Itinerant Band Research Articles

Mechanism of magnetic phase transition in correlated magnetic metal: insight into itinerant ferromagnet Fe3−δGeTe2

Developing a comprehensive magnetic theory for correlated itinerant magnets poses challenges due to the difficulty in reconciling both local moments and itinerant electrons. In this work, we investigate the microscopic process of magnetic phase transition in ferromagnetic metal Fe3−δGeTe2. We find that Hund’s coupling is crucial for establishing ferromagnetic order. During the ferromagnetic transition, we observe the formation of quasiparticle flat bands and an opposing tendency in spectral weight transfer, primarily between the lower and upper Hubbard bands, across the two spin channels. Moreover, our results indicate that one of the inequivalent Fe sites exhibits Mott physics, while the other Fe site exhibits Hund’s physics, attributable to their distinct atomic environments. We suggest that ferromagnetic order reduces spin fluctuations and makes flat bands near the Fermi level more distinct. The hybridization between the distinctly flat bands and other itinerant bands offers a possible way to form heavy fermion behavior in ferromagnets. The complex interactions of competing orders drive correlated magnetic metals to a new frontier for discovering outstanding quantum states.

Mechanism Behind the Enhanced Ferromagnetic Contributions Upon Ru Substitution in Ca3Mn2O7 from X-Ray Absorption Spectroscopies.

We have studied the local structure and electronic and magnetic properties of hybrid improper ferroelectric Ca3Mn2O7 upon Ru substitution at the Mn site by a combination of atomic-selective X-ray absorption spectroscopies in the soft and hard X-ray energy regimes. Ru substitution enhances the macroscopic ferromagnetic contributions, whose origin is here elucidated. In particular, soft X-ray magnetic circular dichroism (XMCD) data indicate that the spin moments of Mn and Ru are aligned in opposite directions, with the effective magnetic moments of Ru being about 1 order of magnitude smaller than for Mn. The XMCD results also demonstrate the presence of an intrinsic ferromagnetic component in the Mn sublattice for Ru contents x ≥ 0.3, although the values of the net Mn magnetic moment are much smaller than expected for a fully saturated Mn4+ magnetic sublattice. Soft and hard X-ray absorption spectroscopy measurements show that Mn keeps a +4 valence state independently of the Ru content, whereas Ru is in an intermediate valence state, ranging between +4.7 (x ≤ 0.1) and +4.4 (x ≥ 0.7). Analysis of the extended X-ray absorption fine structure (EXAFS) signals at the Mn and Ru K-edges suggests the lack of a complete solid solution in the local structure across the entire range of compositions. These findings disclose the existence of charge transfer between Mn and Ru atoms, so the weak ferromagnetic component is attributed to a canting of the antiferromagnetically ordered Mn4+ spins caused by the oxygen-mediated hybridization between localized Mn4+ 3d and itinerant intermediate valence Ru 4d bands, in analogy to the mechanism proposed for ferromagnetism in ordered doubled perovskites with 3d and 4d/5d transition-metal ions. In the present case, disorder prevents the formation of long-range ferromagnetism.

Critical behavior and exchange splitting in the two-dimensional antiferromagnet Mn on Re(0001)

We investigated the temperature-dependent electronic structure of the antiferromagnetic fcc-monolayer Mn on Re(0001) using vacuum-ultraviolet momentum microscopy. At $T=25$ K the collinear, row-wise antiferromagnetic phase of the Mn monolayer results in a spin splitting of states. Density-functional theory, being in good agreement with the experimental results, reveals the spin and orbital projection of the observed electronic bands. The exchange split bands shift in opposite directions with increasing temperature, decreasing the exchange splitting of a pair of itinerant bands from $280\ifmmode\pm\else\textpm\fi{}10$ meV at 25 K down to $185\ifmmode\pm\else\textpm\fi{}10$ meV at the N\'eel temperature ${T}_{\text{N}}=75\ifmmode\pm\else\textpm\fi{}5$ K. The exchange splitting remains constant for $T>{T}_{\text{N}}$. The persisting exchange splitting is attributed to a remaining short-range, fluctuating antiferromagnetic order far above ${T}_{\text{N}}$.

Complex electronic structure evolution of NdSb across the magnetic transition

The rare-earth monopnictide (REM) family, which hosts magnetic ground states with extreme magnetoresistance, has established itself as a fruitful playground for the discovery of interesting topological phases. Here, by using high-resolution angle-resolved photoemission spectroscopy complemented by first-principles density-functional-theory-based modeling, we examine the evolution of the electronic structure of the candidate REM Dirac semimetal NdSb across the magnetic transition. A complex angel-wing-like band structure near the zone center along with arclike features at the zone center and the zone corner are observed. This dramatic reconstruction of the itinerant bands around the zone center is shown to be driven by a magnetic transition: Specifically, the Nd $5d$ electron band backfolds at the $\overline{\mathrm{\ensuremath{\Gamma}}}$ point and hybridizes with the Sb $5p$ hole bands in the antiferromagnetic phase. Our study indicates that antiferromagnetism plays an intricate role in the electronic structure of the REM family.

Electron tunneling spectroscopy of an anisotropic Kitaev quantum spin liquid sandwiched between superconductors

We present the electron tunneling transport and spectroscopic characters of a superconducting (SC) Josephson junction with a barrier of a single anisotropic Kitaev quantum spin liquid (QSL) layer. We find that the dynamical spin-correlation features are well reflected in the direct-current differential conductance $d{I}^{c}/dV$ of single-particle tunneling, including the unique spin gap and dressed itinerant Majorana dispersive band, in addition to an energy shift $2\mathrm{\ensuremath{\Delta}}$ from the two-SC-lead gap. From the spectral characters, we identify different topological quantum phases of the anisotropic Kitaev QSL. We also present the zero-voltage Josephson current ${I}^{s}$, which displays residual features of the anisotropic Kitaev QSL. These results pave a new path to measurement of dynamical spinon or Majorana fermion spectroscopy of Kitaev and other spin-liquid materials.

Interlayer Coupling of a Two-Dimensional Kondo Lattice with a Ferromagnetic Surface in the Antiferromagnet CeCo2P2.

The f-driven temperature scales at the surfaces of strongly correlated materials have increasingly come into the focus of research efforts. Here, we unveil the emergence of a two-dimensional Ce Kondo lattice, which couples ferromagnetically to the ordered Co lattice below the P-terminated surface of the antiferromagnet CeCo2P2. In its bulk, Ce is passive and behaves tetravalently. However, because of symmetry breaking and an effective magnetic field caused by an uncompensated ferromagnetic Co layer, the Ce 4f states become partially occupied and spin-polarized near the surface. The momentum-resolved photoemission measurements indicate a strong admixture of the Ce 4f states to the itinerant bands near the Fermi level including surface states that are split by exchange interaction with Co. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario. Our findings show how rich and diverse the f-driven properties can be at the surface of materials without f-physics in the bulk.

Structural, electronic and magnetic properties of weakly correlated metal Sr2CrTiO6: a first principles study

Structural, electronic and magnetic behaviour of a less-explored material Sr2CrTiO6 has been investigated using ab initio calculations with generalized gradient approximation (GGA) and GGA + U methods, where U is the Hubbard parameter. For each of the three possible Cr/Ti cationic arrangements in the unit cell, considered in this work, the non-magnetic band structure shows distinct traits with significant flat-band regions leading to large t 2g density of states around the Fermi energy. The Cr4+ ion in Sr2CrTiO6, which is a d 2 system, shows a reverse splitting of the t 2g orbitals. The calculated hopping matrix contains non-zero off-diagonal elements between the d xz and d yz orbitals, while the d xy orbitals remain largely unmixed. A minimal tight binding model successfully reproduces the six t 2g bands around the Fermi energy. The Fermi surface shows a two-dimensional nesting feature for the layered arrangement of Cr and Ti atoms. Fixed spin moment studies suggest that the magnetism in this compound cannot be explained by Stoner’s criterion of an itinerant band ferromagnet. In the absence of Hubbard U term, the ground state is a half-metallic ferromagnet. Calculations for the anti-ferromagnetic spin arrangement show re-arrangement of orbital character resulting in (a) narrow d xz /d yz bands and sharp peaks in the density of states at the Fermi energy and (b) highly dispersive d xy bands with a broader density of states around the Fermi energy. The metallicity persists even with increasing U for both the spin arrangements, thus suggesting that Sr2CrTiO6 belongs to the class of weakly correlated metals, while the shifting of O 2p states towards the Fermi energy with U indicates a negative charge-transfer character in Sr2CrTiO6.

Imaging the itinerant-to-localized transmutation of electrons across the metal-to-insulator transition in V2O3.

In solids, strong repulsion between electrons can inhibit their movement and result in a “Mott” metal-to-insulator transition (MIT), a fundamental phenomenon whose understanding has remained a challenge for over 50 years. A key issue is how the wave-like itinerant electrons change into a localized-like state due to increased interactions. However, observing the MIT in terms of the energy- and momentum-resolved electronic structure of the system, the only direct way to probe both itinerant and localized states, has been elusive. Here we show, using angle-resolved photoemission spectroscopy (ARPES), that in V2O3, the temperature-induced MIT is characterized by the progressive disappearance of its itinerant conduction band, without any change in its energy-momentum dispersion, and the simultaneous shift to larger binding energies of a quasi-localized state initially located near the Fermi level.

Multicritical points in a model for 5f-electron systems under pressure and magnetic field

We investigate the evolution of multicritical points under pressure and magnetic field in a model described by two 5f bands (called α and β) that hybridize with a single itinerant conduction band. The interaction is given by the direct Coulomb and the Hund’s rule exchange terms. Three types of orderings are considered: two conventional spin density waves (SDWs) and an exotic SDW, i.e., with no magnetic moment formation. The conventional SDWs phases, are characterized by and , respectively, where and are the intraband staggered magnetizations. The exotic SDW, which has time reversal symmetry, is described by a purely imaginary order parameter. This phase is related to a band mixing given by the spin-flip part of the Hund’s rule exchange interaction. As result, without magnetic field, the phase diagrams of temperature (T) versus pressure (given by the variation of the bandwidth (W)) shows a sequence of phase transitions involving the three phases which gives rise to multicritical points. The presence of the magnetic field (h z ) has drastic effects on part of the phase diagram and the location of the multicritical points.

Unexpected differences between surface and bulk spectroscopic and implied Kondo properties of heavy fermion CeRh2Si2

Ultra-violet angle-resolved photoemission spectroscopy (UV-ARPES) was used to explore the temperature dependence of the Ce-4f spectral responses for surface and bulk in the antiferromagnetic Kondo lattice CeRh2Si2. Spectra were taken from Ce- and Si-terminated surfaces in a wide temperature range, and reveal characteristic 4f patterns for weakly (surface) and strongly (bulk) hybridized Ce, respectively. The temperature dependence of the Fermi level peak differs strongly for both cases implying that the effective Kondo temperature at the surface and bulk can be rather distinct. The greatly reduced crystal–electric-field (CEF) splitting at the surface gives reason to believe that the surface may exhibit a larger effective Kondo temperature because of a higher local-moment effective degeneracy. Further, the hybridization processes could strongly affect the 4f peak intensity at the Fermi level. We derived the k-resolved dispersion of the Kondo peak which is also found to be distinct due to different sets of itinerant bands to which the 4f states of surface and bulk Ce are coupled. Overall our study brings into reach the ultimate goal of quantitatively testing many-body theories that link spectroscopy and transport properties, for both the bulk and the surface, separately. It also allows for a direct insight into the broader problem of Kondo lattices with two different local-moment sublattices, providing some understanding of why the cross-talking between the two Kondo effects is weak.

Magnetic frustration in a metallic fcc lattice

Magnetic frustration in metals is scarce and hard to pinpoint, but exciting due to the possibility of the emergence of fascinating novel phases. The cubic intermetallic compound HoInCu$_4$ with all holmium atoms on an fcc lattice, exhibits partial magnetic frustration, yielding a ground state where half of the Ho moments remain without long-range order, as evidenced by our neutron scattering experiments. The substitution of In with Cd results in HoCdCu$_4$ in a full breakdown of magnetic frustration. Consequently we found a fully ordered magnetic structure in our neutron diffraction experiments. These findings are in agreement with the local energy scales and crystal electric field excitations, which we determined from specific heat and inelastic neutron scattering data. The electronic density of states for the itinerant bands acts as tuning parameter for the ratio between nearest-neighbor and next-nearest-neighbor interactions and thus for magnetic frustration.

Unfolding of antiferromagnetic phases and multicritical points in a two-orbital model for uranium compounds under pressure and magnetic field

We investigate the occurrence of multicritical points under pressure and magnetic field in a model that describes two $5f$ bands (of either $\ensuremath{\alpha}$ or $\ensuremath{\beta}$ characters), which hybridize with a single itinerant conduction band. The $5f$ electrons interact through Coulomb and exchange terms. The antiferromagnetic (AF) order parameter is a N\'eel vector, which is assumed to be fixed by an Ising anisotropy. The applied magnetic field is transverse to the anisotropy axis. Without field, our results for the temperature-pressure phase diagram show that at low temperatures a first-order phase transition occurs between two distinct antiferromagnetic phases, ${\mathrm{AF}}_{1}$ and ${\mathrm{AF}}_{2}$, as the pressure is increased. The two phases are characterized by the gaps of bands $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ given by ${\mathrm{\ensuremath{\Delta}}}_{\ensuremath{\alpha}}$ and ${\mathrm{\ensuremath{\Delta}}}_{\ensuremath{\beta}}$, respectively. The ${\mathrm{AF}}_{1}$ phase occurs when ${\mathrm{\ensuremath{\Delta}}}_{\ensuremath{\beta}}>{\mathrm{\ensuremath{\Delta}}}_{\ensuremath{\alpha}}>0$, while in the ${\mathrm{AF}}_{2}$ phase the gaps satisfy ${\mathrm{\ensuremath{\Delta}}}_{\ensuremath{\alpha}}>{\mathrm{\ensuremath{\Delta}}}_{\ensuremath{\beta}}>0$. The application of a magnetic field produces a drastic change in the phase diagram. The ${\mathrm{AF}}_{1}$ and ${\mathrm{AF}}_{2}$ phases separate, with the latter acquiring a dome shape that is eventually suppressed for large values of the applied field. The evolution of the phase diagram under pressure, without and with magnetic field, shows the presence of multicritical points. Our results show that the evolution of these multicritical points by the simultaneous application of pressure and magnetic field is also drastic, with the suppression of some multicritical points and the emergence of others. We believe that these results may have relevance for the growing field of multicritical points (classical and quantum) in the physics of uranium compounds.

Large Fermi surface expansion through anisotropic mixing of conduction and f electrons in the semimetallic Kondo lattice CeBi

Using angle-resolved photoemission spectroscopy (ARPES) and resonant ARPES, we report evidence of strong anisotropic conduction-f electron mixing (c-f mixing) in CeBi by observing a largely expanded Ce-5d pocket at low temperature, with no change in the Bi-6p bands. The Fermi surface (FS) expansion is accompanied by a pronounced spectral weight transfer from the local 4f 0 peak of Ce (corresponding to Ce3+) to the itinerant conduction bands near the Fermi level. Careful analysis suggests that the observed large FS change (with a volume expansion of the electron pocket up to 40%) can most naturally be explained by a small valence change (~ 1%) of Ce, which coexists with a very weak Kondo screening. Our work therefore provides evidence for a FS change driven by real charge fluctuations deep in the Kondo limit, which is made possible by the low carrier density.

Topological superfluidity with repulsive alkaline-earth atoms in optical lattices

We discuss a realization of topological superfluidity with fermionic atoms in an optical lattice. We consider a situation where atoms in two internal states experience different lattice potentials: one species is localized and the other itinerant, and show how quantum fluctuations of the localized fermions give rise to an attraction and spin–orbit coupling in the itinerant band. At low temperature, these effects stabilize a topological superfluid of mobile atoms even if their bare interactions are repulsive. This emergent state can be engineered with alkaline-earth atoms in a superlattice with a dimerized unit cell. To probe its unique properties we describe protocols that use high spectral resolution and controllability of a narrow clock transition, such as momentum-resolved spectroscopy and supercurrent response to a synthetic (laser-induced) magnetic field.

Spin States Protected from Intrinsic Electron-Phonon Coupling Reaching 100 ns Lifetime at Room Temperature in MoSe2.

We present time-resolved Kerr rotation measurements, showing spin lifetimes of over 100 ns at room temperature in monolayer MoSe2. These long lifetimes are accompanied by an intriguing temperature-dependence of the Kerr amplitude, which increases with temperature up to 50 K and then abruptly switches sign. Using ab initio simulations, we explain the latter behavior in terms of the intrinsic electron-phonon coupling and the activation of transitions to secondary valleys. The phonon-assisted scattering of the photoexcited electron-hole pairs prepares a valley spin polarization within the first few ps after laser excitation. The sign of the total valley magnetization, and thus the Kerr amplitude, switches as a function of temperature, as conduction and valence band states exhibit different phonon-mediated intervalley scattering rates. However, the electron-phonon scattering on the ps time scale does not provide an explanation for the long spin lifetimes. Hence, we deduce that the initial spin polarization must be transferred into spin states, which are protected from the intrinsic electron-phonon coupling, and are most likely resident charge carriers, which are not part of the itinerant valence or conduction band states.

Density-functional theory for plutonium

We review developments in the theoretical description and understanding of plutonium in terms of a metal with itinerant (band) 5f electrons. Within this picture most facets of this remarkable and a...

Effect of laser induced orbital momentum on magnetization switching

The observed magnetization switching by circularly polarized ultrafast laser pulses has been attributed to the inverse Faraday effect in which the induced non-equilibrium orbital momentum serves as an effective magnetic field via spin-orbit coupling for magnetization rotation and switching. We critically examine this scenario by explicitly calculating the magnitude of the induced orbital momentum for generic itinerant band. We show that the calculated induced angular momentum is not large enough for reversing the magnetization by one laser pulse with the order of 100 femtosecond duration. Instead, we propose that each laser pulse is capable to expand a reverse domain a few nano-meters and it takes multiple pulses to complete the magnetization reversal process via domain wall motion.

Pressure dependence of coherence-incoherence crossover behavior in KFe2As2 observed by resistivity and As75 -NMR/NQR

We present the results of $^{75}$As nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and resistivity measurements in KFe$_2$As$_2$ under pressure ($p$). The temperature dependence of the NMR shift, nuclear spin-lattice relaxation time ($T_1$) and resistivity show a crossover between a high-temperature incoherent, local-moment behavior and a low-temperature coherent behavior at a crossover temperature ($T^*$). $T^*$ is found to increase monotonically with pressure, consistent with increasing hybridization between localized $3d$ orbital-derived bands with the itinerant electron bands. No anomaly in $T^*$ is seen at the critical pressure $p_{\rm c}=1.8$ GPa where a change of slope of the superconducting (SC) transition temperature $T_{\rm c}(p)$ has been observed. In contrast, $T_{\rm c}(p)$ seems to correlate with antiferromagnetic spin fluctuations in the normal state as measured by the NQR $1/T_1$ data, although such a correlation cannot be seen in the replacement effects of A in the AFe$_2$As$_2$ (A= K, Rb, Cs) family. In the superconducting state, two $T_1$ components are observed at low temperatures, suggesting the existence of two distinct local electronic environments. The temperature dependence of the short $T_{\rm 1s}$ indicates nearly gapless state below $T_{\rm c}$. On the other hand, the temperature dependence of the long component 1/$T_{\rm 1L}$ implies a large reduction in the density of states at the Fermi level due to the SC gap formation. These results suggest a real-space modulation of the local SC gap structure in KFe$_2$As$_2$ under pressure.

Impact of superconducting gap on the phonon mode mixing itinerant and localized electron states

The influence of superconducting ordering on the optical lattice vibration modified through the mixing of localized states with Hubbard correlation and itinerant band states is analyzed. We consider the situation where the broad electron band is located between lower and upper Hubbard levels. The hardening or softening of phonon dynamics caused by the opening of superconducting gap in the band of itinerant carriers appears in dependence on the disposition of the electron spectrum and chemical potential. The effect varies substantially if the chemical potential approaches the edges of the band of itinerant carriers or the van Hove singularity.

Heavy fermion behavior in the quasi-one-dimensional Kondo lattice CeCo2Ga8

Dimensionality plays an essential role in determining the anomalous non-Fermi liquid properties in heavy fermion systems. So far most heavy fermion compounds are quasi-two-dimensional or three-dimensional. Here we report the synthesis and systematic investigations of the single crystals of the quasi-one-dimensional Kondo lattice CeCo2Ga8. Resistivity measurements at ambient pressure reveal the onset of coherence at T * ≈ 20 K and non-Fermi liquid behavior with linear temperature dependence over a decade in temperature from 2 to 0.1 K. The specific heat increases logarithmically with lowering temperature between 10 and 2 K and reaches 800 mJ/mol K2 at 1 K, suggesting that CeCo2Ga8 is a heavy fermion compound in the close vicinity of a quantum critical point. Resistivity measurements under pressure further confirm the non-Fermi liquid behavior in a large temperature–pressure range. The magnetic susceptibility is found to follow the typical behavior for a one-dimensional spin chain from 300 K down to T *, and first-principles calculations predict flat Fermi surfaces for the itinerant f-electron bands. These suggest that CeCo2Ga8 is a rare example of the quasi-one-dimensional Kondo lattice, but its non-Fermi liquid behaviors resemble those of the quasi-two-dimensional YbRh2Si2 family. The study of the quasi-one-dimensional CeCo2Ga8 family may therefore help us to understand the role of dimensionality on heavy fermion physics and quantum criticality.