Itinerant Band Research Articles

Six-fold crystalline anisotropic magnetoresistance in the (111) LaAlO3/SrTiO3 oxide interface

We measured the magnetoresistance of the 2D electron liquid formed at the (111) LaAlO$_3$/SrTiO$_3$ interface. The hexagonal symmetry of the interface is manifested in a six-fold crystalline component appearing in the anisotropic magnetoresistance (AMR) and planar Hall data, which agree well with symmetry analysis we performed. The six-fold component increases with carrier concentration, reaching 15% of the total AMR signal. Our results suggest the coupling between higher itinerant electronic bands and the crystal as the origin of this effect and demonstrate that the (111) oxide interface is a unique hexagonal system with tunable magnetocrystalline effects.

Superconductivity-induced shift of phonon frequency in the system with itinerant and strongly correlated electrons

We investigate a system consisting of strongly correlated localized and itinerant electron states mixed by optical vibration. The linear vibronic interaction causes the softening of active optical mode and it may induce a structural instability. The modification of renormalized phonon frequency by the gap in the energy of itinerant electrons in the superconducting phase has been established. The disposition of the electron spectrum and chemical potential influences substantially the corresponding effect. It is demonstrated that the presence of van Hove singularity in the center of itinerant electron band introduces qualitative changes into the superconductivity-induced shifts of phonon frequency.

Organometalliclike localization of4d-derived spins in an inorganic conducting niobium suboxide

Based on the refined crystal structure comprised of columns of 3*4 planar blocks of NbO6 octahedra and first principles electronic structure methods, we find that orthorhombic (o)-Nb$_{12}$O$_{29}$ introduces a new class of transition metal oxide. The electronic system consists of a large Nb dimer-based localized orbital comparable in size to those in organometallic compounds, yet is tightly bound and weakly interacting with itinerant electronic bands. These local moments, a rare occurrence for Nb, form one-dimensional spin chains that criss-cross perpendicularly oriented conducting nanowires. The local moment bandwidth is comparable to what is seen in rare earth compounds with extremely localized orbitals. The microscopic origin is traced to the local structure of the NbO6 octahedra and associated orbital+spin ordering. The resulting 1D$_s$*1D$_c$ anisotropic two dimensional Heisenberg-Kondo lattice model (s=spin, c=charge) provides a strongly anisotropic spin-fermion lattice system for further study.

Magnetic switching dynamics due to ultrafast exchange scattering: A model study

We study the heat-induced magnetization dynamics in a toy model of a ferrimagnetic alloy, which includes localized spins antiferromagnetically coupled to an itinerant carrier system with a Stoner gap. We determine the one-particle spin-density matrix including exchange scattering between localized and itinerant bands as well as scattering with phonons. While a transient ferromagnetic-like state can always be achieved by a sufficiently strong excitation, this transient ferromagnetic-like state only leads to magnetization switching for model parameters that also yield a compensation point in the equilibrium M(T) curve.

Distinct itinerant spin-density waves and local-moment antiferromagnetism in an intermetallic ErPd2Si2 single crystal.

Identifying the nature of magnetism, itinerant or localized, remains a major challenge in condensed-matter science. Purely localized moments appear only in magnetic insulators, whereas itinerant moments more or less co-exist with localized moments in metallic compounds such as the doped-cuprate or the iron-based superconductors, hampering a thorough understanding of the role of magnetism in phenomena like superconductivity or magnetoresistance. Here we distinguish two antiferromagnetic modulations with respective propagation wave vectors at Q± = (H ± 0.557(1), 0, L ± 0.150(1)) and QC = (H ± 0.564(1), 0, L), where (H, L) are allowed Miller indices, in an ErPd2Si2 single crystal by neutron scattering and establish their respective temperature- and field-dependent phase diagrams. The modulations can co-exist but also compete depending on temperature or applied field strength. They couple differently with the underlying lattice albeit with associated moments in a common direction. The Q± modulation may be attributed to localized 4f moments while the QC correlates well with itinerant conduction bands, supported by our transport studies. Hence, ErPd2Si2 represents a new model compound that displays clearly-separated itinerant and localized moments, substantiating early theoretical predictions and providing a unique platform allowing the study of itinerant electron behavior in a localized antiferromagnetic matrix.

Emergent topological mirror insulator int2g-orbital systems

Motivated by the itinerant band structure of high-${T}_{c}$ iron pnictides, which exhibit four Dirac cones in the bulk, we demonstrate the prospect of pnictides with transition elements to be topological insulators in two dimensions. In this report, we explore interaction-induced topological phases, in contrast to the spin-orbit-coupling interaction, as the crucial mechanism for tuning Dirac metals into ${Z}_{2}$-topological insulators protected by time reversal and mirror symmetries. We find spontaneous orbital currents generated through nearest-neighbor interorbital Coulomb interaction in the ${t}_{2g}$ manifold of the $d$ orbitals. When spin degrees of freedom are incorporated, spontaneous orbital currents lead to two stable topological phases of the ground state. The first topological insulator is an anomalous orbital Hall phase, characterized by an even Chern number, while the second topological insulator is realized by protected mirror symmetries with a ${Z}_{2}$ index.

Gutzwiller electronic structure calculations applied to transition metals: Kinetic energy gain with ferromagnetic order in bcc Fe

The Gutzwiller projector technique has long been known as a method to include correlations in electronic structure calculations. We describe a model implementation for a Gutzwiller + LDA calculation in a localized-orbital restricted basis framework, emphasizing the protocol step by step and illustrating our specific procedure for this and future applications. We demonstrate the method with a classic problem, the ferromagnetism of bulk bcc Fe, whose nature is attracting fresh interest. In the conventional Stoner-Wohlfarth model, and in spin-polarized LDA calculations, the ferromagnetic ordering of iron sets in so that the electrons can reduce their mutual Coulomb repulsion, at the cost of some increase of electron kinetic energy. This balance may, however, be altered by correlations, which are strong for localized d orbitals. The present localized basis Gutzwiller + LDA calculation demonstrates how the ferromagnetic ordering of Fe may, in fact, entrain a decrease of kinetic energy at the cost of some increase of potential energy. This happens because, as foreshadowed long ago by Goodenough and others and more recently supported by LDA-DMFT calculations, correlations cause e(g) and t(2g) d orbitals to behave differently, with the weakly propagating e(g) states fully spin polarized and almost localized, and only t(2g) states forming a broad partly filled itinerant band. Owing to an intra-atomic Hund's rule exchange that aligns e(g) and t(2g) spins, the propagation of itinerant t(2g) holes is favored when different atomic spins are ferromagnetically aligned. This suggests a strong analogy with double exchange in iron ferromagnetism.

Emergence of a coherent in-gap state in the SmB6 Kondo insulator revealed by scanning tunneling spectroscopy.

We use scanning tunneling microscopy to investigate the (001) surface of a cleaved SmB6 Kondo insulator. Variable temperature dI/dV spectroscopy up to 60K reveals a gaplike density of state suppression around the Fermi level, which is due to the hybridization between the itinerant Sm 5d band and localized Sm 4f band. At temperatures below 40K, a sharp coherence peak emerges within the hybridization gap near the lower gap edge. We propose that the in-gap resonance state is due to a collective excitation in magnetic origin with the presence of spin-orbital coupling and mixed valence fluctuations. These results shed new light on the electronic structure evolution and transport anomaly in SmB6.

Time-Dependent Impurity in Ultracold Fermions: Orthogonality Catastrophe and Beyond

Recent experimental realization of strongly imbalanced mixtures of ultracold atoms opens new possibilities for studying impurity dynamics in a controlled setting. We discuss how the techniques of atomic physics can be used to explore new regimes and manifestations of Anderson's orthogonality catastrophe (OC), which could not be accessed in solid state systems. We consider a system of impurity atoms localized by a strong optical lattice potential and immersed in a sea of itinerant Fermi atoms. Ramsey interference experiments with impurity atoms probe OC in the time domain, while radio-frequency (RF) spectroscopy probes OC in the frequency domain. The OC in such systems is universal for all times and is determined by the impurity scattering length and Fermi wave vector of itinerant fermions. We calculate the universal Ramsey response and RF absorption spectra. In addition to the standard power-law contribution, which corresponds to the excitation of multiple particle-hole pairs near the Fermi surface, we identify a novel contribution to OC that comes from exciting one extra particle from the bottom of the itinerant band. This gives rise to a non-analytic feature in the RF absorption spectra, which evolves into a true power-law singularity with universal exponent 1/4 at the unitarity. Furthermore, we discuss the manifestations of OC in spin-echo experiments, as well as in the energy counting statistic of the Fermi gas following a sudden quench of the impurity state. Finally, systems in which the itinerant fermions have two or more hyperfine states provide an even richer playground for studying non-equilibrium impurity physics, allowing one to explore non-equilibrium OC and to simulate quantum transport through nano-structures. This provides a useful connection between cold atomic systems and mesoscopic quantum transport.

Electron effective mass in n-type electron-induced ferromagnetic semiconductor (In,Fe)As: Evidence of conduction band transport

The electron effective mass (m*) in n-type carrier-induced ferromagnetic semiconductor (In,Fe)As was estimated by using the thermoelectric Seebeck effect. It was found that m* is 0.03 ∼ 0.17m0 depending on the electron concentration, where m0 is the free electron mass. These values are similar to those of electrons in the conduction band of n+ InAs. The Fermi level EF in (In,Fe)As is located at least 0.15 eV above the conduction band bottom. Our results indicate that electron carriers in (In,Fe)As reside in the conduction band, rather than in a hypothetical Fe-related itinerant impurity band.

Pressure effect on transport properties of NdCo2

Electrical resistivity ρ and thermopower S of the Laves phase compound of NdCo2 has been investigated at temperatures from 2 K to 300 K. ρ has been measured under pressures up to 8 GPa and S has been measured under pressures up to 3 GPa. The magnetic transition temperature TC obtained by ρ measurement decreases with increasing pressure. The temperature Tmim where the thermopower S takes minimum at high temperature region increases linearly with increasing pressure. The high-temperature minimum of S is associated with a sharp peak in density of states related mainly to the Co 3d-electron density. Since the width of an itinerant electronic band depends on the extent of the corresponding overlapping of 3d orbitals, the pressure variation of Tmin can be attributed to the broadening of the peak width of 3d electron density of states.

Exciton condensation and its influence on the specific heat

Abstract In rare earth compounds with localized 4f states the observation of bound 4f-hole-5d-electron states, excitons, is questionable. On the other hand the same compounds exhibit p–d excitons, which are derived from itinerant bands. In rare earth compounds, which exhibit intermediate valence, 4f–5d hybridization produces a narrow, several 10 meV wide 4f band. Now 4f–5d excitons are possible and have been observed in TmSe 0.45 Te 0.55 and similar compositions. The special band structure of these materials permits an enormous amount of excitons (≈10 21 cm −3 ), which condense in a first order transition at low temperatures and high pressure. This static and immense concentration of excitons dominates the heat conductivity and the thermal diffusivity and even exhibits the phenomenon of superfluidity in a solid. The measured specific heat shows that phonons couple to these excitons forming exciton polarons.

Weak antiferromagnetic superexchange interaction in fcc C60Hn

A recent density-functional calculation for fcc C${}_{60}$H${}_{n}$ ($n$ $=$ odd) [K. W. Lee and C. E. Lee, Phys. Rev. Lett. 106, 166402 (2011)] proposed the existence of Stoner ferromagnetism based on an itinerant band model. However, our density-functional calculation shows that the antiferromagnetic (AFM) configuration is slightly more stable than the ferromagnetic (FM) one. This preference for antiferromagnetism over ferromagnetism is analogous to the case of a dimer (C${}_{60}$H)${}_{2}$, where each C${}_{60}$H is spin polarized by an intramolecular exchange and the two magnetic moments are antiferromagnetically coupled with each other. The results demonstrate that the underlying mechanism of the magnetic order in fcc C${}_{60}$H${}_{n}$ is associated with the AFM superexchange between the magnetic moments created by H dopants.

Renormalization of spin polarised itinerant electron bands in the normal state of a model ferromagnetic superconductor**Project supported by the National Natural Science Foundation of China (Grant No. 10574063).

This paper studies the normal state properties of itinerant electrons in a toy model, which is constructed according to the model for coexisting ferromagnetism and superconductivity proposed by Suhl [Suhl H 2001 Phys. Rev. Lett. 87 167007]. In this theory with ferromagnetic ordering based on localized spins, the exchange interaction J between conduction electrons and localized spin is taken as the pairing glue for s-wave superconductivity. It shows that this J term will first renormalize the normal state single conduction electron structures substantially. It finds dramatically enhanced or suppressed magnetization of itinerant electrons for positive or negative J. Singlet Cooper pairing can be ruled out due to strong spin polarisation in the J > 0 case while a narrow window for s-wave superconductivity is opened around some ferromagnetic J.

EFFECTS OF PHOTO-EXCITATION AND MAGNON SCATTERING ON FERROMAGNETIC TRANSITION TEMPERATURE OF THE DILUTED MAGNETIC SEMICONDUCTOR (Ga1 - x, Mnx)As

Effects of photo-excitation and spin-wave scattering on magnetization of the diluted magnetic semiconductor (DMS) ( Ga , Mn ) As are theoretically studied. Green function formalism is used to find expression for magnetization and ferromagnetic transition temperature TC starting with a model Hamiltonian consisting of magnons, photons and an interaction of magnons with photons. According to our calculation, there is TC in the absence of magnetic impurity, x = 0, indicating that there could be electronically unpaired influential carriers/holes induced by photon irradiation resulting in residual itinerant band magnetization which can be revealed by experiments. Unusual upturn in magnetization near 0 K temperature values for larger magnon–photon coupling constant is also indicated. Moreover, enhancement of magnetization is established with increase in impurity concentration and even further in the presence of photon–magnon coupling which however decreases due to spin-wave scattering. This leads to the conclusion that at lower temperatures photon irradiation and at higher temperatures spin-wave scattering could affect the system properties predominantly.

Tunneling limit of heavy-fermion point contacts

We present results for a multichannel tunneling model that describes point-contact spectra between a metallic tip and a superconducting heavy-fermion system. We calculate tunneling spectra both in the normal and superconducting state. In point-contact and scanning tunneling spectroscopy many heavy-fermion materials, like CeCoIn5, exhibit an asymmetric differential conductance, dl/dV, combined with a strongly suppressed Andreev reflection signal in the superconducting state. For Andreev reflection to occur a junction has to be in the highly transparent limit. Here we focus on the opposite limit, namely that of low transparency leading to BCS-like dl/dV curves. We discuss the consequences of a multichannel tunneling model for CeCoIn5 assuming itinerant electron bands and localized f electrons.

Electronic structure of URu2Si2 in paramagnetic phase studied by soft x-ray photoemission spectroscopy

We have performed soft x-ray photoemission spectroscopy study on the heavy-fermion compound URu2Si2. We have found that the U 5f states are distributed in the binding energy range between 0.6 eV and the Fermi level. In angle-resolved photoemission spectra, we have observed the U 5f-Ru 4d hybridized band. Our results strongly suggest that the U 5f states form itinerant energy bands in this system.

Point-contact spectroscopy in heavy-fermion superconductors

We develop a microscopic model to calculate point-contact spectra between a metallic tip and a superconducting heavy-fermion system. We apply our tunneling model to the heavy fermion ${\text{CeCoIn}}_{5}$, both in the normal and superconducting states. In point contact and scanning tunneling spectroscopy many heavy-fermion materials, such as ${\text{CeCoIn}}_{5}$, exhibit an asymmetric differential conductance, $dI/dV$, combined with a strongly suppressed Andreev reflection signal in the superconducting state. We argue that both features may be explained in terms of a multichannel tunneling model in the presence of localized states near the interface. We find that it is not sufficient to tunnel into two itinerant bands of light and heavy electrons to explain the Fano line shape of the differential conductance. Localized states in the bulk or near the interface are an essential component for quantum interference to occur when an electron tunnels from the metallic tip of the point contact into the heavy-fermion system.

Paramagnetic limit in ferromagnetic superconductors with triplet pairing

The spin susceptibility in the uranium ferromagnet superconductors is calculated. There is shown that the absence of superconductivity paramagnetic limitation for the field directions perpendicular to the direction of the spontaneous magnetization is explained by the itinerant ferromagnet band splitting rather than by a rotation of magnetization toward the external field direction. The qualitative description of the upper critical field temperature dependence is given.

Electronic structure analysis of UIr using soft x-ray photoemission spectroscopy and band calculation

Uranium ferromagnet UIr is well-known to a pressure-induced superconductor without an inversion symmetry. In order to clarify the U 5f states of ferromagnetically-ordered UIr below Tc ~ 46 K and at ambient pressure, we observed 5f-sensitive soft X-ray photoemission spectra (SXPES), and calculated the band structure by a relativistic LAPW method in a local-spin density approximation. The temperature-dependent angle-integrated SXPES near the Fermi energy show a definite energy-shift of the band structures below and above Tc. The exchange splitting of the 5f bands seems to be consistent with an itinerant band picture like Stoner model. Furthermore the angle-resolved SXPES are compared with the theoretical band structure to investigate the relationship between the band shift and the magnetism.