[Show abstract][Hide abstract] ABSTRACT: We present evidence for the existence of a spontaneous instability towards an
orbital loop-current phase in a multiorbital Hubbard model for the CuO$_2$
planes in cuprates. Contrary to the previously proposed $\theta_{II}$ phase
with intra-unit cell currents, the identified instability is towards a
staggered pattern of intertwined current loops. The orbitally resolved current
pattern thereby shares its staggered character with the proposal of d-density
wave order. The current pattern will cause a Fermi surface reconstruction and
the opening of a pseudogap. We argue that the pseudogap phase with
time-reversal symmetry breaking currents is susceptible to further phase
transitions and therefore offers a route to account for axial incommensurate
charge order and a polar Kerr effect in underdoped cuprates.
[Show abstract][Hide abstract] ABSTRACT: Many cuprate superconductors possess an unusual charge-ordered phase that is characterized by an approximate ${d}_{{x}^{2}$-${}{y}^{2}}$ intraunit cell form factor and a finite modulation wave vector ${\mathbf{q}}^{*}$. We study the effects of impurities on this charge-ordered phase via a single-band model in which bond order is the analog of charge order in the cuprates. Impurities are assumed to be pointlike and are treated within the self-consistent $t$-matrix approximation. We show that suppression of bond order by impurities occurs through the local disruption of the ${d}_{{x}^{2}$-${}{y}^{2}}$ form factor near individual impurities. Unlike $d$-wave superconductors, where the sensitivity of ${T}_{c}$ to impurities can be traced to a vanishing average of the ${d}_{{x}^{2}$-${}{y}^{2}}$ order parameter over the Fermi surface, the response of bond order to impurities is dictated by a few Fermi surface ``hotspots.'' The bond order transition temperature ${T}_{\mathrm{bo}}$ thus follows a different universal dependence on impurity concentration ${n}_{i}$ than does the superconducting ${T}_{c}$. In particular, ${T}_{\mathrm{bo}}$ decreases more rapidly than ${T}_{c}$ with increasing ${n}_{i}$ when there is a nonzero Fermi surface curvature at the hotspots. Based on experimental evidence that the pseudogap is insensitive to Zn doping, we conclude that a direct connection between charge order and the pseudogap is unlikely. Furthermore, the enhancement of stripe correlations in the La-based cuprates by Zn doping is evidence that this charge order is also distinct from stripes.
Physical Review B 03/2015; 91(10). DOI:10.1103/PhysRevB.91.104509 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Many cuprate superconductors possess an unusual charge-ordered phase that is
characterized by an approximate $d_{x^2-y^2}$ intra-unit cell form factor and a
finite modulation wavevector $\bq^\ast$. We study the effects impurities on
this charge ordered phase via a single-band model in which bond order is the
analogue of charge order in the cuprates. Impurities are assumed to be
pointlike and are treated within the self-consistent t-matrix approximation
(SCTMA). We show that suppression of bond order by impurities occurs through
the local disruption of the $d_{x^2-y^2}$ form factor near individual
impurities. Unlike $d$-wave superconductors, where the sensitivity of $T_c$ to
impurities can be traced to a vanishing average of the $d_{x^2-y^2}$ order
parameter over the Fermi surface, the response of bond order to impurities is
dictated by a few Fermi surface "hotspots". The bond order transition
temperature $T_\mathrm{bo}$ thus follows a different universal dependence on
impurity concentration $n_i$ than does the superconducting $T_c$. In
particular, $T_\mathrm{bo}$ decreases more rapidly than $T_c$ with increasing
$n_i$ when there is a nonzero Fermi surface curvature at the hotspots. Based on
experimental evidence that the pseudogap is insensitive to Zn doping, we
conclude that a direct connection between charge order and the pseudogap is
unlikely. Furthermore, the enhancement of stripe correlations in the La-based
cuprates by Zn doping is evidence that this charge order is also distinct from
stripes.
[Show abstract][Hide abstract] ABSTRACT: The verification of topological superconductivity has become a major
experimental challenge. Apart from the very few spin-triplet superconductors
with p-wave pairing symmetry, another candidate system is a conventional,
two-dimensional (2D) s-wave superconductor in a magnetic field with a
sufficiently strong Rashba spin-orbit coupling. Typically, the required
magnetic field to convert the superconductor into a topologically non-trivial
state is however by far larger than the upper critical field H_c2, which
excludes its realization. In this article, we argue that this problem can be
overcome by rotating the magnetic field into the superconducting plane. We
explore the character of the superconducting state upon changing the strength
and the orientation of the magnetic field and show that a topological state,
established for a sufficiently strong out-of-plane magnetic field, indeed
extends to an in-plane field orientation. We present a three-band model
applicable to the superconducting interface between LaAlO_3 and SrTiO_3, which
should fulfil the necessary conditions to realize a topological superconductor.
[Show abstract][Hide abstract] ABSTRACT: In a multiorbital model of the cuprate high-temperature superconductors soft
antiferromagnetic (AF) modes are assumed to reconstruct the Fermi surface to
form nodal pockets. The subsequent charge ordering transition leads to a phase
with a spatially modulated transfer of charge between neighboring oxygen p_x
and p_y orbitals and also weak modulations of the charge density on the copper
d_{x^2-y^2} orbitals. As a prime result of the AF Fermi surface reconstruction,
the wavevectors of the charge modulations are oriented along the crystalline
axes with a periodicity that agrees quantitatively with experiments. This
resolves a discrepancy between experiments, which find axial order, and
previous theoretical calculations, which find modulation wavevectors along the
Brillouin zone (BZ) diagonal. The axial order is stabilized by hopping
processes via the Cu4s orbital, which is commonly not included in model
analyses of cuprate superconductors.
New Journal of Physics 04/2014; 17(1). DOI:10.1088/1367-2630/17/1/013025 · 3.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using realistic multi-orbital tight-binding Hamiltonians and the T-matrix
formalism, we explore the effects of a non-magnetic impurity on the local
density of states in Fe-based compounds. We show that scanning tunneling
spectroscopy (STS) has very specific anisotropic signatures that track the
evolution of orbital splitting (OS) and antiferromagnetic gaps. Both
anisotropies exhibit two patterns that split in energy with decreasing
temperature, but for OS these two patterns map onto each other under 90 degree
rotation. STS experiments that observe these signatures should expose the
underlying magnetic and orbital order as a function of temperature across
various phase transitions.
Physical Review B 08/2013; 88(17). DOI:10.1103/PhysRevB.88.174518 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Two-dimensional electron systems at oxide interfaces are often influenced by a Rashba type spin-orbit coupling, which is tunable by a transverse electric field. Ferromagnetism near the interface can simultaneously induce strong local magnetic fields. This combination of spin-orbit coupling and magnetism leads to asymmetric two-sheeted Fermi surfaces, on which either intra- or inter-band pairing is favored. The superconducting order parameters are derived within a microscopic pairing model realizing both the Bardeen-Cooper-Schrieffer superconductor with inter-band pairing and a mixed parity state with finite-momentum intra-band pairing. We present a phase diagram for the superconducting groundstates and analyze the density of states, the spectra, and the momentum distribution functions of the different phases. The results are discussed in the context of superconductivity and ferromagnetism at LaAlO3-SrTiO3 interfaces and superconductors with broken inversion symmetry.
[Show abstract][Hide abstract] ABSTRACT: The magnetic flux threading a conventional superconducting ring is typically quantized in units of Φ_{0}=hc/2e. The factor of 2 in the denominator of Φ_{0} originates from the existence of two different types of pairing states with minima of the free energy at even and odd multiples of Φ_{0}. Here we show that spatially modulated pairing states exist with energy minima at fractional flux values, in particular, at multiples of Φ_{0}/2. In such states, condensates with different center-of-mass momenta of the Cooper pairs coexist. The proposed mechanism for fractional flux quantization is discussed in the context of cuprate superconductors, where hc/4e flux periodicities were observed.
[Show abstract][Hide abstract] ABSTRACT: We use a two-orbital double-exchange model including Jahn-Teller lattice
distortions, superexchange interactions, and long-range Coulomb (LRC)
interactions to investigate the origin of magnetically disordered interfaces
between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI)
manganites in FM/AFI superlattices. The induced magnetic moment in the AFI
layer varies non-monotonically with increasing AFI layer width as seen in the
experiment. We provide a framework for understanding this non-monotonic
behavior which has a one-to-one correspondence with the magnetization of the FM
interface. The obtained insights provide a basis for improving the tunneling
magnetoresistance in FM/AFI manganite superlattices by avoiding a magnetic dead
layer (MDL) in the FM manganite.
[Show abstract][Hide abstract] ABSTRACT: Charge order in cuprate superconductors is a possible source of anomalous
electronic properties in the underdoped regime. Intra-unit cell charge ordering
tendencies point to electronic nematic order involving oxygen orbitals. In this
context we investigate charge instabilities in the Emery model and calculate
the charge susceptibility within diagrammatic perturbation theory. In this
approach, the onset of charge order is signalled by a divergence of the
susceptibility. Our calculations reveal three different kinds of order: a
commensurate ($q=0$) nematic order, and two incommensurate nematic phases with
modulation wavevectors that are either axial or oriented along the Brillouin
zone diagonal. We examine the nematic phase diagram as a function of the
filling, the interaction parameters, and the band structure. We also present
results for the excitation spectrum near the nematic instability, and show that
a soft nematic mode emerges from the particle-hole continuum at the transition.
The Fermi surface reconstructions that accompany the modulated nematic phases
are discussed with respect to their relevance for magneto-oscillation and
photoemission measurements. The modulated nematic phases that emerge from the
three-band Emery model are compared to those found previously in one-band
models.
Physical Review B 05/2013; 88(15). DOI:10.1103/PhysRevB.88.155132 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study oxygen K-edge x-ray absorption spectroscopy (XAS) and investigate the validity of the Zhang-Rice singlet (ZRS) picture in overdoped cuprate superconductors. Using large-scale exact diagonalization of the three-orbital Hubbard model, we observe the effect of strong correlations manifesting in a dynamical spectral weight transfer from the upper Hubbard band to the ZRS band. The quantitative agreement between theory and experiment highlights an additional spectral weight reshuffling due to core-hole interaction. Our results confirm the important correlated nature of the cuprates and elucidate the changing orbital character of the low-energy quasiparticles, but also demonstrate the continued relevance of the ZRS even in the overdoped region.
[Show abstract][Hide abstract] ABSTRACT: Recent experimental evidence for charge order in cuprates is a possible
source of anomalous electronic properties in the underdoped regime.
Intra-unit cell charge ordering tendencies point to electronic nematic
order involving oxygen orbitals. In this context we investigate charge
instabilities in the Emery model. The charge susceptibilities reveal
three different kinds of nematic order. The first is an intra-unit cell
(q=0) nematic order. The second and the third are incommensurate charge
orders with wavevectors that are either uniaxial or oriented along the
Brillouin zone diagonal. The two latter charge patterns correspond to a
spatially modulated nematic phase. The selection of the leading
instability depends on the filling, the interaction parameters, and
details of the band structure. For these candidate charge orderings we
discuss their possible relevance for the charge ordering signatures in
X-ray and STM experiments.
[Show abstract][Hide abstract] ABSTRACT: We investigate interfaces between ferromagnetic metallic (FM) and
antiferromagnetic insulating (AFI) manganites using a two-orbital
double-exchange model including superexchange interactions, Jahn-Teller
lattice distortions, and long range Coulomb interactions. In FM/AFI
heterostructures the magnetic and the transport properties critically
depend on the thickness of the AFI layers. We focus on superlattices
where the constituent parent FM and AFI manganites have the same
electron density n. For n=0.6, the induced ferromagnetic moment in the
AFI layers sandwiched between FM manganites decreases monotonically with
increasing layer width. For n=0.5 instead, the induced ferromagnetic
moment varies non-monotonously with the layer width. These differences
for n=0.6 and n=0.5 originate from different charge-transfer profiles
and magnetic reconstructions at the FM/AFI interfaces. The width of the
AFI layers furthermore controls the magnitude of the magnetoresistance
and the metal to insulator transition of the FM/AFI heterostructure.
These results are discussed in the context of recent experiments on
LSMO/PCMO [1] and LCMO/PCMO superlattices [2].[4pt] [1] D.
Niebieskikwiat et al., Phys. Rev. Lett. 99, 247207 (2007).[0pt] [2] H.
Li et al. Appl. Phys. Lett. 80, 628 (2002).
[Show abstract][Hide abstract] ABSTRACT: The magnetic flux threading a conventional superconducting ring is
typically quantized in units of φ0=hc/2e. The factor 2
in the denominator of φ0 originates from the existence
of two different types of pairing states with minima of the free energy
at even and odd multiples of φ0. Here we show that
spatially modulated pairing states exist with energy minima at
fractional flux values, in particular at multiples of
φ0/2. In such states condensates with different
center-of-mass momenta of the Cooper pairs coexist. The proposed
mechanism for fractional flux quantization is discussed in the context
of cuprate superconductors, where hc/4e flux periodicities as well as
uniaxially modulated superconducting states were observed.
[Show abstract][Hide abstract] ABSTRACT: In iron-based superconductors, nematicity has been reported in transport
measurements and a broad range of spectroscopies, including
angle-resolved photoemission, neutron scattering, and scanning tunneling
spectroscopy (STS). Several theories have attributed these observed
anisotropies of broken tetragonal symmetry to either pure spin physics
or unequal occupation of the iron d-electron orbitals, referred to as
orbital ordering. We use realistic multi-orbital tight-binding
Hamiltonians and T-matrix formalism to explore the effects of
non-magnetic impurities in an orbitally split and spin density wave
(SDW) state. In each of these, the local density of states around the
impurity in both position space and Fourier-transformed quasiparticle
interference (QPI) have very specific signatures that may be observable
in STS. These allow one to identify and track the evolution of orbital
splitting and SDW gaps in regimes that have not previously been
explored.
[Show abstract][Hide abstract] ABSTRACT: We investigate the electronic reconstruction at the interface between
ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites
in superlattices using a two-orbital double-exchange model including
superexchange interactions, Jahn-Teller lattice distortions, and long range
Coulomb interactions. The magnetic and the transport properties critically
depend on the thickness of the AFI layers. We focus on superlattices where the
constituent parent manganites have the same electron density n = 0.6. The
induced ferromagnetic moment in the AFI layers decreases monotonically with
increasing layer width, and the electron-density profile and the magnetic
structure in the center of the AFI layer gradually return to the bulk limit.
The width of the AFI layers and the charge-transfer profile at the interfaces
control the magnitude of the magnetoresistance and the metal-insulator
transition of the FM/AFI superlattices.
[Show abstract][Hide abstract] ABSTRACT: We study oxygen K-edge x-ray absorption spectroscopy (XAS) and
investigate the validity of the Zhang-Rice singlet (ZRS) picture in
overdoped cuprate superconductors. Using large-scale exact
diagonalization of the three-orbital Hubbard model, we observe the
effect of strong correlations manifesting in a dynamical spectral weight
transfer from the upper Hubbard band to the ZRS band. The quantitative
agreement between theory and experiment highlights an additional
spectral weight reshuffling due to core-hole interaction. Our results
confirm the important correlated nature of the cuprates and elucidate
the changing orbital character of the low-energy quasi-particles, but
also demonstrate the continued relevance of the ZRS even in the
overdoped region.
[Show abstract][Hide abstract] ABSTRACT: Stripe phases are observed experimentally in several copper-based high-Tc
superconductors near 1/8 hole doping. However, the specific characteristics may
vary depending on the degree of dopant disorder and the presence or absence of
a low- temperature tetragonal phase. On the basis of a Hartree-Fock decoupling
scheme for the t-J model we discuss the diverse behavior of stripe phases. In
particular the effect of inhomogeneities is investigated in two distinctly
different parameter regimes which are characterized by the strength of the
interaction. We observe that small concen- trations of impurities or vortices
pin the unidirectional density waves, and dopant disorder is capable to
stabilize a stripe phase in parameter regimes where homogeneous phases are
typically favored in clean systems. The momentum-space results exhibit
universal features for all coexisting density-wave solutions, nearly unchanged
even in strongly disordered systems. These coexisting solutions feature
generically a full energy gap and a particle-hole asymmetry in the density of
states.
New Journal of Physics 08/2012; 15(7). DOI:10.1088/1367-2630/15/7/073049 · 3.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The periodic response of a metallic or a superconducting ring to an external
magnetic flux is one of the most evident manifestations of quantum mechanics.
It is generally understood that the oscillation period hc/2e in the
superconducting state is half the period hc/e in the metallic state, because
the supercurrent is carried by Cooper pairs with a charge 2e. On the basis of
the Bardeen-Cooper-Schrieffer theory we discuss, in which cases this simple
interpretation is valid and when a more careful analysis is needed. In fact,
the knowledge of the oscillation period of the current in the ring provides
information on the electron interactions. In particular, we analyze the
crossover from the hc/e periodic normal current to the hc/2e periodic
supercurrent upon turning on a pairing interaction in a metal ring. Further, we
elaborate on the periodicity crossover when cooling a metallic loop through the
superconducting transition temperature Tc.
[Show abstract][Hide abstract] ABSTRACT: We model disorder in graphene by random impurities treated in a
coherent-potential approximation. Using the analytically solvable Lloyd model
for the disorder distribution, we show that the temperature dependence of the
minimum conductivity as well as the temperature dependence of the resistivity
at high densities and the density dependence of the respective slopes are
consistently explained by a temperature dependent disorder strength $\Gamma$
consisting of a constant plus a $T$-linear contribution. This finding suggests
that at least two contributions to scattering in graphene are important for its
transport properties, and that one of the contributions is due to scattering of
electrons from thermally induced excitations.