Observation Of Quantum Oscillations Research Articles

Mixed state of aπ-striped superconductor

A model of an anti-phase modulated d-wave superconductor has been proposed to describe the decoupling between Cu-O planes in 1/8 doped La$_{2-x}$Ba$_{x}$CuO$_{4}$. Unlike a uniform d-wave superconductor, this model exhibits an extended Fermi surface. Within Bogoliubov-de Gennes theory, we study the mixed state of this model and compare it to the case of a uniform d-wave superconductor. We find a periodic structure of the low-energy density of states, with a period that is proportional to $B$, corresponding to Landau levels that are a coherent mixture of particles and holes. These results are also discussed in the context of experiments which observe quantum oscillations in the cuprates, and are compared to those for models in which the Fermi surface is reconstructed due to translational symmetry breaking in the non-superconducting state and to a model of a Fermi-arc metal.

Electronic topological transition in an n-BiSb semiconductor alloy in the quantum limit range of magnetic fields for H‖C 2

The galvanomagnetic properties of single-crystal samples of the Bi0.93Sb0.07 semiconductor alloy with the electron density n = 1.6 × 1017 cm−3 in magnetic fields up to 14 T at T = 1.6 K have been investigated. The resistivity ρ and Hall coefficient R have been measured as functions of the magnetic field directed along the binary axis of a crystal for a current flowing through a sample along the bisector axis; i.e., the components ρ22 and R 32, 1 have been measured. The strong anisotropy of the electron spectrum of the samples makes it possible to separately observe quantum oscillations of the magnetoresistance ρ22(H) for H ‖ C 2 in low magnetic fields for two equivalent ellipsoids with small extremal cross sections (secondary ellipsoids) and in high magnetic fields for electrons of the ellipsoid with a large extremal cross section (main ellipsoid). An increase in the energy of the electrons of secondary ellipsoids in the quantum limit magnetic fields is accompanied by the flow of electrons to the main ellipsoid; i.e., an electronic topological transition occurs from the three-valley electron spectrum to the single-valley one. After the flow stops, the Fermi energy E F increases from 18 meV to 27.8 meV. With an increase in the quantizing magnetic field, the Fermi energy of the electrons decreases both in the region of quantum oscillations of the resistance that are attributed to the electrons of the secondary ellipsoids and in the region of oscillations associated with the electrons of the main ellipsoid. The Hall coefficient R 32, 1 decreases in high magnetic fields; this behavior indicates the absence of the electron magnetic freezing effect.

Ca3Ru2O7: Density Wave Formation and Quantum Oscillations in the Hall Resistivity

We describe transport measurements in single-crystal, high-purity Ca 3 Ru 2 O 7 . The observation of a large linear magnetoresistance together with low frequency quantum oscillations is shown to be consistent with a small volume Fermi surface incompletely gapped by density wave formation. This complements previous ARPES experiments. The quantum oscillations are more pronounced in the Hall signal than in the longitudinal resistivity. This unusual observation is also explained by the peculiar electronic structure in this material. We remark on the similarity between our observations in Ca 3 Ru 2 O 7 and the observations of quantum oscillations in the underdoped cuprate superconductors.

Redistribution of electrons between ellipsoids in a magnetic field in the quantum limit range in n-Bi-Sb alloys

The magnetoresistivities ρ22(H) and ρ32(H) and the Hall coefficient R32.1 for single-crystal samples of the n-Bi0.93Sb0.07 semiconducting alloy have been measured at low temperatures in magnetic fields up to H=14 T at H∥C2. The samples with three electron concentrations n1=1.25 × 1016 cms-3, n2=3.5×1016 cms-3, and n3=1.6×1017 cms-3 have been studied. The strong anisotropy of the electron spectrum of the alloys has made it possible to observe quantum oscillations of the magnetoresistivity ρ22(H) at H∥C2 for electrons of the secondary ellipsoids with the transition to the quantum limit in high magnetic fields. However, in the same magnetic fields, the quantization condition for electrons of the main ellipsoid is not satisfied. An increase in the energy of electrons of the secondary ellipsoids in the magnetic fields of the quantum limit leads to their migration to the main ellipsoid. After the complete migration, the Fermi energy for the alloy samples with the electron concentrations n1, n2, and n3 increases from 7.0 to 11.3 meV, from 11.0 to 17.1 meV, and from 20.2 to 30.6 meV, respectively. After the migration, the magnetoresistivity for electrons of the main ellipsoid increases with an increase in the magnetic field and the specific features in the behavior of the kinetic coefficients are observed in the vicinity of the magnetic field H=10 T. Therefore, the electronic topological transition from the three-valley electron spectrum to the single-valley electron spectrum occurs in the Bi0.93Sb0.07 single crystals for H∥C2 at low temperatures in the range of magnetic fields of the quantum limit.

Spin-Order Driven Fermi Surface Reconstruction Revealed by Quantum Oscillations in an Underdoped HighTcSuperconductor

We use quantum oscillation measurements to distinguish between spin and orbital components of the lowest energy quasiparticle excitations in YBa(2)Cu(3)O(6.54), each of which couple differently to a magnetic field. Our measurements reveal the phase of the observed quantum oscillations to remain uninverted over a wide angular range, indicating that the twofold spin degeneracy of the Landau levels is virtually unaltered by the magnetic field. The inferred suppression of the spin degrees of freedom indicates a spin-density wave is responsible for creation of the small Fermi surface pockets in underdoped YBa(2)Cu(3)O(6+x)--further suggesting that excitations of this phase are important contributors to the unconventional superconducting pairing mechanism.

Fermi surface ofα-uranium at ambient pressure

We have performed de Haas--van Alphen measurements of the Fermi surface of $\ensuremath{\alpha}$-uranium single crystals at ambient pressure within the ${\ensuremath{\alpha}}_{3}$ charge-density wave (CDW) state from 0.020--10 K and magnetic fields to 35 T using torque magnetometry. The angular dependence of the resulting frequencies and the effective masses were measured. The observation of quantum oscillations within the ${\ensuremath{\alpha}}_{3}$ CDW state gives insight into the effect of the charge-density waves on the Fermi surface. We observed no signature of superconductivity in either transport or magnetization down to 0.020 K at ambient pressure.

Synthesis of the phenomenology of the underdoped cuprates

The underdoped cuprates have a number of interesting and unusual properties that often seem hard to reconcile with one another. In this paper we show how many of these diverse phenomena can be synthesized into a single coherent theoretical picture. Specifically we present a description where a pseudogap and gapless Fermi arcs exist in the normal state above the superconducting transition temperature ($T_c$), but give way to the observed quantum oscillations and other phenomena at low temperature when the superconductivity is suppressed by a magnetic field. We show the consistency between these phenomena and observations of enhanced Nernst and diamagnetic signals above $T_c$. We also develop a description of the vortex core inside the superconducting state and discuss its relation with the high field phenomena.

Spin-Density Wave Fermi Surface Reconstruction in UnderdopedYBa2Cu3O6+x

We consider the reconstruction expected for the Fermi surface of underdoped YBa(2)Cu(3)O(6+x) in the case of a collinear spin-density wave with a characteristic vector Q = (pi[1 +/- 2delta],pi), assuming an incommensurability delta approximately 0.06 similar to that found in recent neutron scattering experiments. A Fermi surface possibly consistent with the multiple observed quantum oscillation frequencies is obtained. From the low band masses expected using this model as compared with experiment, a uniform enhancement of the quasiparticle effective mass over the Fermi surface by a factor of approximately 7 is indicated. Further predictions of the Fermi surface topology are made, which may potentially be tested by experiment to indicate the relevance of this model to underdoped YBa(2)Cu(3)O(6+x).

Dependence of the magnitude and direction of the persistent current on the magnetic flux in superconducting rings

The obtained periodic magnetic-field dependences I c+(Φ/Φ0) and I c−(Φ/Φ0) of the critical current measured in opposite directions on asymmetric superconducting aluminum rings has made it possible to explain previously observed quantum oscillations of dc voltage as a result of alternating current rectification. It was found that a higher rectification efficiency of both single rings and ring systems is caused by hysteresis of the current-voltage characteristics. The asymmetry of current-voltage characteristics providing the rectification effect is due to the relative shifts of the magnetic dependences I c−(Φ/Φ0) = I c+(Φ/Φ0 + Δϕ) of the critical current measured in opposite directions. This shift means that the position of I c+(Φ/Φ0) and I c−(Φ/Φ0) minima does not correspond to n + 0.5 magnetic flux Φ quanta, which is in direct contradiction to measured Little-Parks resistance oscillations. Despite this contradiction, the amplitude I c, an(Φ/Φ0) = I c+(Φ/Φ0) − I c−(Φ/Φ0) of critical current anisotropy oscillations and its variations with temperature correspond to the expected amplitude of persistent current oscillations and its variations with temperature.

Observation of Quantum Oscillations between a Josephson Phase Qubit and a Microscopic Resonator Using Fast Readout

We have detected coherent quantum oscillations between Josephson phase qubits and critical-current fluctuators by implementing a new state readout technique that is an order of magnitude faster than previous methods. These results reveal a new aspect of the quantum behavior of Josephson junctions, and they demonstrate the means to measure two-qubit interactions in the time domain. The junction-fluctuator interaction also points to a possible mechanism for decoherence and reduced fidelity in superconducting qubits.

Quasi-two-dimensional metallic ground state ofCa3Ru2O7

We have succeeded in growing single crystals of ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ using a floating-zone method. The temperature dependence of the electrical resistivity establishes that ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ develops a quasi-two-dimensional metallic ground state below $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, from which the observed quantum oscillation derives. The temperature dependence of specific heat reveals the electronic specific-heat coefficient $\ensuremath{\gamma}$ to be as small as $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕\mathrm{Ru}\phantom{\rule{0.3em}{0ex}}\mathrm{mol}\phantom{\rule{0.3em}{0ex}}{\mathrm{K}}^{2}$. A qualitative difference exists between the field dependences of ${\ensuremath{\rho}}_{ab}$ and ${\ensuremath{\rho}}_{c}$. The field dependence of the resistivity at the metamagnetic transition around $6\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ can be explained from the tunneling magnetoresistance.

Coherent dynamics of a Josephson charge qubit

We have fabricated a Josephson charge qubit by capacitively coupling a single-Cooper-pair box (SCB) to an electrometer based upon a single-electron transistor configured for radio-frequency readout (RF-SET). Charge quantization of 2e is observed and microwave spectroscopy is used to extract the Josephson and charging energies of the box. We perform coherent manipulation of the SCB by using very fast DC pulses and observe quantum oscillations in time of the charge that persist to ~=10ns. The observed contrast of the oscillations is high and agrees with that expected from the finite E_J/E_C ratio and finite rise-time of the DC pulses. In addition, we are able to demonstrate nearly 100% initial charge state polarization. We also present a method to determine the relaxation time T_1 when it is shorter than the measurement time T_{meas}.

Quantum oscillations in two coupled charge qubits.

A practical quantum computer, if built, would consist of a set of coupled two-level quantum systems (qubits). Among the variety of qubits implemented, solid-state qubits are of particular interest because of their potential suitability for integrated devices. A variety of qubits based on Josephson junctions have been implemented; these exploit the coherence of Cooper-pair tunnelling in the superconducting state. Despite apparent progress in the implementation of individual solid-state qubits, there have been no experimental reports of multiple qubit gates--a basic requirement for building a real quantum computer. Here we demonstrate a Josephson circuit consisting of two coupled charge qubits. Using a pulse technique, we coherently mix quantum states and observe quantum oscillations, the spectrum of which reflects interaction between the qubits. Our results demonstrate the feasibility of coupling multiple solid-state qubits, and indicate the existence of entangled two-qubit states.

Fermi surface and magnetic properties ofCexLa1−xSballoys(x=0.5,0.9)

We have investigated the Fermi surface (FS) and magnetic properties of ${\mathrm{Ce}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}\mathrm{Sb}$ alloys for $x=0.9,0.5$ by means of the de Haas--van Alphen effect and high-field magnetization measurements. We successfully observed quantum oscillations in ${\mathrm{Ce}}_{0.5}{\mathrm{La}}_{0.5}\mathrm{Sb}$ and determined its Fermi surface. The other missing \ensuremath{\alpha} FS due to the p-f mixing effect in ${\mathrm{Ce}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}\mathrm{Sb}$ at high concentration, which degenerates for spins in LaSb at the ${X}_{z}$ point, was found in ${\mathrm{Ce}}_{0.5}{\mathrm{La}}_{0.5}\mathrm{Sb}.$ The obtained results indicate that the FS and magnetic properties of ${\mathrm{Ce}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}\mathrm{Sb}$ exhibit a remarkable transformation, strongly dependent on La substitution due to the suppression of the p-f mixing effect. These results indicate that the p-f mixing effect becomes weaker and, consequently, the crystalline electric field effect becomes dominant in ${\mathrm{Ce}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}\mathrm{Sb}$ with increasing La concentration.

De Haas–van Alphen and Shubnikov–de Haas oscillations inRAgSb2(R=Y,La-Nd, Sm)

de Haas--van Alphen and Shubnikov--de Haas oscillations have been used to study the Fermi surface of the simple tetragonal $R{\mathrm{AgSb}}_{2}$ series of compounds with $R=\mathrm{Y},$ La-Nd, and Sm. The high quality of the flux-grown single crystals, coupled with very small extremal cross sections of Fermi surface, allow the observation of quantum oscillations at modest fields $(Hl30\mathrm{kG})$ and high temperatures (up to 25 K in ${\mathrm{SmAgSb}}_{2}$). For $H\ensuremath{\parallel}c,$ the effective masses, determined from the temperature dependence of the amplitudes, are quite small, typically between 0.07 and ${0.5m}_{0}.$ The topology of the Fermi surface was determined from the angular dependence of the frequencies for $R=\mathrm{Y},$ La, and Sm. In ${\mathrm{SmAgSb}}_{2},$ antiferromagnetic ordering below 8.8 K is shown to dramatically alter the Fermi surface. For ${\mathrm{LaAgSb}}_{2}$ and ${\mathrm{CeAgSb}}_{2},$ the effect of applied hydrostatic pressure on the frequencies was also studied. Finally, the experimental data were compared to the Fermi surface calculated within the tight-binding linear muffin-tin orbital approximation. Overall, the calculated electronic structure was found to be consistent with the experimental data.

Collective response of YBa2Cu3O7/Ag weak-link junction arrays

The collective response of superconducting thin-film arrays of 300×300 coupled YBa2Cu3O7/Ag weak-link junctions is investigated as a function of external magnetic fields. Our highT c arrays, which were fabricated using electron-beam and photolithography techniques, are based on a substrate-induced defect method. In this method, a square array of thin-film Ag lines produces a two-dimensional lattice of junctions in an overlaying thin-film array of highT c lines. We find that the zero-field resistive transition of the array is governed by a Kosterlitz-Thouless-type transition. Below the super-conducting transition, we observe quantum oscillations in magnetoresistance curves at both integer and half-integer multiples of the flux quantum. These results suggest that long-range phase coherence may be established in large highT c arrays.

De Haas-van Alphen effect and the Fermi surface in κ-(BEDT-TTF)2I3

The de-Haas-van Alphen effect and the Fermi surface have been studied in the κ-typ organic superconductor (BEDT-TTF)2I3. The originally proposed Fermi surface using the tight-binding band calculation is different from the ones reported afterwards for the other κ-type salts. The Fermi surface has simple 2-dimensional cylindrical shape and no band gap exists at the zone boundary due to the crystal symmetry. The absence of band gap at the boundary is known as the band sticking effect. We confirmed the band sticking effect experimentally using the magnetization measurement. The observed quantum oscillation frequency corresponds to the 100% area of the first Brillouin zone. The Fourier analysis has revealed another frequency corresponding to the 20% area of the first zone other than the fundamental one. This frequency corresponds to the one reported for the other κ-type salts (BEDT-TTF)2Ag(CN)2H2O and (BEDT-TTF)2Cu(NCS)2. The effective mass of the fundamental frequency and the Dingle temperature are 2.7m0 and 0.9 K respectively. The many body renormalization effect has benn discussed considering mass enhancement and strong coupling model for the superconductor.

Magnetotransport Properties and Subband Structure of the Two‐Dimensional Electron Gas in the Inversion Layer of Hg1−xCdxTe Bicrystals

AbstractThe electronic and magnetotransport properties of conduction electrons in the grain boundary interface of p‐type Hg1−xCdxTe bicrystals are investigated. The results clearly demonstrate the existence of a two‐dimensional degenerate n‐type inversion layer in the vicinity of the grain boundary. The observed quantum oscillations of the magnetoresistivity result from a superposition of the Shubnikov‐de Haas effect in several occupied electric subbands. The occupation of higher subbands is presumable depending on the total carrier density ns of the inversion layer. Electron densities, subband energies, and effective masses of these electric subbands in samples with different total densities are determined. The effective masses of lower subbands are markedly different from the band edge values of the bulk material, their values decrease with decreasing electron density and converging to the bulk values at lower densities. This agrees with predictions of the triangular potential well model and a pronounced nonparabolicity of the energy bands in Hg1−xCdxTe. At high magnetic fields (B > 10 T) it is experimentally verified that the Hall resistivity ϱxy is quantized into integer multiplies of h/e2.

Direct measurement of the density of states of a two-dimensional electron gas.

We present a direct measurement of the density of states of a two-dimensional electron gas in a quantizing magnetic field. By measuring the magnetocapacitance of modulation-doped GaAs-(AlGa) As heterostructures at low frequencies, we have observed quantum oscillations resulting from changes in the density of states alone.

Two‐Dimensional Electron Gas in the n‐Inversion Layer of p‐Type InSb Bicrystals

AbstractThe galvanomagnetic properties of conduction electrons in n‐type inversion layers adjacent to the grain boundary in p‐type InSb bicrystals in magnetic fields up to 18 T and temperatures down to 2 K are investigated. The observed quantum oscillations of the magnetoresistivity result from a superposition of the Shubnikov‐de Haas effect in two occupied subbands. The electron densities of subbands nsi (ns1 = 3.5 × 1011 cm−2; nso = 1.1 × 1012 cm−2) and the effective cyclotron mass of the subbands mci (mc1 = (0.016 ± 0.004) m0; mco = (0.026 ± 0.003) m0) are evaluated. Besides the Shubnikov‐de Haas oscillations in high magnetic fields (B > 5 T) the Hall resistivity shows the characteristic features of the quantized Hall plateaus in finite regions of the magnetic field B. Simultaneously the diagonal resistivity ϱxx drops to a minimum. Other characteristic features of a two‐dimensional electron gas confined in the symmetrical potential well of the grain boundary are obtained.