetric double layers of Fe<sub>2</sub>As<sub>2</sub> separated by insulating CaF<sub>2</sub> layers and is known to be highly anisotropic. Our INS study clearly reveals the presence of a neutron spin resonance that exhibits higher intensity at lower momentum transfer (Q) at 5 K compared to 54 K, at an energy of 15 meV. The energy E <sub>R</sub> of the observed spin resonance is broadly consistent with the relationship E <sub>R</sub> = 4.9k <sub>B</sub> T <sub>c</sub>, but is slightly enhanced compared to the values observed in other Fe-based superconductors. We discuss the nature of the electron pairing symmetry by comparing the value of E <sub>R</sub> with that deduced from the total superconducting gap value integrated over the Fermi surface.


Photo-molecular high temperature superconductivity

Physical Review X American Physical Society 10 (2020) 031028

M Buzzi, D Nicoletti, M Fechner, N Tancogne-Dejean, MA Sentef, A Georges, T Biesner, E Uykur, M Dressel, A Henderson, T Siegrist, JA Schlueter, K Miyagawa, K Kanoda, M-S Nam, A Ardavan, J Coulthard, J Tindall, F Schlawin, D Jaksch, A Cavalleri

The properties of organic conductors are often tuned by the application of chemical or external pressure, which change orbital overlaps and electronic bandwidths while leaving the molecular building blocks virtually unperturbed. Here, we show that, unlike any other method, light can be used to manipulate the local electronic properties at the molecular sites, giving rise to new emergent properties. Targeted molecular excitations in the charge-transfer salt κ−(BEDT−TTF)2 Cu[N(CN)2] Br induce a colossal increase in carrier mobility and the opening of a superconducting optical gap. Both features track the density of quasiparticles of the equilibrium metal and can be observed up to a characteristic coherence temperature T∗≃50K, far higher than the equilibrium transition temperature TC=12.5K. Notably, the large optical gap achieved by photoexcitation is not observed in the equilibrium superconductor, pointing to a light-induced state that is different from that obtained by cooling. First-principles calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photomolecular superconductivity.


Information and Decoherence in a Muon-Fluorine Coupled System

PHYSICAL REVIEW LETTERS 125 (2020) 87201

J Wilkinson, S Blundell


Dynamic spin fluctuations in the frustrated A-site spinel CuAl2O4

PHYSICAL REVIEW B 102 (2020) ARTN 014439

H Cho, R Nirmala, J Jeong, PJ Baker, H Takeda, N Mera, SJ Blundell, M Takigawa, DT Adroja, J-G Park


Atomic-scale electronic structure of the cuprate pair density wave state coexisting with superconductivity.

Proceedings of the National Academy of Sciences of the United States of America 117 (2020) 14805-14811

P Choubey, SH Joo, K Fujita, Z Du, SD Edkins, MH Hamidian, H Eisaki, S Uchida, AP Mackenzie, J Lee, JCS Davis, PJ Hirschfeld

The defining characteristic of hole-doped cuprates is <i>d</i>-wave high temperature superconductivity. However, intense theoretical interest is now focused on whether a pair density wave state (PDW) could coexist with cuprate superconductivity [D. F. Agterberg et al., <i>Annu. Rev. Condens. Matter Phys.</i> 11, 231 (2020)]. Here, we use a strong-coupling mean-field theory of cuprates, to model the atomic-scale electronic structure of an eight-unit-cell periodic, <i>d</i>-symmetry form factor, pair density wave (PDW) state coexisting with <i>d</i>-wave superconductivity (DSC). From this PDW + DSC model, the atomically resolved density of Bogoliubov quasiparticle states [Formula: see text] is predicted at the terminal BiO surface of Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8</sub> and compared with high-precision electronic visualization experiments using spectroscopic imaging scanning tunneling microscopy (STM). The PDW + DSC model predictions include the intraunit-cell structure and periodic modulations of [Formula: see text], the modulations of the coherence peak energy [Formula: see text] and the characteristics of Bogoliubov quasiparticle interference in scattering-wavevector space [Formula: see text] Consistency between all these predictions and the corresponding experiments indicates that lightly hole-doped Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8</sub> does contain a PDW + DSC state. Moreover, in the model the PDW + DSC state becomes unstable to a pure DSC state at a critical hole density <i>p</i>*, with empirically equivalent phenomena occurring in the experiments. All these results are consistent with a picture in which the cuprate translational symmetry-breaking state is a PDW, the observed charge modulations are its consequence, the antinodal pseudogap is that of the PDW state, and the cuprate critical point at <i>p</i>* ≈ 19% occurs due to disappearance of this PDW.


Phase-sensitive determination of nodal d-wave order parameter in single-band and multiband superconductors

PHYSICAL REVIEW B 101 (2020) ARTN 214505

J Boeker, MA Sulangi, A Akbari, JCS Davis, PJ Hirschfeld, IM Eremin


Quantum coherent spin-electric control in molecular nanomagnets

arXiv (2020)

A Ardavan, J Liu, J Mrozek, Y Duan, A Ullah, J Baldovi, E Coronado, A Gaita-Arino

Electrical control of spins at the nanoscale offers significant architectural advantages in spintronics, because electric fields can be confined over shorter length scales than magnetic fields. Thus, recent demonstrations of electric-field (E-field) sensitivities in molecular spin materials are tantalising, raising the viability of the quantum analogues of macroscopic magneto-electric devices. However, the E-field sensitivities reported so far are rather weak, prompting the question of how to design molecules with stronger spin-electric couplings. Here we show that one path is to identify an energy scale in the spin spectrum that is associated with a structural degree of freedom with a significant electrical polarisability. We study an example of a molecular nanomagnet in which a small structural distortion establishes clock transitions (i.e. transitions whose energy is to first order independent of magnetic field) in the spin spectrum; the fact that this distortion is associated with an electric dipole on the molecule allows us to control the clock transition energy to an unprecedented degree. We demonstrate coherent electrical control of the quantum spin state and exploit it to manipulate independently the two magnetically-identical but inversion-related molecules in the unit cell of the crystal. Our findings pave the way for the use of molecular spins in quantum technologies and spintronics.


Competing pairing interactions responsible for the large upper critical field in a stoichiometric iron-based superconductor CaKFe4As4

Physical Review B American Physical Society 101 (2020) 134502

M Bristow, W Knafo, P Reiss, W Meier, PC Canfield, SJ Blundell, A Coldea

<p>The upper critical field of multiband superconductors is an important quantity that can reveal details about&nbsp;the nature of the superconducting pairing. Here we experimentally map out the complete upper-critical-field&nbsp;phase diagram of a stoichiometric superconductor, CaKFe4As4, up to 90 T for different orientations of the&nbsp;magnetic field and at temperatures down to 4.2K. The upper critical fields are extremely large, reaching&nbsp;values close to &sim;3 Tc at the lowest temperature, and the anisotropy decreases dramatically with temperature,&nbsp;leading to essentially isotropic superconductivity at 4.2K. We find that the temperature dependence of the&nbsp;upper critical field can be well described by a two-band model in the clean limit with band-coupling parameters&nbsp;favoring intraband over interband interactions. The large Pauli paramagnetic effects together with the presence&nbsp;of the shallow bands is consistent with the stabilization of an FFLO state at low temperatures in this clean&nbsp;superconductor.</p>


Imaging the energy gap modulations of the cuprate pair-density-wave state.

Nature 580 (2020) 65-70

Z Du, H Li, SH Joo, EP Donoway, J Lee, JCS Davis, G Gu, PD Johnson, K Fujita

The defining characteristic<sup>1,2</sup> of Cooper pairs with finite centre-of-mass momentum is a spatially modulating superconducting energy gap Δ(r), where r is a position. Recently, this concept has been generalized to the pair-density-wave (PDW) state predicted to exist in copper oxides (cuprates)<sup>3,4</sup>. Although the signature of a cuprate PDW has been detected in Cooper-pair tunnelling<sup>5</sup>, the distinctive signature in single-electron tunnelling of a periodic Δ(r) modulation has not been observed. Here, using a spectroscopic technique based on scanning tunnelling microscopy, we find strong Δ(r) modulations in the canonical cuprate Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+δ</sub> that have eight-unit-cell periodicity or wavevectors Q ≈ (2π/a<sub>0</sub>)(1/8, 0) and Q ≈ (2π/a<sub>0</sub>)(0, 1/8) (where a<sub>0</sub> is the distance between neighbouring Cu atoms). Simultaneous imaging of the local density of states N(r, E) (where E is the energy) reveals electronic modulations with wavevectors Q and 2Q, as anticipated when the PDW coexists with superconductivity. Finally, by visualizing the topological defects in these N(r, E) density waves at 2Q, we find them to be concentrated in areas where the PDW spatial phase changes by π, as predicted by the theory of half-vortices in a PDW state<sup>6,7</sup>. Overall, this is a compelling demonstration, from multiple single-electron signatures, of a PDW state coexisting with superconductivity in Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+δ</sub>.


Enhancing easy-plane anisotropy in bespoke Ni(II) quantum magnets

Polyhedron 180 (2020)

JL Manson, ZE Manson, A Sargent, DY Villa, NL Etten, WJA Blackmore, SPM Curley, RC Williams, J Brambleby, PA Goddard, A Ozarowski, MN Wilson, BM Huddart, T Lancaster, RD Johnson, SJ Blundell, J Bendix, KA Wheeler, SH Lapidus, F Xiao, S Birnbaum, J Singleton

© 2020 The Authors We examine the crystal structures and magnetic properties of several S = 1 Ni(II) coordination compounds, molecules and polymers, that include the bridging ligands HF2−, AF62− (A = Ti, Zr) and pyrazine or non-bridging ligands F−, SiF62−, glycine, H2O, 1-vinylimidazole, 4-methylpyrazole and 3-hydroxypyridine. Pseudo-octahedral NiN4F2, NiN4O2 or NiN4OF cores consist of equatorial Ni-N bonds that are equal to or slightly longer than the axial Ni-Lax bonds. By design, the zero-field splitting (D) is large in these systems and, in the presence of substantial exchange interactions (J), can be difficult to discriminate from magnetometry measurements on powder samples. Thus, we relied on pulsed-field magnetization in those cases and employed electron-spin resonance (ESR) to confirm D when J ≪ D. The anisotropy of each compound was found to be easy-plane (D > 0) and range from ≈ 8–25 K. This work reveals a linear correlation between the ratio d(Ni-Lax)/d(Ni-Neq) and D although the ligand spectrochemical properties may play an important role. We assert that this relationship allows us to predict the type of magnetocrystalline anisotropy in tailored Ni(II) quantum magnets.


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


Spontaneous rotation of ferrimagnetism driven by antiferromagnetic spin canting

Physical Review Letters American Physical Society 124 (2020) 127201

A Vibhakar, DD Khalyavin, P Manuel, J Liu, AA Belik, R Johnson

Spin-reorientation phase transitions that involve the rotation of a crystal's magnetization have been well characterized in distorted-perovskite oxides such as orthoferrites. In these systems spin reorientation occurs due to competing rare-earth and transition metal anisotropies coupled via f-d exchange. Here, we demonstrate an alternative paradigm for spin reorientation in distorted perovskites. We show that the R_{2}CuMnMn_{4}O_{12} (R=Y or Dy) triple A-site columnar-ordered quadruple perovskites have three ordered magnetic phases and up to two spin-reorientation phase transitions. Unlike the spin-reorientation phenomena in other distorted perovskites, these transitions are independent of rare-earth magnetism, but are instead driven by an instability towards antiferromagnetic spin canting likely originating in frustrated Heisenberg exchange interactions, and the competition between Dzyaloshinskii-Moriya and single-ion anisotropies.


Phase transitions for beginners

CONTEMPORARY PHYSICS (2020)

SJ Blundell


Group theory for physicists, 2nd edition

CONTEMPORARY PHYSICS (2020)

SJ Blundell


Momentum-resolved superconducting energy gaps of Sr2RuO4 from quasiparticle interference imaging.

Proceedings of the National Academy of Sciences of the United States of America 117 (2020) 5222-5227

R Sharma, SD Edkins, Z Wang, A Kostin, C Sow, Y Maeno, AP Mackenzie, JCS Davis, V Madhavan

Sr2RuO4 has long been the focus of intense research interest because of conjectures that it is a correlated topological superconductor. It is the momentum space (k-space) structure of the superconducting energy gap [Formula: see text] on each band i that encodes its unknown superconducting order parameter. However, because the energy scales are so low, it has never been possible to directly measure the [Formula: see text] of Sr2RuO4 Here, we implement Bogoliubov quasiparticle interference (BQPI) imaging, a technique capable of high-precision measurement of multiband [Formula: see text] At T = 90 mK, we visualize a set of Bogoliubov scattering interference wavevectors [Formula: see text] consistent with eight gap nodes/minima that are all closely aligned to the [Formula: see text] crystal lattice directions on both the α and β bands. Taking these observations in combination with other very recent advances in directional thermal conductivity [E. Hassinger et al., Phys. Rev. X 7, 011032 (2017)], temperature-dependent Knight shift [A. Pustogow et al., Nature 574, 72-75 (2019)], time-reversal symmetry conservation [S. Kashiwaya et al., Phys. Rev B, 100, 094530 (2019)], and theory [A. T. Rømer et al., Phys. Rev. Lett. 123, 247001 (2019); H. S. Roising, T. Scaffidi, F. Flicker, G. F. Lange, S. H. Simon, Phys. Rev. Res. 1, 033108 (2019); and O. Gingras, R. Nourafkan, A. S. Tremblay, M. Côté, Phys. Rev. Lett. 123, 217005 (2019)], the BQPI signature of Sr2RuO4 appears most consistent with [Formula: see text] having [Formula: see text] [Formula: see text] symmetry.


The Physics of Pair-Density Waves: Cuprate Superconductors and Beyond

ANNUAL REVIEW OF CONDENSED MATTER PHYSICS, VOL 11, 2020 11 (2020) 231-270

DF Agterberg, JCS Davis, SD Edkins, E Fradkin, DJ Van Harlingen, SA Kivelson, PA Lee, L Radzihovsky, JM Tranquada, Y Wang

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