Chemical Potential Shift Research Articles

Temperature-dependent photoemission on1T-TiSe2: Interpretation within the exciton condensate phase model

The charge-density-wave phase transition of $1T{\text{-TiSe}}_{2}$ is studied by angle-resolved photoemission over a wide temperature range. An important chemical-potential shift which strongly evolves with temperature is evidenced. In the framework of the exciton condensate phase, the detailed temperature dependence of the associated order parameter is extracted. Having a mean-field-like behavior at low temperature, it exhibits a nonzero value above the transition, interpreted as the signature of strong excitonic fluctuations, reminiscent of the pseudogap phase of high-temperature superconductors. Integrated intensity around the Fermi level is found to display a trend similar to the measured resistivity and is discussed within the model.

Doping dependence of the chemical potential and surface electronic structure inYBa2Cu3O6+xandLa2−xSrxCuO4using hard x-ray photoemission spectroscopy

The electronic structure of YBa2Cu3O6+x and La2-xSrxCuO4 for various values of x has been investigated using hard x-ray photoemission spectroscopy. The experimental results establish that the cleaving of YBa2Cu3O6+x compounds occurs predominantly in the BaCuO3 complex leading to charged surfaces at higher x and to uncharged surfaces at lower x values. The bulk component of the core level spectra exhibits a shift in binding energy as a function of x, from which a shift of the chemical potential as a function of hole concentration in the CuO2 layers could be derived. The doping dependence of the chemical potential across the transition from a Mott-Hubbard insulator to a Fermi-liquid-like metal is very different in these two series of compounds. In agreement with previous studies in the literature the chemical potential shift in La2-xSrxCuO4 is close to zero for small hole concentrations. In YBa2Cu3O6+x, similar to all other doped cuprates studied so far, a strong shift of the chemical potential at low hole doping is detected. However, the results for the inverse charge susceptibility at small x shows a large variation between different doped cuprates. The results are discussed in view of various theoretical models. None of these models turns out to be satisfactory.

Quantum Oscillations in the Anomalous Phase inSr3Ru2O7

We report measurements of quantum oscillations detected in the putative nematic phase of Sr3Ru2O7. Improvements in sample purity enabled the resolution of small amplitude de Haas-van Alphen (dHvA) oscillations between two first order metamagnetic transitions delimiting the phase. Two distinct frequencies were observed, whose amplitudes follow the normal Lifshitz-Kosevich profile. Variations of the dHvA frequencies are explained in terms of a chemical potential shift produced by reaching a peak in the density of states, and an anomalous field dependence of the oscillatory amplitude provides information on domains.

Temperature dependence of the excitonic insulator phase model in1T-TiSe2

Recently, detailed calculations of the excitonic insulator phase model adapted to the case of 1\textit{T}-TiSe$_2$ have been presented. Through the spectral function theoretical photoemission intensity maps can be generated which are in very good agreement with experiment [Phys. Rev. Lett. {\bf 99}, (2007) 146403]. In this model, excitons condensate in a BCS-like manner and give rise to a charge density wave, characterized by an order parameter. Here, we assume an analytical form of the order parameter, allowing to perform temperature dependent calculations. The influence of this order parameter on the electronic spectral function, to be observed in photoemission spectra, is discussed. The resulting chemical potential shift and an estimation of the resistivity are also shown.

In situphotoemission study ofNd1−xSrxMnO3epitaxial thin films

We have performed an in situ photoemission study of ${\text{Nd}}_{1\ensuremath{-}x}{\text{Sr}}_{x}{\text{MnO}}_{3}$ (NSMO) thin films grown on ${\text{SrTiO}}_{3}$ (001) substrates by laser molecular-beam epitaxy. The lattice constants of the thin films were relaxed from those of the substrates, and the transport properties were almost the same as those of bulk NSMO. From core-level photoemission studies, we found that the behavior of the chemical-potential shift was not much different from that of the bulk NSMO samples. In the valence-band spectra, finite intensity at the Fermi level was observed even in the insulating phase. The band dispersions of ${\text{Nd}}_{0.6}{\text{Sr}}_{0.4}{\text{MnO}}_{3}$ obtained by angle-resolved photoemission spectroscopy were almost the same as those of ${\text{La}}_{0.6}{\text{Sr}}_{0.4}{\text{MnO}}_{3}$ thin films. These results showed that NSMO is closely related to ${\text{La}}_{1\ensuremath{-}x}{\text{Sr}}_{x}{\text{MnO}}_{3}$ in terms of the band structure, except for a decrease in the coherent spectral weight.

Chemical potential shift ofFe3−xVxSistudied by hard x-ray photoemission

Core-level photoemission spectra of ${\text{Fe}}_{3\ensuremath{-}x}{\text{V}}_{x}\text{Si}$ alloys with inequivalent ${\text{Fe}}_{\text{I}}$ and ${\text{Fe}}_{\text{II}}$ sites are investigated via hard x-ray photoemission spectroscopy over the entire doping range $x=0--1$. All the measured $1s$ core-level peaks are found to shift to higher binding energy with increasing V concentration. First-principles, all electron charge- and spin-self-consistent electronic structure computations within the framework of the local-spin-density approximation are used to interpret the experimental results. The measured size of energy shift in going from $x=0$ to 1 is consistent with the corresponding theoretical value for the ${\text{Fe}}_{\text{II}}$ and $\text{Si}\text{ }1s$ core levels, whereas for the ${\text{Fe}}_{\text{I}}$ and V core levels the computed shifts are generally larger than the experimental values. We ascribe these discrepancies to the effects of the core-hole screening in the final state which are not accounted for in the computations. In a rigid-band model the chemical potential and the core-level binding energies are expected to decrease with V doping as electrons are depleted from the Fermi energy. The observed increase in the binding energy of core levels thus supports a picture of the electronic structure where V doping induces a ``pseudogap'' or a region of reduced density of states in the vicinity of the Fermi energy.

Chemical potential landscape in band filling and bandwidth-control of manganites: Photoemission spectroscopy measurements

We have studied the effects of band filling and bandwidth-control on the chemical potential in perovskite manganites ${R}_{1\ensuremath{-}x}{A}_{x}{\text{MnO}}_{3}$ ($R$: rare earth, $A$: alkaline earth) by measurements of core-level photoemission spectra. A suppression of the doping-dependent chemical potential shift was observed in and around the CE-type charge-ordered composition range, indicating that there is charge self-organization such as stripe formation or its fluctuations. As a function of bandwidth, we observed a downward chemical potential shift with increasing bandwidth due to the reduction of the orthorhombic distortion. After subtracting the latter contribution, we found an upward chemical potential shift in the ferromagnetic metallic region $0.3<x<0.5$, which we attribute to the enhancement of double-exchange interaction involving the Jahn-Teller-split ${e}_{g}$ band.

Electronic structure ofGa1−xCrxNand Si-doping effects studied by photoemission and x-ray absorption spectroscopy

The electronic structure of the magnetic semiconductor ${\text{Ga}}_{1\ensuremath{-}x}{\text{Cr}}_{x}\text{N}$, and the effect of Si doping on it have been investigated by photoemission and soft x-ray absorption spectroscopy. We have confirmed that Cr in GaN is predominantly trivalent when substituting for Ga and that $\text{Cr}\text{ }3d$ states appear within the band gap of GaN just above the $\text{N}\text{ }2p$-derived valence-band maximum. As a result of Si doping, downward shifts of the core levels (except for $\text{Cr}\text{ }2p$) and the formation of new states near the Fermi level were observed, which we attribute to the upward chemical-potential shift and the formation of a small amount of ${\text{Cr}}^{2+}$ species caused by electron doping. Possibility of Cr-rich cluster growth by Si doping are discussed based on the spectroscopic and magnetization data.

Doping evolution of the electronic structure in the single-layer cuprateBi2Sr2−xLaxCuO6+δ: Comparison with other single-layer cuprates

We have performed angle-resolved photoemission and core-level x-ray photoemission studies of the single-layer cuprate ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}\mathrm{Cu}{\mathrm{O}}_{6+\ensuremath{\delta}}$ (Bi2201) and revealed the doping evolution of the electronic structure from the lightly doped to optimally doped regions. We have observed the formation of the dispersive quasiparticle band, evolution of the Fermi ``arc'' into the Fermi surface, and the shift of the chemical potential with hole doping as in other cuprates. The doping evolution in Bi2201 is similar to that in ${\mathrm{Ca}}_{2\ensuremath{-}x}{\mathrm{Na}}_{x}\mathrm{Cu}{\mathrm{O}}_{2}{\mathrm{Cl}}_{2}$ (Na-CCOC), where a rapid chemical potential shift toward the lower Hubbard band of the parent insulator has been observed, but is quite different from that in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ (LSCO), where the chemical potential does not shift, yet the dispersive band and the Fermi arc and/or surface are formed around the Fermi level already in the lightly doped region. The (underlying) Fermi surface shape and band dispersions are quantitatively analyzed using tight-binding fit, and the deduced next-nearest-neighbor hopping integral ${t}^{\ensuremath{'}}$ also confirms the similarity to Na-CCOC and the difference from LSCO.

Chemical potential shift induced by double-exchange and polaronic effects inNd1−xSrxMnO3

We have studied the chemical potential shift as a function of temperature in Nd$_{1-x}$Sr$_x$MnO$_3$ (NSMO) by measurements of core-level photoemission spectra. For ferromagnetic samples ($x=0.4$ and 0.45), we observed an unusually large upward chemical potential shift with decreasing temperature in the low-temperature region of the ferromagnetic metallic (FM) phase. This can be explained by the double-exchange (DE) mechanism if the $e_g$ band is split by dynamical/local Jahn-Teller effect. The shift was suppressed near the Curie temperature ($T_C$), which we attribute to the crossover from the DE to lattice-polaron regimes.

Temperature dependence of the chemical potential in La2-xSrxCuO4

We have performed x-ray photoemission spectroscopy (XPS) measurements on the high-Tc superconductor La2-xSrxCuO4 (LSCO) at various temperatures. From the shifts of core-level spectra, we have deduced the temperature dependence of the chemical potential in this system. In the underdoped region, the chemical potential shifts upwards with increasing temperature, whereas it shows an opposite shift in the overdoped region. These results qualitatively agree with the predictions of the t-J and tight-binding model calculations as well as thermodynamic quantities but a large quantitative discrepancy exists.

Imaginary-Time Formulation of Steady-State Nonequilibrium: Application to Strongly Correlated Transport

We extend the imaginary-time formulation of the equilibrium quantum many-body theory to steady-state nonequilibrium with an application to strongly correlated transport. By introducing the Matsubara voltage, we maintain the finite chemical potential shifts in the Fermi-Dirac function, in agreement with the Keldysh formulation. The formulation is applied to strongly correlated transport in the Kondo regime using the quantum Monte Carlo method.

Temperature-dependent photoemission spectra, spectral weight transfer, and chemical potential shift inPr1−xCaxMnO3: Implications for charge-density modulation

We have studied the temperature dependence of the photoemission spectra of Pr$_{1-x}$Ca$_x$MnO$_3$ (PCMO) with $x=0.25$, 0.3 and 0.5. For $x=0.3$ and 0.5, we observed a gap in the low-temperature CE-type charge-ordered (CO) phase and a pseudogap with a finite intensity at the Fermi level ($E_F$) in the high-temperature paramagnetic insulating (PI) phase. Within the CO phase, the spectral intensity near $E_F$ gradually increased with temperature. These observations are consistent with the results of Monte Carlo simulations on a model including charge ordering and ferromagnetic fluctuations [H. Aliaga {\it et al.} Phys. Rev. B {\bf 68}, 104405 (2003)]. For $x=0.25$, on the other hand, little temperature dependence was observed within the low-temperature ferromagnetic insulating (FI) phase and the intensity at $E_F$ remained low in the high-temperature PI phase. We attribute the difference in the temperature dependence near $E_F$ between the CO and FI phases to the different correlation lengths of orbital order between both phases. Furthermore, we observed a chemical potential shift with temperature due to the opening of the gap in the FI and CO phases. The doping dependent chemical potential shift was recovered at low temperatures, corresponding to the disappearance of the doping dependent change of the modulation wave vector. Spectral weight transfer with hole concentration was clearly observed at high temperatures but was suppressed at low temperatures. We attribute this observation to the fixed periodicity with hole doping in PCMO at low temperatures.

Temperature Dependence of the Chemical Potential in NaxCoO2: Implications for the Large Thermoelectric Power

We have performed a temperature-dependent photoemission study of a Na x CoO 2 ( x ∼0.8) epitaxial thin film prepared by the reactive solid-phase epitaxy method. The chemical potential shift as a function of temperature was derived from the Co 3 d peak shift, and revealed a crossover from the degenerate Fermion state at low temperatures to the correlated hopping state of Co 3+ /Co 4+ mixed-valence at high temperatures. This suggests that the large thermoelectric power at high temperatures should be considered in the correlated hopping picture.

Photoemission Study of Perovskite-Type Manganites with Stripe Ordering

We have studied the chemical potential shift and changes in the electronic density of states near the Fermi level (EF) as functions of temperature and carrier concentration in Pr1−xCaxMnO3 (PCMO) and Nd1−xSrxMnO3 (NSMO) by measurements of photoemission spectra. Suppression of the chemical potential shift as a function of carrier concentration has been observed in PCMO and NSMO near and in the composition range where the CE-type antiferromagnetic charge-ordered (CO) phase appears at low temperatures. This result indicates that there is charge self-organization on a microscopic scale such as stripe formation in this composition range. In the ferromagnetic metallic phase of NSMO, we found a large temperature-dependent chemical potential shift at low temperatures and attributed this to double-exchange mechanism. Suppression of the temperature-dependent chemical potential shift near Curie temperature was observed, possibly associated with the formation of correlated polarons. In the valence band near the EF of PCMO, spectral weight was transferred with hole doping, leading to a finite intensity at EF even in the paramagnetic insulating phase above the CO phase for x≳0.3, and as the temperature was lowered, a clear gap opening was observed in the CO phase.

LDA + GTB (generalized tight-binding) method for the electronic structure calculations of strongly correlated electron systems: Application for the band structure calculations of p-type cuprates

Mean-field theory of the non-superconducting phase of the high- T c cuprates is formulated within the effective t– t′– t″– J model with three-site correlated hoppings. This model with the ab initio calculated parameters results from the LDA + GTB method. The static spin and kinematical correlation functions beyond Hubbard I approximation are calculated self-consistently taking into account hoppings to the first, the second, and the third neighboring sites, as well as the three-site correlated hoppings. The obtained Fermi surface evolves from hole-pockets at low-doping to large hole-type Fermi surface at higher doping concentrations. Calculated doping dependence of the nodal Fermi velocity, the effective mass and the chemical potential shift are in good agreement with experimental data.

Electrical properties of oxide heteroepitaxial p-n junctions: La1−xSrxFeO3∕SrTi0.99Nb0.01O3

Electrical properties of heteroepitaxial p-n junctions composed of correlated electron semiconductors La1−xSrxFeO3 (LSFO) (x=0.4, 0.5, 0.67, and 0.8) and a band semiconductor SrTi0.99Nb0.01O3 were analyzed. The junctions exhibited rectifying current-voltage (I-V) characteristics. The built-in potential (VbiSTO) evaluated from the capacitance-voltage characteristics under reverse bias increased from 0.9eV for x=0.4to1.2eV for x=0.8 junctions, indicating a large downward shift in chemical potential of p-type LSFO due to hole doping. Some of the junctions showed hysteretic I-V characteristics, which have been commonly observed in resistance switching devices, but others did not. LSFO junctions with higher doping (x=0.8) showed higher probability of hysteretic behavior.

Systematic doping evolution of the underlying Fermi surface ofLa2−xSrxCuO4

We have performed a systematic doping dependent study of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) (0.03$\leq x \leq$0.3) by angle-resolved photoemission spectroscopy. In the entire doping range, the underlying ``Fermi surface" determined from the low energy spectral weight approximately satisfies Luttinger's theorem, even down to the lightly-doped region. This is in strong contrast to the result on Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ (Na-CCOC), which shows a strong deviation from Luttinger's theorem. The differences between LSCO and Na-CCOC are correlated with the different behaviors of the chemical potential shift and spectral weight transfer induced by hole doping.

Electronic structures of Fe3–xVxSi probed by photoemission spectroscopy

AbstractThe electronic structures of the Heusler type compounds Fe3–x Vx Si in the concentration range between x = 0 and x = 1 have been probed by photoemission spectroscopy (PES). The observed shift of Si 2p core‐ level and the main valence band structres indicates a chemical potential shift to higher energy with increasing x . It is also clarified that the density of state at Fermi edge is owing to the collaboration of V 3d and Fe 3d derived states. Besides the decrease of the spectral intensity near Fermi edge with increasing x suggests the formation of pseudo gap at large x . (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Chemical potential shift and spectral-weight transfer inPr1−xCaxMnO3revealed by photoemission spectroscopy

We have studied the chemical potential shift and changes in the electronic density of states near the Fermi level $({E}_{F})$ as a function of carrier concentration in ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ (PCMO, $0.2\ensuremath{\leqslant}x\ensuremath{\leqslant}0.65$) through the measurements of photoemission spectra. The results showed that the chemical potential shift was suppressed for $x\ensuremath{\gtrsim}0.3$, where the charge-exchange (CE) type antiferromagnetic charge-ordered state appears at low temperatures. We consider this observation to be related to charge self-organization such as stripe formation on a microscopic scale in this composition range. Together with the previous observation of monotonous chemical potential shift in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$, we conclude that the tendency toward the charge self-organization increases with decreasing bandwidth. In the valence band, spectral weight of the Mn $3d$ ${e}_{g}$ electrons in PCMO was transferred from $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below ${E}_{F}$ to the region near ${E}_{F}$ with hole doping, leading to a finite intensity at ${E}_{F}$ even in the paramagnetic insulating phase for $x\ensuremath{\gtrsim}0.3$, probably related with the tendency toward charge self-organization. The finite intensity at ${E}_{F}$ in spite of the insulating transport behavior is consistent with fluctuations involving ferromagnetic metallic states.