We have measured the full counting statistics (FCS) of current fluctuations in a semiconductor quantum dot (QD) by real-time detection of single electron tunneling with a quantum point contact (QPC). This method gives direct access to the distribution function of current fluctuations. Suppression of the second moment (related to the shot noise) and the third moment (related to the asymmetry of the distribution) in a tunable semiconductor QD is demonstrated experimentally.With this method we demonstrate the ability to measure very low current and noise levels.

We report 115 In and 59 Co Nuclear Magnetic Resonance (NMR) measurements in the heavy fermion superconductor CeCoIn5 above and below Tc. The hyperfine couplings of the 115 In and 59 Co are anisotropic and exhibit dramatic changes below 50K due to changes in the crystal field level populations of the Ce ions. Below Tc the spin susceptibility is suppressed, indicating singlet pairing. PACS Numbers: 74.70.Tx, 76.60.Cq In heavy fermion systems the interplay of magnetism and superconductivity gives rise to a diverse range of ground states including an unconventional form of superconductivity. The recently discovered family of heavy fermion compounds CeMIn 5 , where M = Co, Rh or Ir exemplifies these effects. Whereas the Rh compound undergoes a transition from antiferromagnetic to superconducting under pressure [1], the Ir [2] and Co [3] compounds superconduct at ambient pressure, with the Co system exhibiting the highest known transition temperature (2.3K) for any heavy fermion system. Evidence from heat capacity, thermal transport and µSR indicate that the pairing symmetry in the superconducting state is unconventional and that there are line nodes in the superconducting gap. [4,5] The bulk magnetic susceptibility, χ, of tetragonal CeMIn 5 displays systematic trends consistent with the diversity of observed ground states. In all three cases χ is anisotropic, and is largest for field applied along the c direction. In the ab plane, χ ab is essentially the same for all three materials. However, χ c exhibits a maximum at ∼ 10 K for CeRhIn 5 (T N = 3.8 K), whereas for the superconductors CeIrIn 5 and CeCoIn 5 χ c diverges at low temperatures until T c is reached. For both of these materials χ c also exhibits a plateau-like feature around 50 K, which is less pronounced for the Ir system. The origin of this feature and the relationship between χ c and T c have been sources of debate, however both the plateau and the divergence are intrinsic and independent of field.[3]Here we report a detailed study of site-specific magnetic shifts in CeCoIn 5 using nuclear magnetic resonance (NMR). Measurements in the normal state provide a microscopic measure of the local susceptibility and we find anomalous temperature dependencies. This behavior is likely due to the thermal depopulation of a crystal field (CEF) excitation of the Ce ions. We find remarkably strong departures from the expected proportionality between bulk susceptibility and the NMR Knight shift. We will argue that this effect is indicative of a high degree Ce moment localization, a feature that may play a role in the mechanism for superconductivity in this material. In the superconducting state the temperature dependencies of the shifts reveal a suppression of the spin susceptibility consistent with spin-singlet pairing.Crystals of CeCoIn 5 were grown from an In flux as described in [3]. The tetragonal crystal structure of CeCoIn 5 consists of alternating layers of CeIn 3 and CoIn 2 and so has two inequivalent In sites per unit cell. The In(1) site has axial symmetry ...

We present time-resolved measurements of electron transport through a quantum dot. The measurements were performed using a nearby quantum point contact as a charge detector. The rates for tunneling through the two barriers connecting the dot to source and drain contacts could be determined individually. In the high bias regime, the method was used to probe excited states of the dot. Furthermore, we have detected bunching of electrons, leading to super-Poissonian noise. We have used the framework of full counting statistics (FCS) to model the experimental data. The existence of super-Poissonian noise suggests a long relaxation time for the involved excited state, which could be related to the spin relaxation time.PACS numbers:

The low-temperature magnetoresistance of parabolic quantum wells displays pronounced minima between integer filling factors. Concomitantly the Hall effect exhibits overshoots and plateau-like features next to well-defined ordinary quantum Hall plateaus. These effects set in with the occupation of the second subband. We discuss our observations in the context of single-particle Landau fan charts of a two-subband system empirically extended by a density dependent subband separation and an enhanced spin-splitting g ⋆ . PACS numbers:At low temperatures a two-dimensional electron gas subject to a perpendicular magnetic field gives rise to the integer quantum Hall effect [1]. The Hall effect takes on quantized values ρ xy = h/e 2 ν around magnetic fields B = N s h/eν, with the integer filling factor ν counting the number of occupied Landau levels and N s being the carrier density. In these plateau regions of ρ xy the magnetoresistance ρ xx is exponentially suppressed. Each Landau level is described by a Landau level quantum number n = 0, 1, 2, . . ., a spin quantum number s = ±1/2 and a subband or well index i if several subbands in a quantum well are occupied or a double well system is investigated.Zhang et al.[2] have reported measurements on a quantum well system where the occupation of a second subband leads to additional maxima and minima in ρ xx between integer filling factors, accompanied by over-and undershoots of ρ xy . Here we report similar results obtained on parabolic quantum wells in which the subband occupation can be controlled by front and back gate voltages. We find that the observed phenomena can be qualitatively accounted for using an effective single-particle model where interaction effects beyond the mean field Hartree and exchange interactions are neglected. Previous experiments on double wells showed additional minima as well as hysteretic behavior (see Ref.[3] for a review) which was attributed to more subtle interaction effects [4,5].Our samples are 100 nm wide parabolic Al x Ga 1−x As quantum wells with the Al-content varying from x = 0.4 at the edges to x = 0 in the center of the parabola. Hall bars with Ohmic contacts were fabricated. A Schottky front gate and a back gate [10] allows to tune the electron sheet density N s and with it the number of occupied subbands. At a temperature of 100 mK and zero gate voltages the electron mobility is 16 m 2 /Vs and N s = 3.3 × 10 15 m −2 . While we have investigated many similar samples in the past [6,7,8,9,10]

We report measurements of the longitudinal ( 139 T −1 1 ) and transverse ( 139 T −1 2 ) decay rates of the magnetization of 139 La nuclei performed in a high quality single crystal of La1.65Eu0.2Sr0.15CuO4. We observe a dramatic slowing of the Cu 3d spins manifested as a sharp increase of both 139 T −1 1 and 139 T −1 2 below 30 K. We find that in this temperature range the fluctuations involve a unique time scale τ which diverges as (T − TA) −1.9 with TA ≈ 5 K. This behavior is distinct from the continuous freezing observed in underdoped La1−xSrxCuO4 which involves a distribution of energy barriers. By contrast, in La1.65Eu0.2Sr0.15CuO4, the freezing below 30K is intrinsic to its magnetic ground state and the observed power law supports the existence of a glass forming "charge stripe liquid".Charge inhomogeneities in doped transition metaloxides have aroused great interest because of their possible implication in superconductivity. Indeed, it has been proposed [1,2,3,4] that the superconducting state in underdoped cuprates may consist of a charge stripe phase, in which hole-rich fluctuating stripes act as antiphase boundaries between undoped antiferromagnetic (AF) domains. Experimentally, the presence of a stripe order was inferred from elastic neutron scattering measurements [5] in compounds that are not superconducting: doped lanthanum nickelates and the rare earth codoped lanthanum cuprate La 1.48 Nd 0.4 Sr 0.12 CuO 4 . In the latter, the substitution for La ions by isovalent rare earth ions (Nd or Eu) induces a structural transition from the low temperature orthorhombic (LTO) to the low temperature tetragonal (LTT) phase in which a magnetic ground state occurs in lieu of the superconducting state [6].In both structures, LTO and LTT, the CuO 6 octahedra are tilted inducing a staggered buckling of the CuO 2 plane [7]. The difference between the two phases consists only of a rotation of the tilt axis from [110] HTT in the LTO phase to [100] HTT or [010] HTT alternating along the c axis in the LTT phase. Though specific to lanthanum cuprates, the LTT structure was shown [6,8] to have the remarkable property of generating either a magnetic or superconducting ground state with similar critical temperature at fixed optimal Sr 2+ doping (x=0.15) depending only on the amplitude of the buckling, controlled by the Eu or Nd concentration. This suggests the same physics underlies the two outcomes, but little is known about the fundamental mechanism that connects them. In this context, it is important to gain further insight into the low energy properties of the magnetic ground state in rare earth co-doped lanthanum cuprates.In this Letter, we report NMR measurements performed on 63 Cu and 139 La nuclei in a La 1.65 Eu 0.2 Sr 0.15 CuO 4 single crystal. For the first time, we investigate the slowing down of the Cu 3d spins in this compound by measuring two relaxation rates: 139 T −1 1 and 139 T −1 2 . For a given orientation of the crystal in a static field H 0 , T −1 1 probes the spectral weight of the transverse fluctuati...

We have measured the magneto-resistance of a two-dimensional electron gas (2DEG) under continuous microwave irradiation as a function of electron density and mobility tuned with a metallic top-gate. In the entire range of density and mobility we have investigated, we observe microwave induced oscillations of large amplitude that are B-periodic. These B-periodic oscillations are reminiscent of the ones reported by Kukushkin et al [1] and which were attributed to the presence of edge-magneto-plasmons. We have found that the B-periodicity does not increase linearly with the density in our sample but shows a plateau in the range (2.4-3)×1011 cm −2 . In this regime, the phase of the B-periodic oscillations is found to shift continuously by two periods.

The low temperature behavior of the CsC 60 polymer was investigated by NMR measurements on 13 C and 133 Cs nuclei at ambient and under hydrostatic pressure up to 9 kbar. The existence of a second-order structural phase transition to a spin-singlet (nonmagnetic) ground state at T S 13.8 K is clearly established at ambient pressure. This state coexists with the magnetic order that develops at T N 30 K. The application of pressure first suppresses the magnetic order and a homogeneous nonmagnetic ground state is stabilized at 5 kbar. [S0031-9007(99)08654-8] PACS numbers: 61.48. + c, 76.60. -kThe presence of parallel polymerized chains of C 60 molecules in orthorhombic alkali doped fullerides belonging to the AC 60 series (A K, Rb, Cs) [1] may suggest the possibility of quasi-one-dimensional (1D) electronic properties. In this spirit, the onset of a metal-insulator transition near 40 K (50 K) in polymerized CsC 60 (RbC 60 ) was interpreted in terms of strong electron-electron correlations [2]. Magnetic fluctuations were evident [3] in nuclear magnetic resonance (NMR) experiments, dominating the relaxation rate of 13 C up to 300 K. The temperature dependence of the ESR linewidth [2], which is narrow even in the presence of fluctuations at ambient temperature, was also ascribed to reduced dimensionality [4]. Magnetic fluctuations above the transition temperature and a metal-insulator transition of magnetic nature are consistent with the observed divergent relaxation rate and the concomitant strong broadening of the NMR spectra below 40 K (50 K) for the 13 C and 133 Cs ( 87 Rb) nuclei [3] in CsC 60 (RbC 60 ). An antiferromagnetic resonance [5] below T N ഠ 30 K in CsC 60 and 35 K in RbC 60 has corroborated the conclusions of the NMR studies. From this body of evidence a second-order metal-insulator transition to a magnetic ground state was inferred, in agreement with the suspected 1D spin and charge dynamics. Besides, x-ray diffraction [6] has shown a different chain orientation between KC 60 and RbC 60 but local density approximation calculations [7], performed in the RbC 60 structure [6], predict isotropic 3D energy bands. The corresponding metallic behavior was actually observed only in KC 60 [2]. Such a discrepancy between band calculations and experiments emphazises that despite the polymerization, 1D features in RbC 60 and CsC 60 are not evident.In this Letter, we report a detailed study of 13 C and 133 Cs-NMR in the CsC 60 polymerized fulleride at ambient and under hydrostatic pressure. The analysis of the 133 Cs quadrupolar spin echo enables us to measure the effects of lattice distortions with a great sensitivity. We find a second-order structural transition to a nonmagnetic singlet state at T S 13.8 K (60.2) which coexists with the magnetic phase developed at T N 30 K. The applied pressure suppresses the magnetic fluctuations and stabilizes the homogeneous spin-singlet ground state for which Ts increases up to 20 K at 5 kbar. The measurements were made on two samples, one of them 13 C isotopically enriched, with...

We investigate the local symmetry of the tilting of the CuO6 octahedra in La1.65Eu0.2Sr0.15CuO4 by means of 63 Cu NMR spectroscopy and the Van Vleck susceptibility of the Eu 3+ ions. The Cu NMR central line lineshape is sensitive to local structure through the coupling of the 63 Cu nuclear quadrupole moment to the local electric field gradient. The Eu 3+ Van Vleck susceptibility, as a single ion effect, locally probes the symmetry of the crystal field at the Eu site. Both techniques independently provide clear evidence for a change of the local tilt symmetry at the first order structural transition from the orthorhombic to the low temperature tetragonal phase, in excellent agreement with the average structure obtained by diffraction techniques. We conclude that the symmetry of the average crystal structure accurately represents the symmetry of the octahedral tilt pattern on a local scale.PACS numbers: 61.18. Fs, 74.72.Dn Lanthanum cuprate La 2−x Sr x CuO 4 , the single layer high temperature superconductor, has been extensively studied over the past few years to understand the mechanism of superconductivity. Recently, rare earth codoped lanthanum cuprates La 2−x−y R y Sr x CuO 4 (R = Eu or Nd) have attracted considerable attention because of the subtle interplay between charge stripes and superconductivity.1 In this family of compounds it is possible to tune the optimally doped La 2−x Sr x CuO 4 from a superconducting to a magnetic phase where charge is spatially modulated 1,2 by changing the tilt distortion of the CuO 2 -layers. These modifications of the structure result if La 3+ is partially substituted by smaller R 3+ ions. 3,4 The suppression of superconductivity observed in these doping experiments clearly shows an intimate connection between structure and electronic properties in the high temperature superconductors.3,4 Based on the structural data obtained from diffraction experiments it has been argued that both the symmetry and the magnitude of the tilt distortion of the CuO 2 layers are key factors determining the electronic properties in La 2−x−y R y Sr x CuO 4 4,5 . These neutron and x-ray diffraction techniques, sensitive to the average structure, show that rare earth co-doped lanthanum cuprates can exist in three structural phases dependent upon Sr 2+ and R 3+ doping and upon temperature: the high temperature tetragonal phase (HTT), the low temperature orthorhombic phase (LTO) and the low temperature tetragonal phase (LTT). All three phases can be described by different patterns of rotated CuO 6 octahedra. The CuO 2 plane is flat in the HTT phase and the transition to LTO (spacegroup Abma) consists of a tilt of the CuO 6 octahedra along the [110] direction using the notation for the HTT unit cell (I4/mmm); see Fig. 1. The tilt angle increases gradually as the temperature is lowered and the compound undergoes a first order structural transition to the LTT phase (P 4 2 /ncm) via a discontinuous change of the tilt direction, which is then oriented alternately along [100] and [010] in adjacent CuO ...