Fermi Surface Area Research Articles

In situ carrier tuning in high temperature superconductor Bi2Sr2CaCu2O8+δ by potassium deposition

We report a successful tuning of the hole doping level over a wide range in high temperature superconductor Bi2Sr2CaCu2O8+δ (Bi2212) through successive in situ potassium (K) deposition. By taking high resolution angle-resolved photoemission measurements on the Fermi surface and band structure of an overdoped Bi2212 (Tc = 76K) at different stages of K deposition, we found that the area of the hole-like Fermi surface around the Brillouin zone corner (π,π) shrinks with increasing K deposition. This indicates a continuous hole concentration change from initial ~0.26 to eventual 0.09 after extensive K deposition, a net doping level change of 0.17 that makes it possible to bring Bi2212 from being originally overdoped, to optimally-doped, and eventually becoming heavily underdoped. The electronic behaviors with K deposition are consistent with those of Bi2212 samples with different hole doping levels. These results demonstrate that K deposition is an effective way of in situ controlling the hole concentration in Bi2212. This work opens a good way of studying the doping evolution of electronic structure and establishing the electronic phase diagram in Bi2212 that can be extended to other cuprate superconductors.

Effect of external pressure on the normal and superconducting properties of high‐ cuprates

AbstractThe pressure effects on the normal state electronic structure, the superexchange interaction, and the critical temperature of d‐type superconductivity mediated by magnetic pairing have been studied within the multielectron hybrid scheme LDA+GTB that takes into account electron correlations in planes. We have found the changes of the multiband p–d model parameters at 3% compression of different symmetry: (i) hydrostatic, (ii) along the c‐axis, and (iii) in a–b plane. We have studied the changes of the Fermi surface under external pressure for different hole doping concentration x. In general, this effect is too small except two critical concentrations and where the Lifshitz transitions occur with the change of the Fermi surface topology. In the vicinity of the critical concentration, we have found the giant change of the Fermi surface area up to 100% related to the pressure‐induced Lifshitz transition. The effects of pressure on the antiferromagnetic coupling J and the mean‐field value of are obtained in a good agreement to experimental data.

Pr- and La-doping effects on the magnetic anisotropy in the antiferromagnetic phase of Kondo semiconductorCeRu2Al10

We have studied the Pr- and La-doping effects on the magnetic anisotropy in the antiferro-magnetic (AFM) phase of ${\mathrm{CeRu}}_{2}{\mathrm{Al}}_{10}$. The crystalline electric field (CEF) splitting in ${\mathrm{PrRu}}_{2}{\mathrm{Al}}_{10}$ was found to be as large as $\ensuremath{\sim}800$ K with a singlet ground state. In ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ru}}_{2}{\mathrm{Al}}_{10}$, the CEF level scheme of the Pr ion is not changed with $x$. The AFM moment (${m}_{\mathrm{AF}}$) is rotated from $c$ to $b$ axis in both systems at ${x}_{c}^{\mathrm{sr}}\ensuremath{\sim}0.03$ and $\ensuremath{\sim}0.07$ for Ln=Pr and La, respectively. As the ionic radius of La and Pr is larger and smaller than that of Ce, respectively, these results indicate that the chemical pressure effect is not associated with the rotation of ${m}_{\mathrm{AF}}$, but is caused by the suppression of the $c\ensuremath{-}f$ hybridization originating from the decrease of $4f$ electrons of Ce ions by Ce-site substitution. Since a small amount of Pr or La doping changes easily the magnetization easy axis of all the moments on Ce sites, the origin of the magnetic anisotropy is not the local single ion effect but the bandlike effect through the anisotropic $c\ensuremath{-}f$ hybridization. The magnetic phase diagrams of ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Ln}}_{x}{\mathrm{Ru}}_{2}{\mathrm{Al}}_{10}$ indicate that above ${x}_{c}^{\mathrm{sr}}$, the AFM order with ${m}_{\mathrm{AF}}\ensuremath{\parallel}b$ continues to exist up to ${x}_{c}$, which is $\ensuremath{\sim}0.4$ and $\ensuremath{\sim}0.6$ in Ln=Pr and Ln=La, respectively. This indicates that even in the sample with an AFM transition temperature (${T}_{0}$) near ${x}_{c}$, the anisotropic $c\ensuremath{-}f$ hybridization dominates the AFM order. A large positive transverse magnetoresistance is seen below ${T}_{0}$, but a very small one above ${T}_{0}$. Together with the results of Hall resistivity and the observation of Shubnikov--de Haas oscillation, we propose that there exist large Fermi surfaces above ${T}_{0}$ and small ones below ${T}_{0}$. A gap is opened by the AFM order on almost the area of the large Fermi surface, and small Fermi surfaces are constructed below ${T}_{0}$, although we do not know the mechanism, which might be specific to the AFM order in Kondo semiconductors. The largest suppression of the magnetic scattering below ${T}_{0}$ is observed for the current $I\ensuremath{\parallel}a$ and the smallest one for $I\ensuremath{\parallel}b$. This anisotropy may be associated with the anisotropic $c\ensuremath{-}f$ hybridization, which may contribute to the anisotropic magnetic scattering of the conduction electron below ${T}_{0}$.

Research on Seebeck Coefficient of Thin-Film Thermocouple on Temperature-Testing Cutter

Performance of current temperature sensor is not satisfactory on monitoring transient temperature of cutting edge. A temperature-testing cutter is presented in this paper and analysis of Seebeck coefficient on thin film thermocouple is proposed with experiments. Thin film thermocouple is embedded into the cutting edge using magnetron sputtering technology. According to the fact that electrical conductivity can be replaced by the multiplication of electron mean free path λ and effective Fermi surface area A, a universal formula of thermoelectric power is given based on the diffuse thermoelectric power equation put forward by Mott and Jones. Using the expression of electron mean free path in gas model λ, the relationship of thermoelectric power between thin film and bulk material can be deduced. Result shows that the main influential factor of Seebeck coefficient is film thermal junction thickness. In addition, the Seebeck coefficient of different junction size and film thickness were tested by LabVIEW automatic calibration system. The experimental data indicates that Seebeck coefficient is determined by thermal junction thickness, and the sensor is found to be linear from room temperature up to 600°C while the Seebeck coefficient of thin film thermocouple becomes closer to standard thermocouple as the film thickness increments. However, junction size does not have a major influence on the Seebeck coefficient which is agreed by both theoretical analysis and test.

Exploring intertwined orders in cuprate superconductors

The concept of intertwined orders has been introduced to describe the cooperative relationship between antiferromagnetic spin correlations and electron (or hole) pair correlations that develop in copper-oxide superconductors. This contrasts with systems in which, for example, charge-density-wave (CDW) order competes for Fermi surface area with superconductivity. La2−xBaxCuO4 with x=0.125 provides an example in which the ordering of spin stripes coincides with the onset of two-dimensional superconducting correlations. The apparent frustration of the interlayer Josephson coupling has motivated the concept of the pair-density-wave superconductor, a state that theoretical calculations show to be energetically competitive with the uniform d-wave superconductor. Even at x=0.095, where there is robust superconductivity below 32K in zero field, the coexistence of strong, low-energy, incommensurate spin excitations implies a spatially modulated and intertwined pair wave function. Recent observations of CDW order in YBa2Cu3O6+x and other cuprate families have raised interesting questions regarding the general role of charge modulations and the relation to superconductivity. While there are differences in the doping dependence of the modulation wave vectors in YBa2Cu3O6+x and La2−xBaxCuO4, the maximum ordering strength is peaked at the hole concentration of 1/8 in both cases. There are also possible connections with the quantum oscillations that have been detected about the same hole concentration but at high magnetic fields. Resolving these relationships remains a research challenge.

Magnetotransport in copper-doped noncentrosymmetric BiTeI

BiTeI exhibits large Rashba spin splitting due to its noncentrosymmetric crystal structure. The study of chemical doping effect is important in order to either tune the Fermi level or refine the crystal quality. Here, we report the magneto-transport measurement in high quality BiTeI single crystals with different copper dopings. We found that a small amount of copper doping improves the crystal quality significantly, which is supported by the transport data showing higher Hall mobility and larger amplitude in Shubnikov-de Haas oscillation at low temperature. Two distinct frequencies in Shubnikov-de Haas oscillation were observed giving extremal Fermi surface areas of $A_{S} = 9.1 \times 10^{12}$ cm$^{-2}$ and $A_{L} = 3.47\times 10^{14}$ cm$^{-2}$ with corresponding cyclotron masses $m_S^*$ = 0.0353 $m_e$ and $m_L^*$ = 0.178 $m_e$, respectively. Those results are further compared with relativistic band structure calculations using three reported Te and I refined or calculated positions. Our analysis infers the crucial role of Bi-Te bond length in the observed large bulk Rashba-type spin splitting effect in BiTeI.

Shubnikov-de Haas oscillation in PuIn3

The Fermi surface of PuIn3 is investigated using flux-grown single crystals. Shubnikov-de Haas (SdH) oscillations were detected by means of the skin-depth measurement using a proximity-detector-oscillator circuit. Angular dependence of the SdH frequency which corresponds to the extremal cross-sectional area of Fermi surface agrees well with the previous magnetic susceptibility measurement using conventional field-modulation method. The SdH oscillation suddenly vanishes when the magnetic field is tilted from the cubic [111] direction.

Quantum oscillations in aπ-striped superconductor

Within Bogoliubov--de Gennes theory, a semiclassical approximation is used to study quantum oscillations and to determine the Fermi surface area associated with these oscillations in a model of a $\ensuremath{\pi}$-striped superconductor, where the $d$-wave superconducting order parameter oscillates spatially with period 8 and zero average value. This system has a nonzero density of particle-hole states at the Fermi energy, which form Landau-like levels in the presence of a magnetic field $B$. The Fermi surface is reconstructed via Andreev-Bragg scattering, and the semiclassical motion is along these Fermi surface sections as well as between them via magnetic breakdown. Within the approximation, oscillations periodic in $1/B$ are found in both the positions and widths of the lowest Landau levels. The area corresponding to these quantum oscillations for intermediate pairing interaction strength is similar to that reported for experimental measurements in the cuprates. A comparison is made of this theory to data for quantum oscillations in the specific heat measured by Riggs et al.

Superconductivity at 5.2 K in ZrTe3 polycrystals and the effect of Cu and Ag intercalation

We report the occurrence of superconductivity in polycrystalline samples of ZrTe3 at temperature 5.2 K at ambient pressure. The superconducting state coexists with the charge density wave (CDW) phase, which sets in at 63 K. The intercalation of Cu or Ag does not have any bearing on the superconducting transition temperature but suppresses the CDW state. The feature of a CDW anomaly in these compounds is clearly seen in the DC magnetization data. Resistivity data are analyzed in order to estimate the relative loss of carriers and reduction in the nested Fermi surface area upon CDW formation in ZrTe3 and the intercalated compounds.

Angular Dependence of Magnetic Quantum Oscillations and of Magnetoresistance in Quasi-2D Metals

We study analytically the angular dependence of magnetoresistance and of magnetic quantum oscillation frequencies in layered quasi-two-dimensional (Q2D) metals. The angular dependences of magnetoresistance and of Fermi-surface cross-sectional area are closely related. The harmonic expansion analytical formulas for the angular dependence of Fermi-surface cross-sectional area in an external magnetic field are obtained for various typical crystal symmetries. The simple azimuth-angle dependence of the Yamaji angles is derived for the elliptical in-plane Fermi surface. These formulas correct some previous results and allow the simple and effective interpretation of the magnetic quantum oscillations and magnetoresistance data in cuprate high-temperature superconducting materials, in organic metals and in other Q2D metals. The applicability region of some previous widely used analytical results and of the results obtained is investigated.

Fermi Surface and Mass Enhancement in KFe2As2 from de Haas–van Alphen Effect Measurements

We report on a band structure calculation and de Haas–van Alphen measurements of KFe 2 As 2 . Three cylindrical Fermi surfaces are found. Effective masses of electrons range from 6 to 18 m e , m e being the free electron mass. Remarkable discrepancies between the calculated and observed Fermi surface areas and the large mass enhancement (\({\gtrsim}3\)) highlight the importance of electronic correlations in determining the electronic structures of iron pnicitide superconductors.

Violation of the Onsager relation for quantum oscillations in superconductors

We numerically study the quantum oscillations in superconducting vortex-mixed states with $d$-wave pairing. We show that in the parameter range of an underdoped cuprate superconductor, the commonly held assumption that the period is given by the underlying Fermi-surface area using the Onsager relation becomes invalid. Using this result, we conclude that the interpretation of the recent experimental data on YBCO as a signal of an underlying Fermi surface with four hole pockets created by a $(\ensuremath{\pi},\ensuremath{\pi})$ folding cannot be ruled out.

Evolution of Fermi surface and normal-state gap in the chemically substituted cupratesBi2Sr2−xBixCuO6+δ

We have performed a systematic angle-resolved photoemission study of chemically substituted cuprates ${\text{Bi}}_{2}{\text{Sr}}_{2\ensuremath{-}x}{\text{Bi}}_{x}{\text{CuO}}_{6+\ensuremath{\delta}}$. We observed that the Fermi-surface area shrinks linearly with Bi-substitution content $x$, reflecting the electron doping nature of this chemical substitution. In addition, the spectral linewidth broadens rapidly with increasing $x$ and becomes completely incoherent at the superconducting-insulating boundary. The $d$-wave-like normal-state gap observed in the lightly underdoped region gradually evolves into a large soft gap, which suppresses antinodal spectral weight linearly in both the excitation energy and temperature. Combining with the bulk resistivity data obtained on the same samples, we establish the emergence of the Coulomb gap behavior in the very underdoped regime. Our results reveal the dual roles, doping and disorder, of off-plane chemical substitutions in high-${T}_{c}$ cuprates and elucidate the nature of the quantum electronic states due to strong correlation and disorder.

Magnetic-Field Dependence of theYbRh2Si2Fermi Surface

Magnetic-field-induced changes of the Fermi surface play a central role in theories of the exotic quantum criticality of ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2}$. We have carried out de Haas--van Alphen measurements in the magnetic-field range $8\text{ }\text{ }\mathrm{T}\ensuremath{\le}H\ensuremath{\le}16\text{ }\text{ }\mathrm{T}$, and directly observe field dependence of the extremal Fermi surface areas. Our data support the theory that a low-field ``large'' Fermi surface, including the Yb $4f$ quasihole, is increasingly spin split until a majority-spin branch undergoes a Lifshitz transition and disappears at ${H}_{0}\ensuremath{\approx}10\text{ }\text{ }\mathrm{T}$, without requiring $4f$ localization at ${H}_{0}$.

Fermi Surface Evolution and Luttinger Theorem inNaxCoO2: A Systematic Photoemission Study

We report a systematic angle-resolved photoemission study on Na(x)CoO2 for a wide range of Na concentrations (0.3 < or = x < or = 0.72). In all the metallic samples at different x, we observed (i) only a single holelike Fermi surface centered around gamma and (ii) its area changes with x according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The e'(g) band and the associated small Fermi surface pockets near the K points predicted by band calculations are found to sink below the Fermi energy in a manner almost independent of the doping and temperature.

Pressure-induced low resistive and insulating phases in τ-(EDO-R, R-DMEDT-TTF) 2(AuI 2) 1+y

A sudden drop of resistance below 7 K was observed in the two-dimensional organic τ-conductor, τ-(EDO- R, R-DMEDT-TTF) 2(AuI 2) 1+y. However, this low resistive state was not associated with superconductivity, since the resistance becomes lower by increasing field. The temperature of the resistance anomaly increased by increasing pressure. Further, the low resistive state turns into high resistive one beyond 1.6 GPa. These behaviors have been observed neither in τ-conductors nor in other organic conductors. Although the electronic properties look quite different from other τ-conductors, Fermi surface areas are only 20% different from the isostructural τ-conductors, i.e. τ-(EDO- S,S-DMEDT-TTF) 2(AuBr 2) 1+y or τ-(P- S,S-DMEDT-TTF) 2(AuBr 2) 1+y. Why so much variety of electronic properties are found in each salt in spite of having similar Fermi surface areas, is an attractive problem but unanswered.

Is the resistance upturn around 50 K related to the Fermi surface area in τ-(EDO- R,R-DMEDT-TTF) 2(AuBr 2) 1+y?

From our previous observation, it is known that τ-(EDO- R,R-DMEDT-TTF) 2(AuBr 2) 1+ y ( y ≤ 0.875) exhibits strong sample dependence in the temperature dependence of its resistance R( T). Some samples are metallic down to 4.2 K, while others show upturn below around 50 K. To investigate the origin of the sample dependence, we performed Shubnikov-de Haas (SdH) effect for four samples. Here two samples show metallic behavior, while other samples exhibit nonmetallic below 50 K. From the observation of the SdH oscillation, we detect two series of two-dimensional (2D) Fermi surfaces (FS's) pockets for all samples. Interestingly, we find slight scatter in the area of FS, but no correlation is found between R( T) and FS area. The origin of the slight difference in the FS areas is discussed in terms of possible difference in the band filling.

Comparative magnetotransport and Tcmeasurements under pressure on κ-(BEDT-TTF)2Cu(NCS)2

κ-(BEDT-TTF) 2 Cu(NCS) 2 is one of the best characterized organic superconductors. The resemblance of its pressure-temperature phase diagram to that of the carrier-density-temperature phase diagram in cuprate superconductors has frequently been taken as evidence for similar interaction mechanisms governing superconductivity, even though direct evidence for a correlation between carrier density and T c in organic superconductors had been missing. We have performed and compared several magnetoresistive as well as ac-susceptibility measurements under pressure on single crystals of κ-(BEDT-TTF) 2 Cu(NCS) 2 with different isotopical composition. The isotopical composition of κ-(BEDT-TTP) 2 Cu(NCS) 2 is irrelevant to the pressure dependence of the superconducting state. In general, pressure on κ-(BEDT-TTF) 2 Cu(NCS) 2 is seen to decrease T c and to increase the quasi 2-dimensional Fermi surface area. The exact pressure dependence, however, seems to be strongly influenced by the pressure technique used. However, the apparent divergence of the reported pressure dependences in κ-(BEDT-TTF) 2 Cu(NCS) 2 converges into one dependence of T c against the carrier concentration of the quasi 2-dimensional Fermi surface. Pressure in κ-(BEDT-TTF) 2 Cu(NCS) 2 is seen to transfer carriers from the quasi 1-dimensional to the quasi 2-dimensional Fermi surface section. The present results point to a strong correlation between T c and the quasi 2-dimensional carrier concentration in this organic superconductor.

De Haas–van Alphen measurements of the electronic structure ofLaSb2

We report the results of de Haas--van Alphen measurements in high magnetic fields on the anisotropic compound ${\mathrm{LaSb}}_{2}.$ Our measurements reveal three extremal orbits of the Fermi surface. One is very small and ellipsoidal, while the remaining two are larger and have a nearly cylindrical shape indicating a quasi-two-dimensional Fermi surface. We compare these measurements with the results of energy band calculations and find reasonable agreement with the measured Fermi surface areas, though the calculations show other multiconnected regions of the Fermi surface not observed in the de Haas--van Alphen spectra. The implications of our calculated and measured Fermi surfaces on the mechanisms proposed for the extraordinary large linear magnetoresistance in ${\mathrm{LaSb}}_{2}$ are discussed.

Pressure-dependent magnetoresistance studies of β″-(ET) 2SF 5CH 2CF 2SO 3

We report on the electronic transport properties of the organic superconductor β-(ET) 2 SF 5 CH 2 CF 2 SO 3 at high fields and under hydrostatic pressure. With increasing pressure the superconducting transition temperature decreases in line with a decreasing effective mass. The closed Fermi-surface (FS) area increases strongly but the FS topology remains unchanged.