Publications


Evidence for a J(eff)=0 ground state and defect-induced spin glass behavior in the pyrochlore osmate Y2Os2O7

PHYSICAL REVIEW B 99 (2019) ARTN 174442

NR Davies, CV Topping, H Jacobsen, AJ Princep, FKK Kirschner, MC Rahn, M Bristow, JG Vale, I da Silva, PJ Baker, CJ Sahle, Y-F Guo, D-Y Yan, Y-G Shi, SJ Blundell, DF McMorrow, AT Boothroyd


Magnetic field-induced pair density wave state in the cuprate vortex halo.

Science (New York, N.Y.) 364 (2019) 976-980

SD Edkins, A Kostin, K Fujita, AP Mackenzie, H Eisaki, S Uchida, S Sachdev, MJ Lawler, E-A Kim, JC Séamus Davis, MH Hamidian

High magnetic fields suppress cuprate superconductivity to reveal an unusual density wave (DW) state coexisting with unexplained quantum oscillations. Although routinely labeled a charge density wave (CDW), this DW state could actually be an electron-pair density wave (PDW). To search for evidence of a field-induced PDW, we visualized modulations in the density of electronic states N(r) within the halo surrounding Bi2Sr2CaCu2O8 vortex cores. We detected numerous phenomena predicted for a field-induced PDW, including two sets of particle-hole symmetric N(r) modulations with wave vectors QP and 2QP , with the latter decaying twice as rapidly from the core as the former. These data imply that the primary field-induced state in underdoped superconducting cuprates is a PDW, with approximately eight CuO2 unit-cell periodicity and coexisting with its secondary CDWs.


Probing magnetic order and disorder in the one-dimensional molecular spin chains CuF2(pyz) and [Ln(hfac)3(boaDTDA)] n (Ln  =  Sm, La) using implanted muons.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019) 394002-394002

T Lancaster, BM Huddart, RC Williams, F Xiao, KJA Franke, PJ Baker, FL Pratt, SJ Blundell, JA Schlueter, MB Mills, AC Maahs, KE Preuss

We present the results of muon-spin relaxation ([Formula: see text]SR) measurements on antiferromagnetic and ferromagnetic spin chains. In antiferromagnetic CuF2(pyz) we identify a transition to long range magnetic order taking place at [Formula: see text] K, allowing us to estimate a ratio with the intrachain exchange of [Formula: see text] and the ratio of interchain to intrachain exchange coupling as [Formula: see text]. The ferromagnetic chain [Sm(hfac)3(boaDTDA)] n undergoes an ordering transition at [Formula: see text] K, seen via a broad freezing of dynamic fluctuations on the muon (microsecond) timescale and implying [Formula: see text]. The ordered radical moment continues to fluctuate on this timescale down to 0.3 K, while the Sm moments remain disordered. In contrast, the radical spins in [La(hfac)3(boaDTDA)] n remain magnetically disordered down to T  =  0.1 K suggesting [Formula: see text].


Mott polaritons in cavity-coupled quantum materials

NEW JOURNAL OF PHYSICS 21 (2019) ARTN 073066

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


Magnetic order and enhanced exchange in the quasi-one-dimensional molecule-based antiferromagnet Cu(NO3)2(pyz)3.

Physical chemistry chemical physics : PCCP 21 (2019) 1014-1018

BM Huddart, J Brambleby, T Lancaster, PA Goddard, F Xiao, SJ Blundell, FL Pratt, J Singleton, P Macchi, R Scatena, AM Barton, JL Manson

The quasi-one-dimensional molecule-based Heisenberg antiferromagnet Cu(NO3)2(pyz)3 has an intrachain coupling J = 13.7(1) K () and exhibits a state of long-range magnetic order below TN = 0.105(1) K. The ratio of interchain to intrachain coupling is estimated to be |J'/J| = 3.3 × 10-3, demonstrating a high degree of isolation for the Cu chains.


Local magnetism, magnetic order and spin freezing in the 'nonmetallic metal' FeCrAs.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019) 285803-285803

BM Huddart, MT Birch, FL Pratt, SJ Blundell, DG Porter, SJ Clark, W Wu, SR Julian, PD Hatton, T Lancaster

We present the results of x-ray scattering and muon-spin relaxation ([Formula: see text]SR) measurements on the iron-pnictide compound FeCrAs. Polarized non-resonant magnetic x-ray scattering results reveal the 120° periodicity expected from the suggested three-fold symmetric, non-collinear antiferromagnetic structure. [Formula: see text]SR measurements indicate a magnetically ordered phase throughout the bulk of the material below [Formula: see text] K. There are signs of fluctuating magnetism in a narrow range of temperatures above [Formula: see text] involving low-energy excitations, while at temperatures well below [Formula: see text] behaviour characteristic of freezing of dynamics is observed, likely reflecting the effect of disorder in our polycrystalline sample. Using density functional theory we propose a distinct muon stopping site in this compound and assess the degree of distortion induced by the implanted muon.


Machine learning in electronic-quantum-matter imaging experiments.

Nature 570 (2019) 484-490

Y Zhang, A Mesaros, K Fujita, SD Edkins, MH Hamidian, K Ch'ng, H Eisaki, S Uchida, JCS Davis, E Khatami, E-A Kim

For centuries, the scientific discovery process has been based on systematic human observation and analysis of natural phenomena1. Today, however, automated instrumentation and large-scale data acquisition are generating datasets of such large volume and complexity as to defy conventional scientific methodology. Radically different scientific approaches are needed, and machine learning (ML) shows great promise for research fields such as materials science2-5. Given the success of ML in the analysis of synthetic data representing electronic quantum matter (EQM)6-16, the next challenge is to apply this approach to experimental data-for example, to the arrays of complex electronic-structure images17 obtained from atomic-scale visualization of EQM. Here we report the development and training of a suite of artificial neural networks (ANNs) designed to recognize different types of order hidden in such EQM image arrays. These ANNs are used to analyse an archive of experimentally derived EQM image arrays from carrier-doped copper oxide Mott insulators. In these noisy and complex data, the ANNs discover the existence of a lattice-commensurate, four-unit-cell periodic, translational-symmetry-breaking EQM state. Further, the ANNs determine that this state is unidirectional, revealing a coincident nematic EQM state. Strong-coupling theories of electronic liquid crystals18,19 are consistent with these observations.


Coherent spin manipulation of individual atoms on a surface.

Science (New York, N.Y.) 366 (2019) 509-512

K Yang, W Paul, S-H Phark, P Willke, Y Bae, T Choi, T Esat, A Ardavan, AJ Heinrich, CP 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.


Evidence for a vestigial nematic state in the cuprate pseudogap phase.

Proceedings of the National Academy of Sciences of the United States of America 116 (2019) 13249-13254

S Mukhopadhyay, R Sharma, CK Kim, SD Edkins, MH Hamidian, H Eisaki, S-I Uchida, E-A Kim, MJ Lawler, AP Mackenzie, JCS Davis, K Fujita

The CuO2 antiferromagnetic insulator is transformed by hole-doping into an exotic quantum fluid usually referred to as the pseudogap (PG) phase. Its defining characteristic is a strong suppression of the electronic density-of-states D(E) for energies |E| < [Formula: see text], where [Formula: see text] is the PG energy. Unanticipated broken-symmetry phases have been detected by a wide variety of techniques in the PG regime, most significantly a finite-Q density-wave (DW) state and a Q = 0 nematic (NE) state. Sublattice-phase-resolved imaging of electronic structure allows the doping and energy dependence of these distinct broken-symmetry states to be visualized simultaneously. Using this approach, we show that even though their reported ordering temperatures T DW and T NE are unrelated to each other, both the DW and NE states always exhibit their maximum spectral intensity at the same energy, and using independent measurements that this is the PG energy [Formula: see text] Moreover, no new energy-gap opening coincides with the appearance of the DW state (which should theoretically open an energy gap on the Fermi surface), while the observed PG opening coincides with the appearance of the NE state (which should theoretically be incapable of opening a Fermi-surface gap). We demonstrate how this perplexing phenomenology of thermal transitions and energy-gap opening at the breaking of two highly distinct symmetries may be understood as the natural consequence of a vestigial nematic state within the pseudogap phase of Bi2Sr2CaCu2O8.


Spin dynamics and field-induced magnetic phase transition in the honeycomb Kitaev magnet alpha-Li2IrO3

PHYSICAL REVIEW B 99 (2019) ARTN 054426

S Choi, S Manni, J Singleton, CV Topping, T Lancaster, SJ Blundell, DT Adroja, V Zapf, P Gegenwart, R Coldea


A.C. susceptibility as a probe of low-frequency magnetic dynamics.

Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019) 013001-

CV Topping, SJ Blundell

The experimental technique of a.c. susceptibility can be used as a probe of magnetic dynamics in a wide variety of systems. Its use is restricted to the low-frequency regime and thus is sensitive to relatively slow processes. Rather than measuring the dynamics of single spins, a.c. susceptibility can be used to probe the dynamics of collective objects, such as domain walls in ferromagnets or vortex matter in superconductors. In some frustrated systems, such as spin glasses, the complex interactions lead to substantial spectral weight of fluctuations in the low-frequency regime, and thus a.c. susceptibility can play a unique role. We review the theory underlying the technique and magnetic dynamics more generally and give applications of a.c. susceptibility to a wide variety of experimental situations.


Unconventional Field-Induced Spin Gap in an S=1/2 Chiral Staggered Chain.

Physical review letters 122 (2019) 057207-

J Liu, S Kittaka, RD 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 [Cu(pym)(H_{2}O)_{4}]SiF_{6}·H_{2}O (pym=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 sine-Gordon 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 [Cu(pym)(H_{2}O)_{4}]SiF_{6}·H_{2}O, 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 99 (2019) ARTN 099907

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


Phase transitions, broken symmetry and the renormalization group

in The Routledge Handbook of Emergence, (2019) 237-247

SJ Blundell

© 2019 selection and editorial matter, Sophie Gibb, Robin Findlay Hendry, and Tom Lancaster. All rights reserved. The renormalization group should probably be called the renormalization semigroup, but sometimes, contradictory terminology sticks. The renormalization group procedure provides important insights because it shows quantitatively how fine-scale structure is progressively ignored and the physics of critical phenomena depend on these larger-scale, what students might call "structural", features of the theory. The renormalization group breaks big problems down into small ones. Broken symmetry can be seen as a well-studied paradigm of emergent behaviour in the physical world. By breaking symmetry, these phases forfeit the status of being a "stationary state" of the sort beloved of elementary quantum mechanics treatments. Phase transitions are sharp and there is a clear delineation between the ordered and disordered states. The set of symmetry-breaking phase transitions includes as members those between the ferromagnetic and paramagnetic states and those between the superconducting and normal metal states of certain materials.


Magnetic monopole noise.

Nature 571 (2019) 234-239

R Dusad, FKK Kirschner, JC Hoke, BR Roberts, A Eyal, F Flicker, GM Luke, SJ Blundell, JCS Davis

Magnetic monopoles1-3 are hypothetical elementary particles with quantized magnetic charge. In principle, a magnetic monopole can be detected by the quantized jump in magnetic flux that it generates upon passage through a superconducting quantum interference device (SQUID)4. Following the theoretical prediction that emergent magnetic monopoles should exist in several lanthanide pyrochlore magnetic insulators5,6, including Dy2Ti2O7, the SQUID technique has been proposed for their direct detection6. However, this approach has been hindered by the high number density and the generation-recombination fluctuations expected of such thermally generated monopoles. Recently, theoretical advances have enabled the prediction of the spectral density of magnetic-flux noise from monopole generation-recombination fluctuations in these materials7,8. Here we report the development of a SQUID-based flux noise spectrometer and measurements of the frequency and temperature dependence of magnetic-flux noise generated by Dy2Ti2O7 crystals. We detect almost all of the features of magnetic-flux noise predicted for magnetic monopole plasmas7,8, including the existence of intense magnetization noise and its characteristic frequency and temperature dependence. Moreover, comparisons of simulated and measured correlation functions of the magnetic-flux noise indicate that the motions of magnetic charges are strongly correlated. Intriguingly, because the generation-recombination time constant for Dy2Ti2O7 is in the millisecond range, magnetic monopole flux noise amplified by SQUID is audible to humans.


Manipulating quantum materials with quantum light

PHYSICAL REVIEW B 99 (2019) ARTN 085116

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


Doped Sr2FeIrO6-Phase Separation and a Jeff ≠ 0 State for Ir5.

Inorganic chemistry 57 (2018) 10303-10311

JE Page, CV Topping, A Scrimshire, PA Bingham, SJ Blundell, MA Hayward

High-resolution synchrotron X-ray and neutron powder diffraction data demonstrate that, in contrast to recent reports, Sr2FeIrO6 adopts an I1̅ symmetry double perovskite structure with an a-b-c- tilting distortion. This distorted structure does not tolerate cation substitution, with low levels of A-site (Ca, Ba, La) or Fe-site (Ga) substitution leading to separation into two phases: a stoichiometric I1̅ phase and a cation-substituted, P21/ n symmetry, a-a-c+ distorted double perovskite phase. Magnetization, neutron diffraction, and 57Fe Mössbauer data show that, in common with Sr2FeIrO6, the cation substituted Sr2- xA xFe1- yGa yIrO6 phases undergo transitions to type-II antiferromagnetically ordered states at TN ∼ 120 K. However, in contrast to stoichiometric Sr2FeIrO6, cation substituted samples exhibit a further magnetic transition at TA ∼ 220 K, which corresponds to the ordering of Jeff ≠ 0 Ir5+ centers in the cation-substituted, P21/ n symmetry, double perovskite phases.


Common glass-forming spin-liquid state in the pyrochlore magnets Dy2Ti2 O7 and Ho2Ti2 O7

Physical Review B 98 (2018)

AB Eyvazov, R Dusad, TJS Munsie, HA Dabkowska, GM Luke, ER Kassner, JCS Davis, A Eyal

© 2018 American Physical Society. Despite a well-ordered pyrochlore crystal structure and strong magnetic interactions between the Dy3+ or Ho3+ ions, no long-range magnetic order has been detected in the pyrochlore titanates Ho2Ti2O7 and Dy2Ti2O7. To explore the actual magnetic phase formed by cooling these materials, we measure their magnetization dynamics using toroidal, boundary-free magnetization transport techniques. We find that the dynamical magnetic susceptibility of both compounds has the same distinctive phenomenology, which is indistinguishable in form from that of the dielectric permittivity of dipolar glass-forming liquids. Moreover, Ho2Ti2O7 and Dy2Ti2O7 both exhibit microscopic magnetic relaxation times that increase along the super-Arrhenius trajectories analogous to those observed in glass-forming dipolar liquids. Thus, upon cooling below about 2 K, Dy2Ti2O7 and Ho2Ti2O7 both appear to enter the same magnetic state exhibiting the characteristics of a glass-forming spin liquid.


Phase diagram of Bi2Sr2CaCu2O8+δ revisited.

Nature communications 9 (2018) 5210-

IK Drozdov, I Pletikosić, C-K Kim, K Fujita, GD Gu, JCS Davis, PD Johnson, I Božović, T Valla

In cuprate superconductors, the doping of carriers into the parent Mott insulator induces superconductivity and various other phases whose characteristic temperatures are typically plotted versus the doping level p. In most materials, p cannot be determined from the chemical composition, but it is derived from the superconducting transition temperature, Tc, using the assumption that the Tc dependence on doping is universal. Here, we present angle-resolved photoemission studies of Bi2Sr2CaCu2O8+δ, cleaved and annealed in vacuum or in ozone to reduce or increase the doping from the initial value corresponding to Tc = 91 K. We show that p can be determined from the underlying Fermi surfaces and that in-situ annealing allows mapping of a wide doping regime, covering the superconducting dome and the non-superconducting phase on the overdoped side. Our results show a surprisingly smooth dependence of the inferred Fermi surface with doping. In the highly overdoped regime, the superconducting gap approaches the value of 2Δ0 = (4 ± 1)kBTc.


Magnetic phases of skyrmion-hosting GaV4S8-ySey (y=0, 2, 4, 8) probed with muon spectroscopy

PHYSICAL REVIEW B 98 (2018) ARTN 054428

KJA Franke, BM Huddart, TJ Hicken, F Xiao, SJ Blundell, FL Pratt, M Crisanti, JAT Barker, SJ Clark, A Stefancic, MC Hatnean, G Balakrishnan, T Lancaster

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