Publications (137)426.35 Total impact

Article: Instability towards Staggered Loop Currents in the ThreeOrbital Model for Cuprate Superconductors
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ABSTRACT: We present evidence for the existence of a spontaneous instability towards an orbital loopcurrent phase in a multiorbital Hubbard model for the CuO$_2$ planes in cuprates. Contrary to the previously proposed $\theta_{II}$ phase with intraunit 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 ddensity wave order. The current pattern will cause a Fermi surface reconstruction and the opening of a pseudogap. We argue that the pseudogap phase with timereversal 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. 
Article: The Effect of Pointlike Impurities on d_{x^2y^2} Charge Density Waves in Cuprate Superconductors
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ABSTRACT: Many cuprate superconductors possess an unusual chargeordered phase that is characterized by an approximate $d_{x^2y^2}$ intraunit cell form factor and a finite modulation wavevector $\bq^\ast$. We study the effects impurities on this charge ordered phase via a singleband 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 selfconsistent tmatrix approximation (SCTMA). We show that suppression of bond order by impurities occurs through the local disruption of the $d_{x^2y^2}$ 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 $d_{x^2y^2}$ 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 Labased cuprates by Zn doping is evidence that this charge order is also distinct from stripes. 
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ABSTRACT: The verification of topological superconductivity has become a major experimental challenge. Apart from the very few spintriplet superconductors with pwave pairing symmetry, another candidate system is a conventional, twodimensional (2D) swave superconductor in a magnetic field with a sufficiently strong Rashba spinorbit coupling. Typically, the required magnetic field to convert the superconductor into a topologically nontrivial 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 outofplane magnetic field, indeed extends to an inplane field orientation. We present a threeband model applicable to the superconducting interface between LaAlO_3 and SrTiO_3, which should fulfil the necessary conditions to realize a topological superconductor. 
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ABSTRACT: In a multiorbital model of the cuprate hightemperature 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^2y^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:10.1088/13672630/17/1/013025 · 3.67 Impact Factor 
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ABSTRACT: Using realistic multiorbital tightbinding Hamiltonians and the Tmatrix formalism, we explore the effects of a nonmagnetic impurity on the local density of states in Febased 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; DOI:10.1103/PhysRevB.88.174518 · 3.66 Impact Factor 
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ABSTRACT: Twodimensional electron systems at oxide interfaces are often influenced by a Rashba type spinorbit coupling, which is tunable by a transverse electric field. Ferromagnetism near the interface can simultaneously induce strong local magnetic fields. This combination of spinorbit coupling and magnetism leads to asymmetric twosheeted Fermi surfaces, on which either intra or interband pairing is favored. The superconducting order parameters are derived within a microscopic pairing model realizing both the BardeenCooperSchrieffer superconductor with interband pairing and a mixed parity state with finitemomentum intraband 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 LaAlO3SrTiO3 interfaces and superconductors with broken inversion symmetry.Journal of Physics Condensed Matter 08/2013; 25(36):362201. DOI:10.1088/09538984/25/36/362201 · 2.22 Impact Factor 
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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 centerofmass 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.Physical Review Letters 07/2013; 111(4):047003. DOI:10.1103/PhysRevLett.111.047003 · 7.73 Impact Factor 
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ABSTRACT: We use a twoorbital doubleexchange model including JahnTeller lattice distortions, superexchange interactions, and longrange 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 nonmonotonically with increasing AFI layer width as seen in the experiment. We provide a framework for understanding this nonmonotonic behavior which has a onetoone 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.Physical review. B, Condensed matter 06/2013; 88(11). DOI:10.1103/PhysRevB.88.115136 · 3.66 Impact Factor 
Article: Spatially Modulated Electronic Nematicity in the ThreeBand Model of Cuprate Superconductors
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ABSTRACT: Charge order in cuprate superconductors is a possible source of anomalous electronic properties in the underdoped regime. Intraunit 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 particlehole continuum at the transition. The Fermi surface reconstructions that accompany the modulated nematic phases are discussed with respect to their relevance for magnetooscillation and photoemission measurements. The modulated nematic phases that emerge from the threeband Emery model are compared to those found previously in oneband models.Physical Review B 05/2013; 88(15). DOI:10.1103/PhysRevB.88.155132 · 3.66 Impact Factor 
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ABSTRACT: We study oxygen Kedge xray absorption spectroscopy (XAS) and investigate the validity of the ZhangRice singlet (ZRS) picture in overdoped cuprate superconductors. Using largescale exact diagonalization of the threeorbital 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 corehole interaction. Our results confirm the important correlated nature of the cuprates and elucidate the changing orbital character of the lowenergy quasiparticles, but also demonstrate the continued relevance of the ZRS even in the overdoped region.Physical review. B, Condensed matter 04/2013; 87(16). DOI:10.1103/PhysRevB.87.165144 · 3.66 Impact Factor 
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ABSTRACT: Recent experimental evidence for charge order in cuprates is a possible source of anomalous electronic properties in the underdoped regime. Intraunit 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 intraunit 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 Xray and STM experiments. 
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ABSTRACT: We investigate interfaces between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites using a twoorbital doubleexchange model including superexchange interactions, JahnTeller 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 nonmonotonously with the layer width. These differences for n=0.6 and n=0.5 originate from different chargetransfer 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). 
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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 centerofmass 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. 
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ABSTRACT: In ironbased superconductors, nematicity has been reported in transport measurements and a broad range of spectroscopies, including angleresolved 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 delectron orbitals, referred to as orbital ordering. We use realistic multiorbital tightbinding Hamiltonians and Tmatrix formalism to explore the effects of nonmagnetic 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 Fouriertransformed 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. 
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ABSTRACT: We investigate the electronic reconstruction at the interface between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in superlattices using a twoorbital doubleexchange model including superexchange interactions, JahnTeller 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 electrondensity 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 chargetransfer profile at the interfaces control the magnitude of the magnetoresistance and the metalinsulator transition of the FM/AFI superlattices.Physical review. B, Condensed matter 02/2013; 87(15). DOI:10.1103/PhysRevB.87.155152 · 3.66 Impact Factor 
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ABSTRACT: We study oxygen Kedge xray absorption spectroscopy (XAS) and investigate the validity of the ZhangRice singlet (ZRS) picture in overdoped cuprate superconductors. Using largescale exact diagonalization of the threeorbital 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 corehole interaction. Our results confirm the important correlated nature of the cuprates and elucidate the changing orbital character of the lowenergy quasiparticles, but also demonstrate the continued relevance of the ZRS even in the overdoped region. 
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ABSTRACT: Stripe phases are observed experimentally in several copperbased highTc 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 HartreeFock decoupling scheme for the tJ 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 momentumspace results exhibit universal features for all coexisting densitywave solutions, nearly unchanged even in strongly disordered systems. These coexisting solutions feature generically a full energy gap and a particlehole asymmetry in the density of states.New Journal of Physics 08/2012; 15(7). DOI:10.1088/13672630/15/7/073049 · 3.67 Impact Factor 
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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 BardeenCooperSchrieffer 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. 
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ABSTRACT: We model disorder in graphene by random impurities treated in a coherentpotential 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. 
Article: Superconductivity and magnetism in the presence of interfaceinduced Rashba spinorbit coupling
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ABSTRACT: Two dimensional electron systems at oxide interfaces are often influenced by a Rashba type spinorbit coupling (SOC), which is tunable by a transverse electric field. Ferromagnetism at the interface can simultaneously induce strong local magnetic fields. This combination of SOC and magnetism leads to anisotropic twosheeted Fermi surfaces, on which superconductivity with finitemomentum pairing is favored. The superconducting order parameter is derived within a generalized pairing model realizing both, the FFLO superconductor in the limit of vanishing SOC and a mixedparity pairing state with zero pair momentum if the magnetism vanishes. The nature of the pairing state is discussed in the context of interface superconductivity and ferromagnetism at LAOSTO interfaces [1,2]. [4pt] [1] Lu Li, C. Richter, J. Mannhart, and R. C. Ashoori, Nature Physics 7, 762 (2011) [0pt] [2] J. A. Bert, B. Kallisky, C. Bell, M. Kim, Y. Hikita, H. Y. Hwang, and K. A. Moler, Nature Physics 7, 767 (2011)
Publication Stats
3k  Citations  
426.35  Total Impact Points  
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Institutions

1997–2015

Universität Augsburg
 Institute of Physics
Augsberg, Bavaria, Germany


1992–2007

Forschungszentrum Jülich
Jülich, North RhineWestphalia, Germany


1987–1997

University of Cologne
 Institute for Theoretical Physics
Köln, North RhineWestphalia, Germany


1993

University of California, Davis
 Department of Physics
Davis, California, United States


1990–1991

University of California, Santa Barbara
 Department of Physics
Santa Barbara, California, United States 
Los Alamos National Laboratory
 Nuclear Materials Science Group
Los Alamos, California, United States
