Overdoped Samples Research Articles

Evolution of the cyclotron mass with doping in La2−xSrxCuO4

The recent observation of cyclotron resonance in optimally-doped La$_{2-x}$Sr$_x$CuO$_4$ using time-domain THz spectroscopy in high magnetic field has given new possibilities for the study of cuprate superconductors. One can measure the cyclotron mass in more disordered cuprates possesing short scattering times therefore expanding the study to materials and dopings in which quantum oscillations have not been observed. Here we present the measurement of the carrier mass of the hole doped cuprate La$_{2-x}$Sr$_x$CuO$_4$ across a a range of dopings spanning the slightly underdoped ($x=0.13$) to highly overdoped ($x=0.26$), near to the termination of the superconducting dome. These results reveal a systematic increase of $m_c$ with doping, up to values greater than thirteen times the bare electron mass. This is in contrast with those masses extracted from the heat capacity, which show a peak near the pseudogap critical point $p^*$ and/or Lifshitz transition. The cyclotron frequency is linear in field up to 31 T for all dopings giving no evidence for field induced Fermi surface reconstructions. The cyclotron mass is found to be positive for all dopings, but with a magnitude systematically below the heat capacity mass for under and optimally doped samples, while exceeding it for overdoped samples. Among other aspects, these results are surprising as photoemission reveals a Lifshitz transition in the middle of our doping range and the sign of the cyclotron mass determined from a finite frequency resonance is -- in conventional theories -- a {\it topological quantity} only sensitive to whether or not the Fermi surface is closed around holes or electrons. We see no sign of a divergence of the mass near $p^*$ nor near the Lifshitz transition, showing that any singularity -- if it exists -- is not strong enough to affect the cyclotron mass.

Normal-State Transport Properties of Infinite-Layer Sr1-xLaxCuO2 Electron-Doped Cuprates in Optimal- and Over-Doped Regimes.

Transport properties of electron-doped cuprate SrLaCuO thin films have been investigated as a function of doping. In particular, optimal- and over-doped samples were obtained by tuning the Sr:La stoichiometric ratio. Optimal-doped samples show a non-Fermi liquid behavior characterized by linear dependence of the resistivity from room temperature down to intermediate temperature (about 150–170 K). However, by approaching temperatures in the superconducting transition, a Fermi-liquid behavior-characterized by a -scaling law-was observed. Once established, the transition from a linear-T to a quadratic- behavior was successfully traced back in over-doped samples, even occurring at lower temperatures. In addition, the over-doped samples show a crossover to a linear-T to a logarithmic dependence at high temperatures compatible with anti-ferromagnetic spin fluctuations dominating the normal state properties of electron-doped cuprates.

Linear dichroism infrared resonance in overdoped, underdoped, and optimally doped cuprate superconductors

By measuring the polarization changes in THz, infrared, and visible radiation over an extended energy range (3-2330 meV), we observe symmetry-breaking in cuprate high temperature superconductors over wide energy, doping, and temperature ranges. We measure the polarization rotation and ellipticity of transmitted radiation though thin films as the sample is rotated. We observe a two-fold rotational symmetry in the Faraday angle, which is associated with linear dichroism (LD) and occurs when electromagnetic radiation polarized along one direction is absorbed more strongly than radiation polarized in the perpendicular direction. Such polarization anisotropies can be generally associated with symmetry breakings. We measure the amplitude of the LD signal and study its temperature, energy, and doping dependence. The LD signal shows a resonant behavior with a peak in the few hundred meV range, which is coincident with the mid-infrared optical feature that has been associated with the formation of the pseudogap state. The strongest LD signal is found in under-doped films, although it is also observed in optimally- and over-doped samples. The LD signal is consistent with nematic charge ordering as well as novel magnetoelectric effects.

Absence of μSR evidence for magnetic order in the pseudogap phase of Bi2+xSr2−xCaCu2O8+δ

We present an extended zero-field muon spin relaxation (ZF-$\mu$SR) study of overdoped Bi$_{2+x}$Sr$_{2-x}$CaCu$_2$O$_{8+\delta}$ (Bi2212) single crystals, intended to elucidate the origin of weak quasistatic magnetism previously detected by $\mu$SR in the superconducting and normal states of optimally-doped and overdoped samples. New results on heavily-overdoped single crystals show a similar monotonically decreasing ZF-$\mu$SR relaxation rate with increasing temperature that persists above the pseudogap (PG) temperature $T^*$ and does not evolve with hole doping ($p$). Additional measurements using an ultra-low background apparatus confirm that this behavior is an intrinsic property of Bi2212, which cannot be due to magnetic order associated with the PG phase. Instead we show that the temperature-dependent relaxation rate is most likely caused by structural changes that modify the contribution of the nuclear dipole fields to the ZF-$\mu$SR signal. Our results for Bi2212 emphasize the importance of not assuming the nuclear-dipole field contribution is independent of temperature in ZF-$\mu$SR studies of high-temperature (high-$T_c$) cuprate superconductors, and do not support a recent $\mu$SR study of YBa$_2$Cu$_3$O$_{6+x}$ that claims to detect magnetic order in the PG phase.

Doping Dependence of the Second Magnetization Peak, Critical Current Density, and Pinning Mechanism in BaFe2–xNixAs2 Pnictide Superconductors

A series of high quality BaFe$_{2-x}$Ni$_x$As$_2$ pnictide superconductors were studied using magnetic relaxation and isothermal magnetic measurements in order to study the second magnetization peak (SMP) and critical current behaviour in Ni-doped 122 family. The temperature dependence of the magnetic relaxation rate suggests a pinning crossover, whereas, it's magnetic field dependence hints a vortex-lattice structural phase-transition. The activation energy ($U$) estimated using the magnetic relaxation data was analyzed in detail for slightly-underdoped, slightly-overdoped and an overdoped samples, using Maley's method and collective creep theory. Our results confirm that the SMP in these samples is due to the collective (elastic) to plastic creep crossover as has been observed for the other members of 122-family. In addition, we also investigated the doping dependence of the critical current density ($J_c$) and the vortex-pinning behaviour in these compounds. The observed $J_c$ is higher than the threshold limit (10$^5$ A/cm$^2$) considered for the technological potential and even greater than 1 MA/cm$^2$ for slightly underdoped Ni-content, x = 0.092 sample. The pinning characteristics were analyzed in terms of the models developed by Dew-Hughes and Griessen $et$ $al$, which suggest the dominant role of $\delta l$-type pinning.

Nematic fluctuations in iron-based superconductors studied by resistivity change under uniaxial pressure

Antiferromagnetic, nematic and superconducting phases have been widely found in iron-based superconductors. The study on their relationships is thus crucial for understanding the low-energy physics and high-temperature superconductivity. The so-called nematic phase represents a spontaneous in-plane rotational symmetry breaking of the electronic states, which results in strong in-plane anisotropic properties. We have developed a uniaxial pressure device, which enables us to obtain nematic susceptibility by studying the resistivity change under uniaxial pressure at low temperature. In this paper, we brief two of our recent researches on nematic fluctuations in iron-based superconductors. The first research shows the presence of a nematic quantum critical point in BaFe2-xNixAs2, which exhibits several characteristics, including the zero mean-field nematic transition temperature x=0.11, broad hump feature in the nematic susceptibility in overdoped samples, strongest nematic susceptibility along the (100) direction at x=0.11, and the divergence of zero-temperature nematic susceptibility at x=0.11 for uniaxial pressure along both the (110) and (100) directions. We further study the nematic susceptibility in many other iron-based superconductors and find that the ordered moment at zero temperature linearly scales with nematic Curie constant, which is obtained from the Curie-Weiss-like temperature dependence of nematic susceptibility in these materials. Accordingly, we propose a universal phase diagram for iron-based superconductors, where superconductivity is achieved by suppressing the long-range antiferromagnetic order in a hypothetical parent compound though the enhancement of nematic fluctuations by doping, including both carrier doping and isovalent doping. Our results suggest that nematic fluctuations play a very important role in both the antiferromagnetism and superconductivity in iron-based superconductors.

STM measurements of Ni impurity effects in Bi2212

We report on the STS measurements for the temperature dependences of density-of-states for optimally-doped and overdoped samples. In the tunnelling spectra above TC in an overdoped sample, we found two types of a pseudogap; the large one, which had a typical pseudogap size and decreased with increasing a temperature, and the small one, which seemed almost temperature independent in our measured temperature range. The large gap seemed to switch to the small one with some overlap region with increasing a temperature. Similar behaviour was found in the optimally doped samples with an enhanced gap size and the temperature range. We identified the large gap as a conventional pseudogap and the small one as a new pseudogap, which is associated with Ni impurity. This shows a peculiarity of Ni impurity, and suggests an unusual interaction between an impurity and the Cu-O electronic system.

Dependence of the critical temperature in overdoped copper oxides on superfluid density.

The physics of underdoped copper oxide superconductors, including the pseudogap, spin and charge ordering and their relation to superconductivity, is intensely debated. The overdoped copper oxides are perceived as simpler, with strongly correlated fermion physics evolving smoothly into the conventional Bardeen-Cooper-Schrieffer behaviour. Pioneering studies on a few overdoped samples indicated that the superfluid density was much lower than expected, but this was attributed to pair-breaking, disorder and phase separation. Here we report the way in which the magnetic penetration depth and the phase stiffness depend on temperature and doping by investigating the entire overdoped side of the La2-xSrxCuO4 phase diagram. We measured the absolute values of the magnetic penetration depth and the phase stiffness to an accuracy of one per cent in thousands of samples; the large statistics reveal clear trends and intrinsic properties. The films are homogeneous; variations in the critical superconducting temperature within a film are very small (less than one kelvin). At every level of doping the phase stiffness decreases linearly with temperature. The dependence of the zero-temperature phase stiffness on the critical superconducting temperature is generally linear, but with an offset; however, close to the origin this dependence becomes parabolic. This scaling law is incompatible with the standard Bardeen-Cooper-Schrieffer description.

Distinct doping dependence of critical temperature and critical current density in Ba1-xKxFe2As2 superconductor.

Since the high transition temperature (High-Tc) superconductivity was discovered in the series of materials containing iron (Fe), their potential for the applications has been extensively scrutinized. In particular, a lot of effort has been made in achieving the high current-carrying ability by revealing the vortex pinning behavior. Here, we report on the critical current density (Jc) for the pristine Ba1−xKxFe2As2 single crystals with various K concentrations (0.25 ≤ x ≤ 0.52) determined by the magnetization hysteresis loop measurements. The x-dependence of Jc is characterized by a spike-like peak at x ~ 0.30, which corresponds to the under-doped region. This behavior is distinct from a moderate Tc dome with a broad maximum spanning from x ~ 0.3 to 0.5. For the under-doped samples, with increasing magnetic field (H), a second magnetization peak in Jc is observed, whereas for the optimally- and over-doped samples, Jc monotonically decreases with H. This result emphasizes that fine tuning of doping composition is important to obtain strong flux pinning. The origin of the characteristic doping dependence of Jc is discussed in connection with the orthorhombic phase domain boundary, as well as the chemical inhomogeneity introduced by the dopant substitutions.

Multiband thermal transport in the iron-based superconductorBa1−xKxFe2As2

We present results of precise measurements of the thermal and electrical transport in the optimally- and over-doped Ba1-xKxFe2As2 single crystals (x = 0.35, 0.55, 0.88) and compare them to the previously reported data on Ba(Fe1-yCoy)2As2. A contraction of the electron pocket is observed upon substitution potassium for barium, but even at the extreme doping (x = 0.88) there is still a noticeable contribution from negative charge carriers to the electronic transport. The size of the electron pocket in all K-doped samples is small enough to cause a significant enhancement of the respective Hall-Lorenz number. Another observed characteristic is the emergence of a maximum in the transverse thermal conductivity below the superconducting critical temperature of the optimally- (x = 0.35) and slightly over-doped (x = 0.55) samples. The evolution of this anomaly from the optimally electron-doped Ba(Fe0.94Co0.06)2As2 to hole-overdoped Ba0.45K0.55Fe2As2 suggests formation of a uniform superconducting gap on the electron pocket in the former and regions of a depressed gap on the hole-pocket in the latter.

Doping Induced Gap Anisotropy in Iron-Based Superconductors: a Point-Contact Andreev Reflection Study of BaFe2−xNixAs2 Single Crystals**Supported by the National Basic Research Program of China under Grant Nos 2011CBA00100 and 2012CB821404, and the Project of International Team on Superconductivity and Novel Electronic Materials of Chinese Academy of Sciences.

We report a systematic investigation on c-axis point-contact Andreev reflection (PCAR) in BaFe2−xNixAs2 superconducting single crystals from underdoped to overdoped regions (0.075≤ x ≤0.15). At low temperatures, an in-gap sharp peak at low-bias voltage is observed in PCAR for overdoped samples, in contrast to the case of underdoped junctions, in which an in-gap plateau is observed. The variety of the conductance spectra with doping can be well described by using a generalized Blonder–Tinkham–Klapwijk formalism with an angle-dependent gap. This gap shows a clear crossover from a nodeless in the underdoped side to a nodal feature in the overdoped region. This result provides evidence of the doping-induced evolution of the superconducting order parameter when the inter-pocket and intra-pocket scattering are tuned through doping, as expected in the s± scenario.

Crossover in the magnetic response of single-crystallineBa1−xKxFe2As2and Lifshitz critical point evidenced by Hall effect measurements

We report the doping evolution of magnetic susceptibility $\chi (T)$ and Hall coefficient $R_H$ in high quality Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($0.13 \leq x \leq 1$) single crystals. It is found that the normal-state magnetic susceptibility of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ compounds undergoes a crossover from linear-$T$ dependence in the undoped and underdoped samples into KFe$_2$As$_2$-type magnetic response in the overdoped samples with increasing K content. Hall coefficient $R_H$ of underdoped sample $x$=0.22 shows a rapid increase above spin-density-wave transition temperature $T_{SDW}$. Below $T_{SDW}$, it increases slowly. $R_H$ of optimally doped and slightly overdoped samples ($0.34 \leq x \leq 0.65$) shows relative weak temperature dependence and saturation tendency below 150 K. However, $R_H$ of K heavily overdoped samples ($0.80 \leq x \leq 1$) increases rapidly below 150 K. Meanwhile, Hall angle $cot \theta _H$ displays a concave temperature dependence within the doping range $0.22 \leq x \leq 0.55$, whereas it changes to a convex temperature dependence within the doping range $0.65 \leq x \leq 1$. The dramatic change coincides with Lifshitz transition occurred around the critical doping $x$=0.80, where ARPES measurements had confirmed that electron pocket disappears with excess hole doping in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ system. It is suggested that the characteristic temperature $T$* at around $120 \sim 150$ K observed in susceptibility and Hall coefficient, as well as previously reported resistivity data, may indicate an incoherence-coherence crossover in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ system.

Resonant x-ray scattering study of charge-density wave correlations inYBa2Cu3O6+x

We report the results a comprehensive study of charge density wave (CDW) correlations in untwinned YBCO6+x single crystals with 0.4<x<0.99 using Cu-L3 edge resonant x-ray scattering (RXS). Evidence of CDW formation is found for 0.45<x<0.93, but not for samples with x<0.44 that exhibit incommensurate spin-density-wave order, and in slightly overdoped samples with x=0.99. This suggests the presence of two proximate zero-temperature CDW critical points at doping pc1~0.08 and pc2~0.18. The CDW reflections are observed at incommensurate in-plane wave vectors (d_a, 0) and (0, d_b). Both decrease linearly with increasing doping, in agreement with recent reports on Bi-based high-Tc superconductors, but in sharp contrast to the behavior of the 214 family. The CDW intensity and correlation length exhibit maxima at p~0.12, coincident with a plateau in the superconducting transition temperature Tc. The onset temperature of the CDW reflections depends non-monotonically on p, with a maximum of~160 K for p~0.12. The RXS reflections exhibit a uniaxial intensity anisotropy. We further observe a depression of CDW correlations upon cooling below Tc, and (for samples with p> 0.09) an enhancement of the signal when an external magnetic field up to 6 T is applied in the superconducting state. For samples with p~0.08, where prior work has revealed a field-enhancement of incommensurate magnetic order, the RXS signal is field-independent. This supports a previously suggested scenario in which incommensurate charge and spin orders compete against each other, in addition to individually competing against. We discuss the relationship of these results to stripe order 214, the pseudogap phenomenon, superconducting fluctuations, and quantum oscillations.

Phase diagram and physical properties of NaFe1−xCuxAs single crystals

A series of high quality NaFe$_{1-x}$Cu$_x$As single crystals has been grown by a self-flux technique, which were systematically characterized via structural, transport, thermodynamic, and high pressure measurements. Both the structural and magnetic transitions are suppressed by Cu doping, and bulk superconductivity is induced by Cu doping. Superconducting transition temperature ($T_c$) is initially enhanced from 9.6 to 11.5 K by Cu doping, and then suppressed with further doping. A phase diagram similar to NaFe$_{1-x}$Co$_x$As is obtained except that insulating instead of metallic behavior is observed in extremely overdoped samples. $T_c$'s of underdoped, optimally doped, and overdoped samples are all notably enhanced by applying pressure. Although a universal maximum transition temperature ($T_c^{max}$) of about 31 K under external pressure is observed in underdoped and optimally doped NaFe$_{1-x}$Co$_x$As, $T_c^{max}$ of NaFe$_{1-x}$Cu$_x$As is monotonously suppressed by Cu doping, suggesting that impurity potential of Cu is stronger than Co in NaFeAs. The comparison between Cu and Co doping effect in NaFeAs indicates that Cu serves as an effective electron dopant with strong impurity potential, but part of the doped electrons are localized and do not fill the energy bands as predicted by the rigid-band model.

Progressive slowing down of spin fluctuations in underdoped LaFeAsO1−xFx

The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO$_{1-x}$F$_x$ was studied over a broad doping, temperature, and magnetic field range (x = 0 - 0.15, T up to 480K, H up to 30T) by means of As nuclear magnetic resonance (NMR). An enhanced spin-lattice relaxation rate divided by temperature, 1/T1T, in underdoped superconducting samples (x = 0.045, 0.05 and 0.075) suggests the presence of antiferromagnetic spin fluctuations, which are strongly reduced in optimally-doped (x = 0.10) and completely absent in overdoped (x = 0.15) samples. In contrast to previous analysis, Curie-Weiss fits are shown to be insufficient to describe the data over the whole temperature range. Instead, a BPP-type model is used to describe the occurrence of a peak in 1/T1T clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition.

Doping Evolution of Nodal Band Renormalization in Bi2Sr2CuO6+δ Superconductor Revealed by Laser-Based Angle-Resolved Photoemission Spectroscopy

High resolution laser-based angle-resolved photoemission measurements are carried out on Bi2Sr2CuO6+δ superconductor covering a wide doping range from heavily underdoped to heavily overdoped samples. Two obvious energy scales are identified in the nodal dispersions: one is the well-known 50–80 meV high energy kink and the other is <10 meV low energy kink. The high energy kink increases monotonously in its energy scale with increasing doping and shows weak temperature dependence, while the low energy kink exhibits a non-monotonic doping dependence with its coupling strength enhanced sharply below Tc. These systematic investigations on the doping and temperature dependence of these two energy scales favor electron-phonon interactions as their origin. They point to the importance in involving the electron-phonon coupling in understanding the physical properties and the superconductivity mechanism of high temperature cuprate superconductors.

Microstructural properties of over-doped GaN-based diluted magnetic semiconductors grown by MOCVD

We have grown transition metal (Fe, Mn) doped GaN thin films on c-oriented sapphire by metal-organic chemical vapor deposition. By varying the flow of the metal precursor, a series of samples with different ion concentrations are synthesized. Microstructural properties are characterized by using a high-resolution transmission electron microscope. For Fe over-doped GaN samples, hexagonal Fe3N clusters are observed with Fe3N(0002) parallel to GaN (0002) while for Mn over-doped GaN, hexagonal Mn6N2.58 phases are observed with Mn6N2.58(0002) parallel to GaN(0002). In addition, with higher concentration ions doping into the lattice matrix, the partial lattice orientation is distorted, leading to the tilt of GaN(0002) planes. The magnetization of the Fe over-doped GaN sample is increased, which is ascribed to the participation of ferromagnetic iron and Fe3N. The Mn over-doped sample displays very weak ferromagnetic behavior, which probably originates from the Mn6N2.58.

Scaling of the physical properties in Ba(Fe,Ni)2As2single crystals: Evidence for quantum fluctuations

We report on local magnetization, tunnel diode oscillators, and specific-heat measurements in a series of Ba(Ni${}_{x}$Fe${}_{1\ensuremath{-}x}$)${}_{2}$As${}_{2}$ single crystals ($0.26\ensuremath{\le}x\ensuremath{\le}0.74$). We show that the London penetration depth $\ensuremath{\lambda}(T)=\ensuremath{\lambda}(0)+\ensuremath{\Delta}\ensuremath{\lambda}(T)$ scales as $\ensuremath{\lambda}(0)\ensuremath{\propto}1/{T}_{c}^{0.85\ifmmode\pm\else\textpm\fi{}0.2}$, $\ensuremath{\Delta}\ensuremath{\lambda}(T)\ensuremath{\propto}{T}^{2.3\ifmmode\pm\else\textpm\fi{}0.3}$ (for $T&lt;{T}_{c}/3$), and $\ensuremath{\partial}\ensuremath{\Delta}\ensuremath{\lambda}/\ensuremath{\partial}{T}^{2}\ensuremath{\propto}1/{T}_{c}^{2.8\ifmmode\pm\else\textpm\fi{}0.3}$ in both underdoped and overdoped samples. Moreover, the slope of the upper critical field [${H}_{c2}^{\ensuremath{'}}=\ensuremath{-}{(d{H}_{c2}/dT)}_{|T\ensuremath{\rightarrow}{T}_{c}}$] decreases with ${T}_{c}$ in overdoped samples but increases with decreasing ${T}_{c}$ in underdoped samples. The remarkable variation of $\ensuremath{\lambda}(0)$ with ${T}_{c}$ and the nonexponential temperature dependence of $\ensuremath{\Delta}\ensuremath{\lambda}$ clearly indicates that pair-breaking effects are important in this system. We show that the observed scalings strongly suggest that those pair-breaking effects could be associated with quantum fluctuations near three-dimensional superconducting critical points.

Impurity and magnetic field effects on the stripes in cuprates

Impurity and magnetic field effects on the stripes are reviewed not only in the La-based cuprates but also in the other high-Tc cuprates. It has turned out that the so-called 1/8 anomaly takes place not only in the La-based cuprates but also in the hole-doped high-Tc cuprates universally, when adequate pinning centers are introduced. Impurity-induced magnetism tends to emerge at low temperatures in a wide range of hole concentration in LSCO, Bi2201 and YBCO where superconductivity appears. Accordingly, it is possible that the dynamically fluctuating stripes exist universally in the hole-doped high-Tc cuprates and play an important role in the appearance of the high-Tc superconductivity. For the hole-doped high-Tc cuprates, it has turned out that magnetic field effects on the stripes are small in samples where the stripes are well stabilized statically in zero field, while field-induced magnetism is observed under magnetic fields parallel to the c-axis in underdoped samples where magnetism is not well developed in zero field. The field-induced magnetism has been observed in some optimally doped samples also, but it has never been observed in overdoped samples. Field-induced CDW has been observed in slightly overdoped BSCCO. These results are understood fundamentally in terms of the theoretical phase diagram as a function of magnetic field, based on the Ginzburg–Landau theory with competing antiferromagnetic and superconducting order parameters. It is also possible to understand that the field-induced magnetism and CDW are due to pinning of the dynamically fluctuating stripes by vortex cores as in the case of the pinning by impurities. In the electron-doped high-Tc cuprates, on the other hand, impurity-induced magnetism has never been observed. The nature of the field-induced magnetism observed in the electron-doped cuprates is different from that in the hole-doped ones.

75As Nuclear Magnetic Resonance Studies on Ba(Fe1−xNix)2As2 Single Crystals under High Pressure

75As nuclear magnetic resonance (NMR) was measured for Ba(Fe1−xNix)2As2 single crystals with x = 0.05 and x = 0.1 under 0 and 1.5 GPa, respectively. For the optimal doped sample with x = 0.05, the superconducting transition temperature, Tc, is strongly suppressed from 18 to 5 K, while for the over-doped sample with x = 0.1, it is turned from a superconducting ground state to a disordered paramagnetic state under 1.5 GPa. Our experimental results show that the antiferromagnetic spin fluctuations, as well as Tc, are suppressed. The experimental results can be explained with the two-band model. As a result, the electronic band is shifted downwards with an increase in pressure, and the electrons become the dominant carriers in the system.