Severe Dirac Mass Gap Suppression in Sb2Te3-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 Cr0.08(Bi0.1Sb0.9)1.92Te3 to that of its nonmagnetic parent (Bi0.1Sb0.9)2Te3, to explore the cause. In (Bi0.1Sb0.9)2Te3, we find spatially random variations of the Dirac energy. Statistically equivalent Dirac energy variations are detected in Cr0.08(Bi0.1Sb0.9)1.92Te3 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 Sb2Te3-based FMTI materials to very low temperatures.

Observation of a neutron spin resonance in the bilayered superconductor CsCa2Fe4As4F2.

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 CsCa2Fe4As4F2 above and below its superconducting transition temperature T c ≈ 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 Fe2As2 separated by insulating CaF2 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 R of the observed spin resonance is broadly consistent with the relationship E R = 4.9k B T c, 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 R 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


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 d-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., Annu. Rev. Condens. Matter Phys. 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, d-symmetry form factor, pair density wave (PDW) state coexisting with d-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 Bi2Sr2CaCu2O8 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 Bi2Sr2CaCu2O8 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 p*, 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 p* ≈ 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

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


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


SJ Blundell

Group theory for physicists, 2nd edition


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


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

Optimization of superconducting properties of the stoichiometric CaKFe4As4

Superconductor Science and Technology IOP Press 33 (2019) 025003

SJ Singh, SJ Cassidy, M Bristow, S Blundell, SJ Clarke, A Coldea

CaKFe4As4 (1144) is a unique stoichiometric iron-based superconductor which harbours high upper critical fields and large critical current densities. In this work, we describe a study to optimize the synthesis conditions of stoichiometric polycrystalline samples of CaKFe4As4 and asses their structural, magnetic and transport properties. The samples were prepared over a wide temperature range (900-1100°C) and the pure phase formation is centered around 955°C. Outside this temperature region, impurity phases of KFe2As2 and CaFe2As2 can also form. Magnetic susceptibility and resistivity measurements establish that the critical temperature reaches ~34 K for the optimum synthesis conditions and the critical current reaches 2 × 104 A-cm−2. The post-annealing process demonstrates the stability of the 1144 phase up to 500°C, however, under higher temperature annealing, phase degradation occurs. Our study indicates that the formation of phase-pure 1144 occurs over a much narrower window and its highly prone to multi-phase formation as compared with the 122 family. As a result, the superconducting properties are enhanced for the pure 1144 phase but they are likely to be affected by the inter and intra-granular behaviour originating from the microstructural nature of polycrystalline CaKFe4As4, similar to other iron-based superconductors. Based on our study, we construct the phase diagram for polycrystalline 1144 and compared it with that reported for 1144 single crystal.

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

Exsolution of SrO during the topochemical conversion of LaSr3CoRuO8 to the oxyhydride LaSr3CoRuO4H4

Inorganic Chemistry American Chemical Society 58 (2019) 14863-14870

L Jin, M Batuk, FKK Kirschner, F Lang, SJ Blundell, J Hadermann, M Hayward

Reaction of the n = 1 Ruddlesden-Popper oxide LaSr3CoRuO8 with CaH2 yields the oxyhydride phase LaSr3CoRuO4H4 via a topochemical anion exchange. Close inspection of the X-ray and neutron powder diffraction data in combination with HAADF-STEM images reveals that the nanoparticles of SrO are exsolved from the system during the reaction, with the change in cation stoichiometry accommodated by the inclusion of n &gt; 1 (Co/Ru)nOn+1H2n "perovskite" layers into the Ruddlesden-Popper stacking sequence. This novel pseudotopochemical process offers a new route for the formation of n &gt; 1 Ruddlesden-Popper structured materials. Magnetization data are consistent with a LaSr3Co+Ru2+O4H4 (Co+, d8, S = 1; Ru2+, d6, S = 0) oxidation/spin state combination. Neutron diffraction and μ+SR data show no evidence for long-range magnetic order down to 2 K, suggesting the diamagnetic Ru2+ centers impede the Co-Co magnetic-exchange interactions.

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.