Coherent electron spin manipulation in a dilute oriented ensemble of molecular nanomagnets: pulsed EPR on doped single crystals.

Chem Commun (Camb) 50 (2014) 91-93

F Moro, D Kaminski, F Tuna, GFS Whitehead, GA Timco, D Collison, REP Winpenny, A Ardavan, EJL McInnes

Doping a Cr7Zn molecular nanomagnet into a diamagnetic and isostructural host allows pulsed X-band EPR on single crystals, including relaxation and nutation experiments on the S = 3/2 ground state.

The effect of the demagnetizing field in cylindrical samples in high transverse field mu+SR experiments


F Lang, SJ Blundell

Probing the magnetic phases in the Ni-V alloy close to the disordered ferromagnetic quantum critical point with mu SR


A Schroeder, R Wang, PJ Baker, FL Pratt, SJ Blundell, T Lancaster, I Franke, JS Moeller

Direct phase-sensitive identification of a d-form factor density wave in underdoped cuprates

Proceedings of the National Academy of Sciences of the United States of America 111 (2014)

K Fujita, MH Hamidian, SD Edkins, CK Kim, Y Kohsaka, M Azuma, M Takano, H Takagi, H Eisaki, SI Uchida, A Allais, MJ Lawler, EA Kim, S Sachdev, JC Séamus Davis

The identity of the fundamental broken symmetry (if any) in the underdoped cuprates is unresolved. However, evidence has been accumulating that this state may be an unconventional density wave. Here we carry out site-specific measurements within each CuO2 unit cell, segregating the results into three separate electronic structure images containing only the Cu sites [Cu(r)] and only the x/y axis O sites [Ox (r) and Oy( r)]. Phase-resolved Fourier analysis reveals directly that the modulations in the Ox(r) and Oy(r) sublattice images consistently exhibit a relative phase of π. We confirm this discovery on two highly distinct cuprate compounds, ruling out tunnel matrix-element and materials-specific systematics. These observations demonstrate by direct sublattice phaseresolved visualization that the density wave found in underdoped cuprates consists of modulations of the intraunit-cell states that exhibit a predominantly d-symmetry form factor.

Simultaneous transitions in cuprate momentum-space topology and electronic symmetry breaking

Science 344 (2014) 612-616

K Fujita, CK Kim, I Lee, J Lee, MH Hamidian, IA Firmo, S Mukhopadhyay, H Eisaki, S Uchida, MJ Lawler, EA Kim, JC Davis

The existence of electronic symmetry breaking in the underdoped cuprates and its disappearance with increased hole density p are now widely reported. However, the relation between this transition and the momentum-space (k →-space) electronic structure underpinning the superconductivity has not yet been established. Here, we visualize the Q→ = 0 (intra-unit-cell) and Q→ ≠ 0 (density-wave) broken-symmetry states, simultaneously with the coherent k→-space topology, for Bi2Sr2CaCu2O8+δ samples spanning the phase diagram 0.06 ≤ p ≤ 0.23. We show that the electronic symmetry-breaking tendencies weaken with increasing p and disappear close to a critical doping pc = 0.19. Concomitantly, the coherent k →-space topology undergoes an abrupt transition, from arcs to closed contours, at the same pc. These data reveal that the k →-space topology transformation in cuprates is linked intimately with the disappearance of the electronic symmetry breaking at a concealed critical point.

Iron-Based superconductors: Enigmatic nematic

Nature Physics 10 (2014) 184-185

JC Davis, PJ Hirschfeld

Quantifying magnetic exchange in doubly-bridged Cu-X(2)-Cu (X = F, Cl, Br) chains enabled by solid state synthesis of CuF(2)(pyrazine).

Chemical communications (Cambridge, England) 49 (2013) 3558-3560

SH Lapidus, JL Manson, J Liu, MJ Smith, P Goddard, J Bendix, CV Topping, J Singleton, C Dunmars, JF Mitchell, JA Schlueter

Solid state techniques involving pressure and temperature have been used to synthesize the fluoride member of the CuX(2)(pyrazine) (X = F, Cl, Br) family of coordination polymers that cannot be crystallized by solution methods. CuF(2)(pyrazine) exhibits unique trans doubly-bridged Cu-F(2)-Cu chains that provide an opportunity to quantify magnetic superexchange in an isostructural Cu-X(2)-Cu series.

$μ$SR study of magnetic order in the organic quasi-one-dimensional ferromagnet F4BImNN

Phys. Rev. B American Physical Society 88 (2013) 064423-064423

SJ Blundell, JS Möller, T Lancaster, PJ Baker, FL Pratt, G Seber, PM Lahti

Magnetic fluctuations and spin freezing in nonsuperconducting LiFeAs derivatives

PHYSICAL REVIEW B 88 (2013) ARTN 060401

JD Wright, MJ Pitcher, W Trevelyan-Thomas, T Lancaster, PJ Baker, FL Pratt, SJ Clarke, SJ Blundell

Another dimension: Investigations of molecular magnetism using muon-spin relaxation

Physica Scripta 88 (2013)

T Lancaster, SJ Blundell, FL Pratt

We review examples of muon-spin relaxation measurements on molecule-based magnetic coordination polymers, classified by their magnetic dimensionality. These include the one-dimensional s = 1/2 spin chain Cu(pyz)(NO 3)2 and the two-dimensional s = 1/2 layered material [Cu(HF2)(pyz)2]BF4. We also describe some of the more exotic ground states that may become accessible in the future given the ability to tune the interaction strengths of our materials through crystal engineering. © 2013 The Royal Swedish Academy of Sciences.

Playing quantum hide-and-seek with the muon: Localizing muon stopping sites

Physica Scripta 88 (2013)

JS Möller, P Bonfà, D Ceresoli, F Bernardini, SJ Blundell, T Lancaster, R De Renzi, N Marzari, I Watanabe, S Sulaiman, MI Mohamed-Ibrahim

The most fundamental limitations of a muon-spin relaxation experiment can be the lack of knowledge of the implantation site of the muon and the uncertainty about the muon's perturbation of its host. Here we review some of the work done on the 'muon site problem' in the solid state and highlight some recent applications of electronic structure calculations that have successfully characterized the quantum states of muons in a number of insulating compounds containing fluorine, in a number of pnictide superconductors, and in ZnO. © 2013 The Royal Swedish Academy of Sciences.

Superconducting fluctuations in organic molecular metals enhanced by Mott criticality


M Nam, C Meziere, B Batail, A Ardavan

Evolution of magnetic interactions in a pressure-induced Jahn-Teller driven magnetic dimensionality switch

Phys. Rev. B American Physical Society 87 (2013) 241102-241102

S Ghannadzadeh, JS Möller, PA Goddard, T Lancaster, F Xiao, SJ Blundell, A Maisuradze, R Khasanov, JL Manson, SW Tozer, D Graf, JA Schlueter

AC magnetic measurement of LiFeAs at pressures up to 5.2 GPa: The relation between T<inf>c</inf> and the structural parameters

Journal of the Korean Physical Society 63 (2013) 445-447

S Yamaguchi, N Yamaguchi, M Mito, H Deguchi, PJ Baker, SJ Blundell, MJ Pitcher, DR Parker, SJ Clarke

The pressure effects on the 111-type Fe-based superconductor LiFeAs were investigated through AC susceptibility measurements and X-ray diffraction experiments, and revealed a correlation between the superconducting transition temperature (T c) and the As-Fe-As bond angle (α) rather than the height of As from the Fe layers (h As). As the pressure was increased, T c of 17 K at P = 0 GPa decreased down to 10 K at P = 5.2 GPa. According to a previous report from an X-ray diffraction experiment, α changes from 101.5° at 0 GPa to 97.8° at 17 GPa. The obtained change in T c is consistent with Lee et al.'s plot of T c as a function of α, and from this result, we conclude that T c will fall to zero at around α = 98°. © 2013 The Korean Physical Society.

Magnetic transition and spin dynamics in the triangular Heisenberg antiferromagnet $α$-KCrO$_2$

Phys. Rev. B American Physical Society 88 (2013) 180401-180401

F Xiao, T Lancaster, PJ Baker, FL Pratt, SJ Blundell, JS Möller, NZ Ali, M Jansen

Concepts relating magnetic interactions, intertwined electronic orders, and strongly correlated superconductivity.

Proceedings of the National Academy of Sciences of the United States of America 110 (2013) 17623-17630

JCS Davis, D-H Lee

Unconventional superconductivity (SC) is said to occur when Cooper pair formation is dominated by repulsive electron-electron interactions, so that the symmetry of the pair wave function is other than an isotropic s-wave. The strong, on-site, repulsive electron-electron interactions that are the proximate cause of such SC are more typically drivers of commensurate magnetism. Indeed, it is the suppression of commensurate antiferromagnetism (AF) that usually allows this type of unconventional superconductivity to emerge. Importantly, however, intervening between these AF and SC phases, intertwined electronic ordered phases (IP) of an unexpected nature are frequently discovered. For this reason, it has been extremely difficult to distinguish the microscopic essence of the correlated superconductivity from the often spectacular phenomenology of the IPs. Here we introduce a model conceptual framework within which to understand the relationship between AF electron-electron interactions, IPs, and correlated SC. We demonstrate its effectiveness in simultaneously explaining the consequences of AF interactions for the copper-based, iron-based, and heavy-fermion superconductors, as well as for their quite distinct IPs.

Evidence from tunneling spectroscopy for a quasi-one-dimensional origin of superconductivity in Sr<inf>2</inf>RuO<inf>4</inf>

Physical Review B - Condensed Matter and Materials Physics 88 (2013)

IA Firmo, S Lederer, C Lupien, AP Mackenzie, JC Davis, SA Kivelson

To establish the mechanism of unconventional superconductivity in Sr 2RuO4, a prerequisite is direct information concerning the momentum-space structure of the energy gaps Δi(k), and in particular whether the pairing strength is stronger ("dominant") on the quasi-one-dimensional (α and β) or on the quasi-two-dimensional (γ) Fermi surfaces. We present scanning tunneling microscopy measurements of the density of states spectra in the superconducting state of Sr 2RuO4 for 0.1Tc < T < Tc and analyze them along with published thermodynamic data using a simple phenomenological model. We show that our observation of a single superconducting gap scale with maximum value 2Δ≈5Tc along with a spectral shape indicative of line nodes is consistent, within a weak-coupling model, with magnetically mediated odd-parity superconductivity generated by dominant, near-nodal, Cooper pairing on the α and β bands. © 2013 American Physical Society.

Correlated Electrons in Quantum Matter

CONTEMPORARY PHYSICS 54 (2013) 117-118

SJ Blundell

Weak magnetic transitions in pyrochlore Bi$_2$Ir$_2$O$_7$

Phys. Rev. B American Physical Society 87 (2013) 180409-180409

PJ Baker, JS Möller, FL Pratt, W Hayes, SJ Blundell, T Lancaster, TF Qi, G Cao

A two-step approach to the synthesis of N@C<inf>60</inf> fullerene dimers for molecular qubits

Chemical Science 4 (2013) 2971-2975

SR Plant, M Jevric, JJL Morton, A Ardavan, AN Khlobystov, GAD Briggs, K Porfyrakis

We report the two-step synthesis of a highly soluble fullerene dimer, both for short reaction times and at the microscale. We apply this reaction scheme to starting materials that contain 15N@C60 and 14N@C60, and we demonstrate how, if applied to highly pure N@C60 in the future, this scheme may be used to produce (14N@C60)2 or (15N@C60)2 dimers in one step, and crucially 14N@C60-15N@C60 dimers in a second step. Such dimers represent isolated electron spin pairs that may be used to demonstrate entanglement between the spins. Additionally, CW EPR spectroscopy of the 15N@C60-C60 dimer in the solid state reveals permanent zero-field splitting (D = 14.6 MHz and E = 0.56 MHz). © 2013 Royal Society of Chemistry.