Publications


Persistent coherence of quantum superpositions in an optimally doped cuprate revealed by 2D spectroscopy

Science Advances American Association for the Advancement of Science 6 (2020) eaaw9932-

F Novelli, JO Tollerud, D Prabhakaran, JA Davis

Quantum materials displaying intriguing magnetic and electronic properties could be key to the development of future technologies. However, it is poorly understood how the macroscopic behavior emerges in complex materials with strong electronic correlations. While measurements of the dynamics of excited electronic populations have been able to give some insight, they have largely neglected the intricate dynamics of quantum coherence. Here, we apply multidimensional coherent spectroscopy to a prototypical cuprate and report unprecedented coherent dynamics persisting for ~500 fs, originating directly from the quantum superposition of optically excited states separated by 20 to 60 meV. These results reveal that the states in this energy range are correlated with the optically excited states at ~1.5 eV and point to nontrivial interactions between quantum many-body states on the different energy scales. In revealing these dynamics and correlations, we demonstrate that multidimensional coherent spectroscopy can interrogate complex quantum materials in unprecedented ways.


SquidLab-A user-friendly program for background subtraction and fitting of magnetization data.

The Review of scientific instruments 91 (2020) 023901-

MJ Coak, C Liu, DM Jarvis, S Park, MJ Cliffe, PA Goddard

We present an open-source program free to download for academic use with a full user-friendly graphical interface for performing flexible and robust background subtraction and dipole fitting on magnetization data. For magnetic samples with small moment sizes or sample environments with large or asymmetric magnetic backgrounds, it can become necessary to separate background and sample contributions to each measured raw voltage measurement before fitting the dipole signal to extract magnetic moments. Originally designed for use with pressure cells on a Quantum Design MPMS3 SQUID magnetometer, SquidLab is a modular object-oriented platform implemented in Matlab with a range of importers for different widely available magnetometer systems (including MPMS, MPMS-XL, MPMS-IQuantum, MPMS3, and S700X models) and has been tested with a broad variety of background and signal types. The software allows background subtraction of baseline signals, signal preprocessing, and performing fits to dipole data using Levenberg-Marquardt non-linear least squares or a singular value decomposition linear algebra algorithm that excels at picking out noisy or weak dipole signals. A plugin system allows users to easily extend the built-in functionality with their own importers, processes, or fitting algorithms. SquidLab can be downloaded, under Academic License, from the University of Warwick depository (wrap.warwick.ac.uk/129665).


Super resolution convolutional neural network for feature extraction in spectroscopic data

Review of Scientific Instruments AIP Publishing 91 (2020) 033905

H Peng, X Gao, Y He, Y Ji, Y Chen

Two dimensional (2D) peak finding is a common practice in data analysis for physics experiments, which is typically achieved by computing the local derivatives. However, this method is inherently unstable when the local landscape is complicated, or the signal-to-noise ratio of the data is low. In this work, we propose a new method in which the peak tracking task is formalized as an inverse problem, thus can be solved with a convolutional neural network (CNN). In addition, we show that the underlying physics principle of the experiments can be used to generate the training data. By generalizing the trained neural network on real experimental data, we show that the CNN method can achieve comparable or better results than traditional derivative based methods. This approach can be further generalized in different physics experiments when the physical process is known.


Spontaneous Rotation of Ferrimagnetism Driven by Antiferromagnetic Spin Canting

PHYSICAL REVIEW LETTERS 124 (2020) ARTN 127201

AM Vibhakar, DD Khalyavin, P Manuel, J Liu, AA Belik, RD Johnson


Strong quantum fluctuations from competition between magnetic phases in a pyrochlore iridate

PHYSICAL REVIEW B 101 (2020) ARTN 104404

H Jacobsen, CD Dashwood, E Lhotel, D Khalyavin, P Manuel, R Stewart, D Prabhakaran, DF McMorrow, AT Boothroyd


Quantum oscillations probe the Fermi surface topology of the nodal-line semimetal CaAgAs

Physical Review Research American Physical Society 2 (2020) 012055(R)

YH Kwan, P Reiss, Y Han, M Bristow, D Prabhakaran, D Graf, A McCollam, S Ashok Parameswaran, AI Coldea

Nodal semimetals are a unique platform to explore topological signatures of the unusual band structure that can manifest by accumulating a nontrivial phase in quantum oscillations. Here we report a study of the de Haas–van Alphen oscillations of the candidate topological nodal line semimetal CaAgAs using torque measurements in magnetic fields up to 45 T. Our results are compared with calculations for a toroidal Fermi surface originating from the nodal ring. We find evidence of a nontrivial π phase shift only in one of the oscillatory frequencies. We interpret this as a Berry phase arising from the semiclassical electronic Landau orbit which links with the nodal ring when the magnetic field lies in the mirror (ab) plane. Furthermore, additional Berry phase accumulates while rotating the magnetic field for the second orbit in the same orientation which does not link with the nodal ring. These effects are expected in CaAgAs due to the lack of inversion symmetry. Our study experimentally demonstrates that CaAgAs is an ideal platform for exploring the physics of nodal line semimetals and our approach can be extended to other materials in which trivial and nontrivial oscillations are present.


Anomalous high-magnetic field electronic state of the nematic superconductors FeSe1-xSx

Phys. Rev. Research 2, 013309 (2020) (2020)

M Bristow, P Reiss, AA Haghighirad, Z Zajicek, SHIV Singh, T Wolf, D Graf, W Knafo, A McCollam, AMALIA Coldea

Understanding superconductivity requires detailed knowledge of the normal electronic state from which it emerges. A nematic electronic state that breaks the rotational symmetry of the lattice can potentially promote unique scattering relevant for superconductivity. Here, we investigate the normal transport of superconducting FeSe$_{1-x}$S$_x$ across a nematic phase transition using high magnetic fields up to 69 T to establish the temperature and field-dependencies. We find that the nematic state is an anomalous non-Fermi liquid, dominated by a linear resistivity at low temperatures that can transform into a Fermi liquid, depending on the composition $x$ and the impurity level. Near the nematic end point, we find an extended temperature regime with $T^{1.5}$ resistivity. The transverse magnetoresistance inside the nematic phase has as a $H^{1.55}$ dependence over a large magnetic field range and it displays an unusual peak at low temperatures inside the nematic phase. Our study reveals anomalous transport inside the nematic phase, driven by the subtle interplay between the changes in the electronic structure of a multi-band system and the unusual scattering processes affected by large magnetic fields and disorder


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.


Mode-resolved detection of magnetization dynamics using x-ray diffractive ferromagnetic resonance

Nano Letters American Chemical Society (2019)

T Hesjedal, G Van Der Laan, Y Sun, Y Chai, S Zhang, D Burn, K Zhai

Collective spin excitations of ordered magnetic structures o er great potential for the development of novel spintronic devices. The present approach is to rely on micromagnetic models to explain the origins of dynamic modes observed by ferromagnetic resonance (FMR) studies, since experimental tools to directly reveal the origins of the complex dynamic behavior are lacking. Here we demonstrate a new approach which combines resonant magnetic x-ray diffraction with FMR, thereby allowing for a reconstruction of the real-space spin dynamics of the system. This new diffractive FMR (DFMR) technique builds on x-ray detected FMR (XFMR) that allows for element-selective dynamic studies, giving unique access to specific wave components of static and dynamic coupling in magnetic heterostructures. In combination with diffraction, FMR is elevated to the level of a modal spectroscopy technique, potentially opening new pathways for the development of spintronic devices.


Robust perpendicular skyrmions and their surface-confinement

Nano Letters American Chemical Society (2020)

S Zhang, D Burn, N Jaouen, J-Y Chauleau, A Haghighirad, Y Wang, W Wang, G Van Der Laan, T Hesjedal

Magnetic skyrmions are two-dimensional magnetization swirls that stack in the form of tubes in the third dimension, and which are proposed as prospective information carriers for nonvolatile memory devices due to their unique topological properties. From resonant elastic x-ray scattering measurements on Cu2OSeO3 with an in-plane magnetic field we find that a state of perpendicularly ordered skyrmions forms - in stark contrast to the well-studied bulk state. The surface state is stable over a wide temperature range, unlike the bulk state in out-of-plane fields which is confined in a narrow region of the temperature-field phase diagram. In contrast to ordinary skyrmions found in the bulk, the surface state skyrmions result from the presence of magnetic interactions unique to the surface which stabilize them against external perturbations. The surface-guiding makes the robust state particular interesting for racetrack-like devices, ultimately allowing for much higher storage densities due to the smaller lateral footprint of the perpendicular skyrmions.


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


Phase transitions for beginners

CONTEMPORARY PHYSICS (2020)

SJ Blundell


Group theory for physicists, 2nd edition

CONTEMPORARY PHYSICS (2020)

SJ Blundell


Tailoring Hybrid Anomalous Hall Response in Engineered Magnetic Topological Insulator Heterostructures

Nano Letters: a journal dedicated to nanoscience and nanotechnology American Chemical Society (2020)

P Chen, Y Zhang, Q Yao, F Tian, L Li, Z Qi, X Liu, L Liao, C Song, J Wang, J Xia, G Li, DM Burn, G van der Laan, T HESJEDAL, S ZHANG, X Kou

Engineering the anomalous Hall effect (AHE) in the emerging magnetic topological insulators (MTIs) has great potentials for quantum information processing and spintronics applications. In this letter, we synthesize the epitaxial Bi2Te3/MnTe magnetic heterostructures and observe pronounced AHE signals from both layers combined together. The evolution of the resulting hybrid AHE intensity with the top Bi2Te3 layer thickness manifests the presence of an intrinsic ferromagnetic phase induced by the topological surface states at the heterolayer-interface. More importantly, by doping the Bi2Te3 layer with Sb, we are able to manipulate the sign of the Berry phase-associated AHE component. Our results demonstrate the un-paralleled advantages of MTI heterostructures over magnetically doped TI counterparts, in which the tunability of the AHE response can be greatly enhanced. This in turn unveils a new avenue for MTI heterostructure-based multifunctional applications.


Tailoring the topological surface state in ultrathin α -Sn(111) films

Physical Review B: Condensed Matter and Materials Physics American Physical Society (2019)

VA Rogalev, F Reis, F Adler, M Bauernfeind, J Erhardt, L Dudy, LB Duffy, THORSTEN Hesjedal, M Hoesch, G Bihlmayer, R Claessen, J Schäfer, G Bihlmayer, J Schäfer, A Kowalewski, Scholz, THORSTEN Hesjedal, L Duffy, M Bauernfeind, VA Rogalev, J Erhardt, M Hoesch, F Adler, L Dudy

We report on the electronic structure of α -Sn films in the very low thickness regime grown on InSb(111)A. High-resolution low photon energy angle-resolved photoemission spectroscopy allows for the direct observation of the linearly dispersing two-dimensional (2D) topological surface state (TSS) that exists between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200 meV below the Fermi level in 10-nm-thick films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The crossover to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin-zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of α -Sn films where the hybridization gap in the TSS coexists with the topologically nontrivial electronic structure and one can expect the presence of a one-dimensional helical edge state.


Fractionalized pair density wave in the pseudogap phase of cuprate superconductors

PHYSICAL REVIEW B 100 (2019) ARTN 224511

D Chakraborty, M Grandadam, MH Hamidian, JCS Davis, Y Sidis, C Pepin


High-resolution resonant inelastic x-ray scattering study of the electron-phonon coupling in honeycomb alpha-Li2IrO3

PHYSICAL REVIEW B 100 (2019) ARTN 224303

J Vale, C Dashwood, E Paris, L Veiga, M Garcia-Fernandez, A Nag, A Walters, K-J Zhou, I-M Pietsch, A Jesche, P Gegenwart, R Coldea, T Schmitt, D McMorrow

© 2019 American Physical Society. The excitations in honeycomb α-Li2IrO3 have been investigated with high-resolution resonant inelastic X-ray scattering (RIXS) at the O K edge. The low-energy response is dominated by a fully resolved ladder of excitations, which we interpret as being due to multiphonon processes in the presence of strong electron-phonon coupling (EPC). At higher energies, the orbital excitations are shown to be dressed by phonons. The high quality of the data permits a quantitative test of the analytical model for the RIXS cross section, which has been proposed to describe EPC in transition-metal oxides (TMOs). We find that the magnitude of the EPC is comparable to that found for a range of 3d TMOs. This indicates that EPC may be of equal importance in determining the phenomenology displayed by corresponding 5d-based systems.


Chemical tuning between triangular and honeycomb structures in a 5d spin-orbit Mott insulator

Physical Review B American Physical Society 100 (2019) 214113

R Johnson, I Broeders, K Mehlawat, Y Li, Y Singh, R Valenti, R Coldea

We report structural studies of the spin-orbit Mott insulator family K x Ir y O 2 , with triangular layers of edge-sharing IrO 6 octahedra bonded by potassium ions. The potassium content acts as a chemical tuning parameter to control the amount of charge in the Ir-O layers. Unlike the isostructural families with Ir replaced by Co or Rh ( y = 1 ), which are metallic over a range of potassium compositions x , we instead find insulating behavior with charge neutrality achieved via iridium vacancies, which order in a honeycomb supercell above a critical composition x c . By performing density functional theory calculations we attribute the observed behavior to a subtle interplay of crystal-field environment, local electronic correlations, and strong spin-orbit interaction at the Ir 4 + sites, making this structural family a candidate to display Kitaev magnetism in the experimentally unexplored regime that interpolates between triangular and honeycomb structures.


Tailoring the topological surface state in ultrathin alpha -Sn(111) films

Physical Review B: Condensed Matter and Materials Physics American Physical Society (2019)

VA Rogalev, F Reis, F Adler, M Bauernfeind, J Erhardt, L Dudy, LB Duffy, T Hesjedal, M Hoesch, G Bihlmayer, J Schaefer, R Claessen

We report on the electronic structure of α-Sn films in the low thickness regime grown on InSb(111)A. High-resolution angle-resolved photoemission (ARPES), enhanced at low photon energies, allows for the direct observation of the linearly dispersing 2D topological surface states (TSSs) that exist between the second valence band and the conduction band. The Dirac point of this TSS was found to be 200meV below the Fermi level in 10-nm-thick films, which enables the observation of the hybridization gap opening at the Dirac point of the TSS for thinner films. The cross-over to a quasi-2D electronic structure is accompanied by a full gap opening at the Brillouin zone center, in agreement with our density functional theory calculations. We further identify the thickness regime of α-Sn films where the hybridization gap in TSS coexists with the topologically non-trivial electronic structure which must result in a presence of 1D helical edge states.

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