Enhancement Of Effective Mass Research Articles

Pressure induced superconductivity on the border of magnetic order in MnP.

We report the discovery of superconductivity on the border of long-range magnetic order in the itinerant-electron helimagnet MnP via the application of high pressure. Superconductivity with T(sc)≈1 K emerges and exists merely near the critical pressure P(c)≈8 GPa, where the long-range magnetic order just vanishes. The present finding makes MnP the first Mn-based superconductor. The close proximity of superconductivity to a magnetic instability suggests an unconventional pairing mechanism. Moreover, the detailed analysis of the normal-state transport properties evidenced non-Fermi-liquid behavior and the dramatic enhancement of the quasiparticle effective mass near P(c) associated with the magnetic quantum fluctuations.

Fermi Surface Instabilities in Ferromagnetic Superconductor URhGe

The field-reentrant (field-reinforced) superconductivity on ferromagnetic superconductors is one of the most interesting topics in unconventional superconductivity. The enhancement of effective mass and the induced ferromagnetic fluctuations play key roles for reentrant superconductivity. However, the associated change of the Fermi surface, which is often observed at (pseudo-) metamagnetic transition, can also be a key ingredient. In order to study the Fermi surface instability, we performed Hall effect measurements in the ferromagnetic superconductor URhGe. The Hall effect of URhGe is well explained by two contributions, namely by the normal Hall effect and by the large anomalous Hall effect due to skew scattering. The large change in the Hall coefficient is observed at low fields between the paramagnetic and ferromagnetic states for H // c-axis (easy-magnetization axis) in the orthorhombic structure, indicating that the Fermi surface is reconstructed in the ferromagnetic state below the Curie temperature (T_Curie=9.5K). At low temperatures (T << T_Curie), when the field is applied along the b-axis, the reentrant superconductivity was observed in both the Hall resistivity and the magnetoresistance below 0.4K. Above 0.4K, a large jump with the first-order nature was detected in the Hall resistivity at a spin-reorientation field H_R ~ 12.5T, demonstrating that the marked change of the Fermi surface occurs between the ferromagnetic state and the polarized state above H_R. The results can be understood by the Lifshitz-type transition, induced by the magnetic field or by the change of the effective magnetic field.

Normal-state correlated electronic structure of tetragonal FeSe superconductor

Tetragonal FeSe, a prototype iron-chalcogenide superconductor, shows signatures of a strange incoherent normal state. Motivated thereby, we use LDA+DMFT to show how multi-band correlations generate a low-energy pseudogap in the normal state, giving an incoherent metal in good semi-quantitative agreement with observations. Anomalous responses in the normal state, including orbital-dependent effective mass enhancement and photoemission lineshape, are consistently understood.

Correlated band structure of superconducting NdFeAsO0.9F0.1: Dynamical mean-field study

We report the LDA + DMFT (method combining local density approximation with dynamical mean-field theory) results for spectral properties of the superconductor NdFeAsO0.9F0.1 in the paramagnetic phase. The calculated momentum-resolved spectral functions are in good agreement with angle-resolved photoemission spectra (ARPES). The obtained effective quasiparticle mass enhancement (m*/m = 1.4) is smaller than the one in isostructural parent compound LaFeAsO which critical temperature under the same fluorine doping (LaFeAsO0.9F0.1) is two times lower. Our results demonstrate that in quaternary FeAs-based superconductors of the same class, changes of the crystal structure caused by substitution of one rare-earth atom, implicitly result in reduction of the electronic correlation strength.

Understanding of the Temperature–Pressure Phase Diagram of β-Pyrochlore Oxides: A Role of Anhamonicity on Superconductivity

We develop a physical understanding for how anhamonicity of lattice vibration affects transition temperature, \(T_{\text{c}}\), of superconductivity mediated by phonons, via specific heat measurements on β-pyrochlore oxide superconductors under high pressure and in magnetic fields. We find that \(T_{\text{c}}\) demonstrates unimodal dependence on pressure, where \(T_{\text{c}}\) increases more than three times by pressure. On the other hand, an electron–phonon coupling constant, λ, is monotonically enhanced as pressure is applied, leading to the enhancement of effective mass of conduction electrons. From the analysis based on the McMillan equation, moreover, an average phonon frequency, ωlog, is found to reflect a downward trend on all the pressure regions, which is consistent with the results of the previous inelastic neutron scattering and specific heat measurements. The downward trend of ωlog means that the application of pressure enhances a magnitude of anhamonicity, contrary to a common sense of pres...

High-resolution angle-resolved photoemission study of electronic structure and electron self-energy in palladium

In this study, we investigate the electronic structure and electron self-energy of palladium single crystals using polarization-dependent high-resolution angle-resolved photoemission spectroscopy. The observed Fermi surfaces and energy-band dispersions agree with those given by the band-structure calculation. A detailed comparison between the observed and theoretical band dispersions of the ${\ensuremath{\Sigma}}_{1}$ band forming the electronlike Fermi surface indicates an electron-electron coupling parameter of ${\ensuremath{\lambda}}_{ee}\ensuremath{\sim}0.02$. Near the Fermi level, a kink structure in the energy-band dispersion exists at $\ensuremath{\sim}\ensuremath{-}20$ meV, in agreement with the Debye energy. The electron-phonon coupling parameter is estimated to be ${\ensuremath{\lambda}}_{ep}=0.39\ifmmode\pm\else\textpm\fi{}0.05$ at 8 K for the ${\ensuremath{\Sigma}}_{1}$ band, which is consistent with the theoretical values of ${\ensuremath{\lambda}}_{ep}=0.35--0.41$. Furthermore, analyses of the self-energy indicate a possible contribution from the electron-paramagnon interaction in the energy range of $\ensuremath{-}50\ensuremath{\sim}\ensuremath{-}150$ meV. The evaluated electron-paramagnon coupling parameter is ${\ensuremath{\lambda}}_{em}\ensuremath{\sim}0.06$ for the ${\ensuremath{\Sigma}}_{1}$ band. We found that the magnitudes of ${\ensuremath{\lambda}}_{ep}$ and ${\ensuremath{\lambda}}_{em}$ depend on the Fermi surface points. The total effective mass enhancement factor is estimated to be $1+{\ensuremath{\lambda}}_{ep}+{\ensuremath{\lambda}}_{ee}+{\ensuremath{\lambda}}_{em}\ensuremath{\sim}1.5$ for the ${\ensuremath{\Sigma}}_{1}$ band, which is close to the values ${m}^{*}/{m}_{b}\ensuremath{\sim}1.5--1.7$ given by the de Haas--van Alphen and electron specific-heat measurements.

Spectral properties of LiFeAs: An LDA+DMFT study

Spectral properties of LiFeAs superconductor are investigated within the LDA+DMFT method. Calculated distribution of the spectral weight in the k-space is in good agreement with angle-resolved photoemission (ARPES) spectra. Calculated effective electron mass enhancement factor m*/m ≈ 3 is close to the one estimated from comparison of density-functional theory results with ARPES spectra. Our results demonstrate that inclusion into consideration of dynamical Coulomb correlations between the electrons plays a key role in understanding of the spectral properties of LiFeAs.

New view of the occupied band structure of Mo(112)

We present a comprehensive examination of the occupied surface-weighted band structure of Mo(112) along the two high-symmetry directions of the surface Brillouin zone, both from theoretical and experimental perspectives. The band structures are found to be significantly different for the states along the two high-symmetry directions and for the states with even and odd reflection parities with respect to the mirror planes. The present study suggests the existence of a number of surface-weighted bands along both high-symmetry directions. The complexity of the band structure near the Fermi level may impose potential difficulties in experimental determination of the electron-phonon coupling parameters based on the effective mass enhancement distortion (or kink) in the energy-band dispersion, in the vicinity of the Fermi level, for several surface resonance bands of Mo(112).

Anderson lattice with explicit Kondo coupling revisited: metamagnetism and the field-induced suppression of the heavy fermion state

We apply the extended (statistically consistent, SCA) Gutzwiller-type approach to the periodic Anderson model (PAM) in an applied magnetic field and in the strong-correlation limit. The finite-U corrections are included systematically by transforming the PAM into the form with the Kondo-type interaction and the residual hybridization, both appearing at the same time and on equal footing. This effective Hamiltonian represents the essence of our Anderson–Kondo lattice model. We show that in ferromagnetic phases the low-energy single-particle states are strongly affected by the presence of the applied magnetic field. We also find that for large values of hybridization strength the system enters the so-called locked heavy fermion state introduced earlier. In this state the chemical potential lies in the majority-spin hybridization gap and, as a consequence, the system evolution is insensitive to further increase of the applied field. However, for a sufficiently strong magnetic field, the system transforms from the locked state to the fully spin-polarized phase. This is accompanied by a metamagnetic transition, as well as by a drastic reduction of the effective mass of the quasiparticles. In particular, we observe no effective mass enhancement in the fully polarized state. The findings are in overall agreement with experimental results for the Ce compounds in high magnetic fields. The mass enhancement for the spin-minority electrons may also diminish with the increasing field, unlike for the quasiparticle states in a single narrow band in the same limit of strong correlations.

Anomalous Resistivity and the Electron–Polaron Effect in the Two-Band Hubbard Model with One Narrow Band

We search for anomalous normal and superconductive behavior in the two-band Hubbard model with one narrow band. We analyze the influence of the electron–polaron effect and the Altshuler–Aronov effect on effective mass enhancement and scattering times of heavy and light components in the clean case. We find anomalous behavior of resistivity at high temperatures $T > W_{h}^{*}$ both in 3D and 2D situations. The SC instability in the model is governed by an enhanced Kohn–Luttinger effect for p-wave pairing of heavy electrons via polarization of light electrons.

Doping evolution of the optical scattering rate and effective mass of Bi2Sr2−xLaxCuO6

We determined the optical conductivity of Bi(2)Sr(2-x)La(x)CuO(6) at dopings covering the phase diagram from the underdoped to the overdoped regimes. The frequency dependent scattering rate shows a pseudogap extending into the overdoped regime. We found that the effective mass enhancement calculated from the optical conductivity is constant throughout the phase diagram. Conversely, the effective optical charge density varies almost linearly with doping. Our results suggest that the low frequency electrodynamics of Bi(2)Sr(2-x)La(x)CuO(6) is not strongly affected by the long range Mott transition.

Effective mass enhancement and spin-glass behaviour in CeCu4MnyAl1−y compounds

We report on the magnetic ac/dc susceptibility and specific heat measurements for the CeCu4MnyAl1−y series of compounds with 0 ≤ y ≤ 1. All compounds investigated crystallize in the hexagonal CaCu5-type structure with the space group P6/mmm. The results reveal that the frustration of the interactions dominates and leads to a spin-glass (SG) behaviour with a linear change of the freezing temperature Tf as a function of y. The SG state has been confirmed by the frequency dependence of the ac magnetic susceptibility, the relaxation of the remanent magnetization and the split of the field-cooled–zero-field-cooled dc magnetic susceptibility. The electronic specific heat coefficient γ is enhanced for all y and increases with the Al content. Below a threshold at y ≈ 0.3, γ is large even if determined for temperatures above Tf; moreover the paramagnetic Curie–Weiss temperature θp changes sign to negative. These observations indicate the possible presence of the heavy fermion (HF) state, at least below y ≈ 0.3, and the possible coexistence of the SG and HF states down to y = 0, i.e. for the CeCu4Al compound. A tentative magnetic phase diagram for CeCu4MnyAl1−y has been constructed.

Sudden slowing down of charge carrier dynamics at the Mott metal–insulator transition in κ-(D8-BEDT-TTF)2Cu[N(CN)2]Br

We investigate the dynamics of correlated charge carriers in the vicinity of the Mott metal–insulator (MI) transition in the quasi-two-dimensional organic charge-transfer salt κ-(D8-BEDT-TTF)2Cu[N(CN)2]Br by means of fluctuation (noise) spectroscopy. The observed 1/f-type fluctuations are quantitatively very well described by a phenomenological model based on the concept of non-exponential kinetics. The main result is a correlation-induced enhancement of the fluctuations accompanied by a substantial shift of spectral weight to low frequencies in the vicinity of the Mott critical endpoint. This sudden slowing down of the electron dynamics, observed here in a pure Mott system, may be a universal feature of MI transitions. Our findings are compatible with an electronic phase separation in the critical region of the phase diagram and offer an explanation for the not yet understood absence of effective mass enhancement when crossing the Mott transition.

Coupling parameters of many-body interactions for the Al(100) surface state: A high-resolution angle-resolved photoemission spectroscopy study

We examined the dimensionless coupling parameters of many-body interactions for a free-electron-like surface-derived state in Al(100) by means of high-resolution angle-resolved photoemission spectroscopy. A kink structure was found to exist in the energy-band dispersion near the Fermi level (${E}_{F}$), which was attributed to the electron-phonon interaction. At 50 K, the coupling parameters of the electron-phonon and electron-electron interactions were estimated as ${\ensuremath{\lambda}}_{ep}=0.67\ifmmode\pm\else\textpm\fi{}0.05$ and ${\ensuremath{\lambda}}_{ee}\ensuremath{\sim}0.003$, respectively, indicating that the effective mass enhancement was mainly derived from the electron-phonon interaction. The temperature dependence of the kink structure, as measured by ${\ensuremath{\lambda}}_{ep}(T)$, was consistent with a theoretical calculation based on the Eliashberg function. A quasiparticle peak with a width of $15--20$ meV was found near ${E}_{F}$, which was explained well by the simulated spectral function incorporating the self-energy evaluated in this study. We found that the electrons at the surface were strongly scattered by the defects at the surface and that the linewidth was significantly broadened (${\ensuremath{\Gamma}}_{0}=0.238\ifmmode\pm\else\textpm\fi{}0.006$ eV).

Optical conductivity of geometrical frustrated electronic systems

We study optical conductivity of the one-band Hubbard model on a two-dimensional isotropic triangular lattice at half filling by using the cellular dynamical mean field theory and particularly investigate its behavior near the critical end point of the first order Mott transition. In the metallic phase, the enhancement of effective mass and the significant frequency dependence of scattering rate is identified near the Mott transition. From the results in the insulating state near the Mott transition, we find that the frequency dependence of optical conductivity decays quite smoothly toward zero frequency and exhibits a power-law-like behavior in an intermediate frequency region. We also study optical conductivity of the Hubbard model on a kagomé lattice near the critical end point of the Mott transition. It shows a larger incoherent peak both in the metallic and insulating phases and a stronger frequency dependence of effective mass and scattering rate.

Disordered polyhalide anion effect on the Fermi surface of the incommensurate organic superconductor (MDT-TSF)I0.77Br0.52

The effect of disorder in polyhalide anions is investigated in terms of the degree of charge transfer and the Fermi surface in the incommensurate organic superconductor (MDT-TSF)I${}_{0.77}$Br${}_{0.52}$, where MDT-TSF is methylenedithio-tetraselenafulvalene. The Raman spectra suggest that the infinite polyhalide chains include the asymmetric trihalide anion I${}_{2}$Br${}^{\ensuremath{-}}$ and that the degree of charge transfer from the donor molecule to the anion is $0.429$, estimated from the lattice constants. The Shubnikov--de Haas (SdH) oscillation is in good agreement with this charge transfer degree. The beating behavior of the SdH oscillation gives a large interlayer transfer integral, which is consistent with a corrugated conducting sheet structure. The randomness of the disordered anions reduces the periodicity of the incommensurate anion potential; this destroys the Fermi surface reconstruction by the incommensurate anion potential. The factor dominating the superconducting transition temperature is not the randomness of the polyhalide anions but the effective cyclotron mass enhancement, i.e., the many-body effect.

Quasiparticles beyond the Fermi liquid and heavy fermion criticality

We give a self-consistent theory of the scale dependent effective mass enhancement m*/m of quasiparticles by 3D antiferromagnetic (AFM) spin fluctuations in the presence of disorder at an AFM quantum critical point. The coupling of fermionic and bosonic degrees of freedom in the critical regime is described in terms of a critical quasiparticle theory. Using the fact that even in the "non-Fermi liquid" regime the quasiparticle width does not exceed the quasiparticle energy, we adopt relations from Fermi liquid theory to determine the dependence of the spin fluctuation spectrum on m*/m, from which the self energy and hence m*/m may be calculated. The self-consistent equation for m*/m has a strong coupling solution provided the initial value is sufficiently large. We argue that in YbRh2Si2, quasi-2D Gaussian fluctuations existing over a wide range drive the system into the 3D strongly coupled fluctuation regime. We find critical exponents of the temperature dependence of the specific heat coefficient \gamma\propto T^{-1/4} and of the resistivity \rho(T)=\rho(0)+ A T^{3/4} in good agreement with experiments on YbRh2Si2 in the temperature range T < 0.3K.

Experimental investigation of interaction of very low frequency electromagnetic waves with metallic nanostructure

The interaction of low-frequency electromagnetic waves with metallic nanostructure consisting of nanoparticles has been investigated. The existence of helicons in metallic nanostructure is predicted based on the enhancement of effective mass of the electrons in metallic mesostructures in low-frequency electromagnetic field. The enhancement of the effective mass of electrons subjected to low-frequency electromagnetic field results in the suppression of the cyclotron frequency. When the excitation frequency approaches the cyclotron frequency, helicons can be observed, which in typical metals occur at MHz frequencies. With the decrease in the cyclotron frequency in nanostructures, the helicons can be observed at low frequencies. We present an experimental setup to detect and image helicon resonances in metallic nanostructures at low frequencies and room temperature conditions. The approach is based on modifying an existing atomic force microscope to image surface topography and the magnetic field images of helicon waves simultaneously in metallic nanoparticles. The magnetic field images of the helicon waves and different resonance modes are presented for platinum nanoparticles deposited using through thin film ablation. The contrast in the magnetic field images is explained based on the generation of resonance modes due to helicon wave propagation in the nanostructure. The features in the helicon resonance images are examined as a function of the particle size and frequency of the electromagnetic wave.

Sharp Valence Change as Origin of Drastic Change of Fermi Surface and Transport Anomalies in CeRhIn5 under Pressure

The drastic changes of Fermi surfaces as well as transport anomalies near P = Pc ~ 2.35 GPa in CeRhIn5 are explained theoretically from the viewpoint of sharp valence change of Ce. It is pointed out that the key mechanism is the interplay of magnetic order and Ce-valence fluctuations. It is shown that the antiferromagnetic state with "small" Fermi surfaces changes to the paramagnetic state with "large" Fermi surfaces with huge enhancement of effective mass of electrons with keeping finite c-f hybridization. This naturally explains the de Haas-van Alphen measurement and also the transport anomalies of T-linear resistivity emerging simultaneously with the residual resistivity peak at P = Pc in CeRhIn5.

Trends in Heavy Fermion Matter

A brief review on major advances in heavy fermion physics is presented including the Ce metal phase diagram, the huge effective mass detected in CeAl3, and the successive discoveries of unconventional superconductivity in CeCu2Si2 and three U based compounds, UBe13, UPt3 and URu2Si2. In order to track the origin of the huge effective mass, the case of intermediate valence compounds is discussed with emphasis on the differences between Yb and Ce materials. The formation of the effective mass is analyzed by two regular- and singular-part contributions. Examples are given for both, antiferromagnetic (CeRu2Si2 series) and ferromagnetic tricriticalities (UGe2). Pressure and magnetic-field studies on the ferromagnetic superconductor URhGe illustrate the role of the singular effective mass enhancement on the superconducting pairing. The discovery of the Ce-115 material gives the opportunity to study deeply the interplay of antiferromagnetism and superconductivity. This is clearly demonstrated by field re-entrant AF inside the SC phase just below the superconducting upper critical field (Hc2) for CeCoIn5 or on both sides of Hc2 within a restricted pressure window for CeRhIn5. The present status of the search for the hidden-order parameter of URu2Si2 is given and we emphasize that it may correspond to a lattice unit-cell doubling which leads to a drastic change in the band structure and spin dynamics, with the possibility of competition between multipolar ordering and antiferromagnetism.