In zero magnetic field, conductance measurements of clean one-dimensional (1D) constrictions defined in GaAs/AlGaAs heterostructures show twenty-six quantized ballistic plateaux, as well as a structure close to $0.7(2e^2/h)$. In an in-plane magnetic field all the 1D subbands show Zeeman splitting and in the wide channel limit the $g$-factor is $\mid g \mid = 0.4$, close to that of bulk GaAs. For the last subband spin-splitting originates from the structure at $0.7(2e^2/h)$, indicating spin polarization at $B=0$. The measured enhancement of the $g$-factor as the subbands are depopulated suggests that the ``0.7 structure'' is induced by electron-electron interactions.Comment: Ten pages with four ps figures. Accepted for publication in Phys. Rev. Lett. (17. June 1996

We have investigated the transport properties of one-dimensional (1D) constrictions defined by split-gates in high quality GaAs/AlGaAs heterostructures. In addition to the usual quantized conductance plateaus, the equilibrium conductance shows a structure close to 0.7(2e 2 /h), and in consolidating our previous work [K. J. Thomas et al., Phys. Rev. Lett. 77, 135 (1996)] this 0.7 structure has been investigated in a wide range of samples as a function of temperature, carrier density, in-plane magnetic field B and source-drain voltage V sd . We show that the 0.7 structure is not due to transmission or resonance effects, nor does it arise from the asymmetry of the heterojunction in the growth direction. All the 1D subbands show Zeeman splitting at high B , and in the wide channel limit the g-factor is | g |≈ 0.4, close to that of bulk GaAs. As the channel is progressively narrowed we measure an exchange-enhanced g-factor. The measurements establish that the 0.7 structure is related to spin, and that electron-electron interactions become important for the last few conducting 1D subbands.

We report a detailed study, using neutron scattering, transport and magnetization measurements, of the interplay between superconducting (SC) and spin density wave (SDW) order in La2CuO4+y. Both kinds of order set in below the same critical temperature. However, the SDW order grows with applied magnetic field, whereas SC order is suppressed. Most importantly, the field dependence of the SDW Bragg peak intensity has a cusp at zero field, as predicted by a recent theory of competing SDW and SC order. This leads us to conclude that there is a repulsive coupling between the two order parameters. The question of whether the two kinds of order coexist or microscopically phase separate is discussed.

We report conductance measurements of a ballistic one-dimensional (1D) wire defined in the lower two-dimensional electron gas of a GaAs/AlGaAs double quantum well. At low temperatures there is an additional structure at 0.7(2e 2 /h) in the conductance, which tends to e 2 /h as the electron density is decreased. We find evidence for complete spin polarization in a weakly disorderd 1D wire at zero magnetic field through the observation of a conductance plateau at e 2 /h, which strengthens in an in-plane magnetic field and disappears with increasing electron density. In all cases studied, with increasing temperature structure occurs at 0.6(2e 2 /h). We suggest that the 0.7 structure is a many-body spin state excited out of, either the spin-polarized electron gas at low densities, or the spin-degenerate electron gas at high densities.One-dimensional (1D) semiconductor systems can be fabricated by a variety of techniques. Some of the best quality devices, as determined by the clarity of the quantized plateaus in the conductance characteristics, are obtained by electrostatically squeezing a two-dimensional electron gas (2DEG) at a GaAs/AlGaAs interface using a split-gate defined by electron-beam lithography. 1The conductance, measured as a function of the split-gate voltage, exhibits plateaus quantized at integer multiples of 2e2 /h, a result that is well understood as the adiabatic transmission of spin-degenerate 1D subbands. However, after the last 1D subband has been depopulated, an additional structure in the conductance has been measured at 0.7(2e 2 /h). One of the most revealing properties of this so-called 0.7 structure is its evolution into the spin-split plateau at e 2 /h in a strong in-plane magnetic field. There is also an enhancement of the g-factor as the 1D carrier density is reduced. Both results suggest that there is a possible spin polarization of the 1D electron gas at zero magnetic field. Hartree-Fock calculations 3 of electrons confined in a cylindrical wire show that correlation effects are weak, and that at low electron densities exchange interactions will drive a spontaneous spin polarization. A spin polarization at zero magnetic field would give an extra plateau in the conductance at e 2 /h rather than 0.7(2e 2 /h). To explain this discrepancy various theories 4-8 invoking spin have been put forward. Recent quantum Monte Carlo calculations 9 show that in 1D the paramagnetic state is always lower in energy than the ferromagnetic state, so it is not clear whether the Hartree-Fock calculations are in conflict with the Lieb-Mattis prediction 10 that there is no ferromagnetic order in a 1D system. The role of disorder in 1D systems is little understood, but it has been shown 11 within mean-field theory that for dimensions d ≤ 2 a disordered system may exhibit a partial spin polarization, even though the system without disorder is paramagnetic.The 0.7 structure is distinctly different from the conductance plateaus measured 12 at multiples of α(2e 2 /h) in long wires fabricated by overgrowth on a clea...

We present data from an induced gallium arsenide (GaAs) quantum wire that exhibits an additional conductance plateau at 0.5(2e2/h), where e is the charge of an electron and h is Planck's constant, in zero magnetic field. The plateau was most pronounced when the potential landscape was tuned to be symmetric by using low-temperature scanning-probe techniques. Source-drain energy spectroscopy and temperature response support the hypothesis that the origin of the plateau is the spontaneous spin-polarization of the transport electrons: a ferromagnetic phase. Such devices may have applications in the field of spintronics to either generate or detect a spin-polarized current without the complications associated with external magnetic fields or magnetic materials.

We study the low-temperature transport properties of 1D quantum wires as the confinement strength V conf and the carrier density n 1D are varied using a combination of split gates and a top gate in GaAs=AlGaAs heterostructures. At intermediate V conf and n 1D , we observe a jump in conductance to 4e 2 =h, suggesting a double wire. On further reducing n 1D , plateau at 2e 2 =h returns. Our results show beginnings of the formation of an electron lattice in an interacting quasi-1D quantum wire. In the presence of an in-plane magnetic field, mixing of spin-aligned levels of the two wires gives rise to more complex states. DOI: 10.1103/PhysRevLett.102.056804 PACS numbers: 73.21.Hb, 71.70.Àd, 72.25.Dc, 73.23.Ad The use of modern semiconductor technology has permitted the fabrication of low-dimensional electron systems, which exhibit quantum transport properties particular to their dimensionality. In the case of a 1D electron gas [1,2], the quantization of ballistic resistance has long been demonstrated in devices with strong confinement [3,4], whereas the regime of weak confinement has largely been overlooked in experimental investigations of quantum wires. However, it has recently been emphasized that interaction can cause a lateral spread of the electron distribution when the confinement weakens [5]. Here, we report on a behavior of the conductance of quantum wires with varying carrier concentration and confinement, which leads us to suggest a spatial redistribution of the electron system to form a lattice. In previous work, we showed that, at sufficiently low carrier concentrations (but in a strictly 1D regime), the system could enter the spin-incoherent regime [6]. The devices used for this work operate at higher carrier concentration, where we observe interaction-induced bifurcation of the 1D system into two distinct rows [7].By strongly confining electrons to one dimension, the transverse wave functions are spatially quantized in accordance with what is essentially a ''particle in the box'' model. With little or no electron scattering, for example, in short quantum wires, transport is ballistic and the conductance accords well with the predictions of noninteracting theory, where each subband contributes a conductance of 2e 2 =h. The effects of the electron-electron interactions on the quantization are small [8], except for the structure at 0:7 Â 2e 2 =h [9], and occasionally that at 0:5 Â 2e 2 =h [10][11][12]. A plateau at 0:5 Â 2e 2 =h, attributed to the spinincoherent regime, has recently been observed [6]. At sufficiently low electron densities n 1D a B ( 1, where a B is the effective Bohr radius, Coulomb energy dominates kinetic energy and interactions make possible the formation of a Wigner lattice [13] in which the electrons localize at equidistant sites along a line [14,15]. For a line of electrons, such ordering can occur simply in the absence of disorder without being accompanied by a change in the topology of the charge distribution. Experimental observations of Wigner crystallization are few and f...

We report conductance measurements of ballistic one-dimensional (1D) wires defined in GaAs/AlGaAs heterostructures in an in-plane magnetic field, B. When the Zeeman energy is equal to the 1D subband energy spacing, the spin-split subband N upward arrow intersects (N+1) downward arrow, where N is the index of the spin-degenerate 1D subband. At the crossing of N=1 upward arrow and N=2 downward arrow subbands, there is a spontaneous splitting giving rise to an additional conductance structure evolving from the 1.5(2e(2)/h) plateau. With further increase in B, the structure develops into a plateau and lowers to 2e(2)/h. With increasing temperature and magnetic field the structure shows characteristics of the 0.7 structure. Our results suggest that at low densities a spontaneous spin splitting occurs whenever two 1D subbands of opposite spins cross.

We have utilized the resonant x-ray diffraction technique at the Mn L-edge in order to directly compare magnetic and orbital correlations in the Mn sub-lattice of Pr0.6Ca0.4MnO3. The resonant line shape is measured below TOO ∼ 240 K at the orbital ordering wave vector (0, ,0,0). Comparing the width of the super-lattice peaks at the two wavevectors, we find that the correlation length of the magnetism exceeds that of the orbital order by nearly a factor of two. Furthermore, we observe a large (∼ 3 eV) shift in spectral weight between the magnetic and orbital line shapes, which cannot be explained within the classic Goodenough picture of a charge-ordered ground state. To explain the large shift, we calculate the resonant line shapes for orbital and magnetic diffraction based on a relaxed charge-ordered model.In a number of manganites, including Pr 1−x Ca x MnO 3 , La 1−x Ca x MnO 3 and La 2−x Sr x MnO 4 , the dynamics resulting in a charge-ordered, insulating state in the vicinity of half-doping are still not well understood. In his seminal work on exchange interactions in manganites, Goodenough considered charge and orbital order at halfdoping as a precursor to the magnetic CE ground state [1]. In this picture, charge ordering at T CO results in a checkerboard pattern of Mn 3+ and Mn 4+ sites (Figure 1). The Mn 3+ sites each have one e g electron and are thus Jahn-Teller (JT) active, while the e g levels are empty on the Mn 4+ sites. A cooperative JT distortion and orbital ordering of the Mn 3+ sites at T OO = T CO occurs concomitantly with the charge ordering. The in-plane JT distortions favor occupation of 3x 2 -r 2 and 3y 2 -r 2