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M. R. Norman

Publications and source records attributed to M. R. Norman.

At least 73 records · Page 4Linked to original sources
arXiv2012

Orbital currents, anapoles, and magnetic quadrupoles in CuO

We show that orbital currents in a CuO2 plane, if present, should be described by two independent parity and time-reversal odd order parameters, a toroidal dipole (anapole) and a magnetic quadrupole. Based on this, we derive the resonant X-ray diffraction cross-section for monoclinic CuO at the antiferromagnetic wavevector and show that the two order parameters can be disentangled. From our analysis, we examine a recent claim of detecting anapoles in CuO.

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arXiv2011

Cuprates - An Overview

A brief overview is given of the problem of high temperature superconductivity in the cuprates, with an emphasis on theoretical ideas.

cond-mat.supr-con↗
arXiv2011

The Challenge of Unconventional Superconductivity

During the past few decades, several new classes of superconductors have been discovered. Most of these do not appear to be related to traditional superconductors. As a consequence, it is felt by many that for these materials, superconductivity arises from a different source than the electron-ion interactions that are at the heart of conventional superconductivity. Developing a rigorous theory for any of these classes of materials has proven to be a difficult challenge, and will continue to be one of the major problems in physics in the decades to come.

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arXiv2011

Effect of Fermi Surface Nesting on Resonant Spin Excitations in Ba1-xKxFe2As2

We report inelastic neutron scattering measurements of the resonant spin excitations in Ba1-xKxFe2As2 over a broad range of electron band filling. The fall in the superconducting transi- tion temperature with hole doping coincides with the magnetic excitations splitting into two incom- mensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s+- symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight caused by the weakening of electron-electron correlations.

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arXiv2011

Revealing the ortho-II Band Folding in YBa2Cu3O7-delta Films

We present an angle-resolved photoelectron spectroscopy study of YBa2Cu3O7-delta films in situ grown by pulsed laser deposition. We have successfully produced underdoped surfaces with ordered oxygen vacancies within the CuO chains resulting in a clear ortho-II band folding of the Fermi surface. This indicates that order within the CuO chains affects the electronic properties of the CuO2 planes. Our results highlight the importance of having not only the correct surface carrier concentration, but also a very well ordered and clean surface in order that photoemission data on this compound be representative of the bulk.

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arXiv2011

Probing the pseudogap phase of cuprates by a Giaever transformer

We develop a theory of the rectification effect in a double-layer system where both layers are superconductors, or one of the layers is a normal metal. The Coulomb interaction is assumed to provide the dominant coupling between the layers. We find that superconducting fluctuations strongly enhance the drag conductivity, with rectification most pronounced when both layers are superconductors. In view of their distinct dependence on temperature near Tc and layer separation, drag measurements based on a Giaever transformer could distinguish whether rectification occurs due to fluctuating pairs or inductively coupled fluctuating vortices.

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arXiv2010

The modulated spin liquid: a new paradigm for URu$_2$Si$_2$

We argue that near a Kondo breakdown critical point, a spin liquid with spatial modulations can form. Unlike its uniform counterpart, we find that this occurs via a second order phase transition. The amount of entropy quenched when ordering is of the same magnitude as for an antiferromagnet. Moreover, the two states are competitive, and at low temperatures are separated by a first order phase transition. The modulated spin liquid we find breaks $Z_4$ symmetry, as recently seen in the hidden order phase of URu$_2$Si$_2$. Based on this, we suggest that the modulated spin liquid is a viable candidate for this unique phase of matter.

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arXiv2010

Nematic spin fluid in the tetragonal phase of BaFe2As2

Magnetic interactions are generally believed to play a key role in mediating electron pairing for superconductivity in iron arsenides; yet their character is only partially understood. Experimentally, the antiferromagnetic (AF) transition is always preceded by or coincident with a tetragonal to orthorhombic structural distortion. Although it has been suggested that this lattice distortion is driven by an electronic nematic phase, where a spontaneously generated electronic liquid crystal state breaks the C4 rotational symmetry of the paramagnetic state, experimental evidence for electronic anisotropy has been either in the low-temperature orthorhombic phase or the tetragonal phase under uniaxial pressure that breaks this symmetry. Here we use inelastic neutron scattering to demonstrate the presence of a large in-plane spin anisotropy above TN in the unstressed tetragonal phase of BaFe2As2. In the low-temperature orthorhombic phase, we find highly anisotropic spin waves with a large damping along the AF a-axis direction. On warming the system to the paramagnetic tetragonal phase, the low-energy spin waves evolve into quasi-elastic excitations, while the anisotropic spin excitations near the zone boundary persist. These results strongly suggest that the spin nematicity we find in the tetragonal phase of BaFe2As2 is the source of the electronic and orbital anisotropy observed above TN by other probes, and has profound consequences for the physics of these materials.

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arXiv2010

The Nernst effect from fluctuating pairs in the pseudogap phase

The observation of a large Nernst signal in cuprates above the superconducting transition temperature has attracted much attention. A potential explanation is that it originates from superconducting fluctuations. Although the Nernst signal is indeed consistent with gaussian fluctuations for overdoped cuprates, gaussian theory fails to describe the temperature dependence seen for underdoped cuprates. Here, we consider the vertex correction to gaussian theory resulting from the pseudogap. This yields a Nernst signal in good agreement with the data.

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arXiv2010

Anisotropic Neutron Spin Resonance in Superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$

We use polarized inelastic neutron scattering to show that the neutron spin resonance below $T_c$ in superconducting BaFe$_{1.9}$Ni$_{0.1}$As$_2$ ($T_c=20$ K) is purely magnetic in origin. Our analysis further reveals that the resonance peak near 7~meV only occurs for the planar response. This challenges the common perception that the spin resonance in the pnictides is an isotropic triplet excited state of the singlet Cooper pairs, as our results imply that only the $S_{001}=\pm1$ components of the triplet are involved.

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arXiv2010

Spin Zeros and the Origin of Fermi Surface Reconstruction in the Cuprates

Two recent quantum oscillation studies find contradictory results concerning the existence of spin zeros - zeros of the oscillatory signal induced by Zeeman splitting of the Landau levels. We discuss these experiments in light of calculations of the oscillations assuming a spin density wave state. We find that the lack of spin zeros reported in one of the experiments is consistent with either hole or electron pockets in such a state, if the staggered moment is perpendicular to the external field. An analysis for field directions near the planes might be able to differentiate between the two. On the other hand, if spin zeros exist as reported in the other experiment, then the staggered moment would have to have a substantial longitudinal component. We suggest several experiments to test whether this is indeed the case.

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arXiv2010

Transport implications of Fermi arcs in the pseudogap phase of the cuprates

We derive the fermionic contribution to the longitudinal and Hall conductivities within a Kubo formalism, using a phenomenological Greens function which has been previously developed to describe photoemission data in the pseudogap phase of the cuprates. We find that the in-plane electrical and thermal conductivities are metallic-like, showing a universal limit behavior characteristic of a d-wave spectrum as the scattering rate goes to zero. In contrast, the c-axis resistivity and the Hall number are insulating-like, being divergent in the same limit. The relation of these results to transport data in the pseudogap phase is discussed.

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arXiv2010

A proposal to detect vortices above the superconducting transition temperature

We propose a simple experiment to determine whether vortices persist above the superconducting transition temperature Tc in the pseudogap phase of high temperature cuprate superconductors. This involves using a magnetic dot to stabilize a vortex in a thin cuprate film beneath the dot. We calculate the magnetic field profile as a function of distance from the dot if a vortex is present, and discuss possible measurements that could be done to detect this. Finally, we comment on the temperature range where a stable vortex should be observable.

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arXiv2010

Lifshitz Transition in Underdoped Cuprates

Recent studies show that quantum oscillations thought to be associated with a density wave reconstructed Fermi surface disappear at a critical value of the doping for YBa2Cu3O6+y, and the cyclotron mass diverges as the critical value is approached from the high doping side. We argue that the phenomenon is due to a Lifshitz transition where the pockets giving rise to the quantum oscillations connect to form an open (quasi-1d) Fermi surface. The estimated critical doping is close to that found by experiment, and the theory predicts a logarithmic divergence of the cyclotron mass with a coefficient comparable to that observed in experiment.

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arXiv2010

Spin Hamiltonian of Hyperkagome Na4Ir3O8

We derive the spin Hamiltonian for the quantum spin liquid Na4Ir3O8, and then estimate the direct and superexchange contributions between near neighbor iridium ions using a tight binding parametrization of the electronic structure. We find a magnitude of the exchange interaction comparable to experiment for a reasonable value of the on-site Coulomb repulsion. For one of the two tight binding parametrizations we have studied, the direct exchange term, which is isotropic, dominates the total exchange. This provides support for those theories proposed to describe this novel quantum spin liquid that assume an isotropic Heisenberg model.

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arXiv2010

Electronic Structure of Hyperkagome Na4Ir3O8

We investigate the electronic structure of the frustrated magnet Na4Ir3O8 using density functional theory. Due to strong spin-orbit coupling, the hyperkagome lattice is characterized by a half-filled complex of states, making it a cubic iridium analogue of the high temperature superconducting cuprates. The implications of our results for this unique material are discussed.

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arXiv2010

The nature of spectral gaps due to pair formation

Several phenomenological self-energies have been presented to describe the pseudogap in cuprates. Here, we offer a derivation of the self-energy in two dimensions due to pair formation and compare it to photoemission data. We then use our results to address several questions of interest, including the existence of magneto-oscillations in the presence of the pseudogap, and the two length scale nature of vortices in underdoped cuprates.

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