Spin-orbit effects in a GaAs quantum dot in a parallel magnetic field

We analyze the effects of spin-orbit coupling on fluctuations of the conductance of a quantum dot fabricated in a GaAs heterostructure. Counterintuitively we argue that spin-orbit effects may become important in the presence of a large parallel magnetic field B( parallel), even if they are negligible for B( parallel) = 0. This should be manifest in the level repulsion of a closed dot, and in reduced conductance fluctuations in dots with a small number of open channels in each lead, for large B( parallel). Our picture is consistent with the experimental observations of Folk et al.     

Magnetic dissipation and fluctuations in individual nanomagnets measured by ultrasensitive cantilever magnetometry

Cantilever magnetometry with moment resolution better than 10(4)micro(B) was used to study individual nanomagnets. By using the fluctuation-dissipation theorem to interpret measurements of field-induced cantilever damping, the low frequency spectral density of magnetic fluctuations could be determined with resolution better than 1micro(B) Hz-1/2. Cobalt nanowires exhibited significant magnetic dissipation and the associated magnetic fluctuations were found to have 1/f frequency dependence. In individual submicron rare-earth alloy magnets, the dissipation/fluctuation was very small and not distinguishable from that of a bare silicon cantilever.     

Magnetic quantum dot: a magnetic transmission barrier and resonator

We study the ballistic edge-channel transport in quantum wires with a magnetic quantum dot, which is formed by two different magnetic fields B(*) and B0 inside and outside the dot, respectively. We find that the electron states located near the dot and the scattering of edge channels by the dot strongly depend on whether B(*) is parallel or antiparallel to B0. For parallel fields, two-terminal conductance as a function of channel energy is quantized except for resonances, while, for antiparallel fields, it is not quantized and all channels can be completely reflected in some energy ranges. All these features are attributed to the characteristic magnetic confinements caused by nonuniform fields.     

Element-specific hard X-ray micro-magnetometry of magnetic modifications in Co–Pt dots fabricated by ion etching

We developed a micro-magnetometry with a 2.5 μm spatial resolution based on micro X-ray magnetic circular dichroism (XMCD) technique in order to study magnetic properties of dot arrays for bit-patterned media. This micro-magnetometer was applied to the magnetic characterization of Co–Pt dot arrays fabricated by ion beam etching. As the dot size became small, the intensity of XMCD drastically decreased for dots fabricated by Ga-focused ion beam. This suggested that the dot edges were damaged magnetically by implantation of Ga ions. The damaged width of the dot edge was estimated to be about 13 nm from the decrease in XMCD intensities. This damaged edge width agreed with the ion-implanted area estimated by Monte-Carlo simulation. The less-damaged effect of Ar ion etching was verified by the XMCD measurement of Co–Pt dots with diameter of 20 and 70 nm. It was concluded that ions with inertness, lower energy and smaller atomic number should be used to fabricate dot arrays with an areal density of 1 Tbit/in².     

Optical properties of individual manganese-doped quantum dots

The magnetic state of a single magnetic ion (Mn²⁺) embedded in an individual quantum dot is optically probed using micro-spectroscopy. The fine structure of a confined exciton in the exchange field of a single Mn²⁺ ion (S=) is analyzed in detail. The exciton–Mn²⁺ exchange interaction shifts the energy of the exciton depending on the Mn²⁺ spin component and six emission lines are observed at zero magnetic field. The emission spectra of individual quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton–Mn²⁺ exchange interaction (dependent on the Mn position) and the anisotropic part of the electron–hole exchange interaction (related to the asymmetry of the quantum dot).     

Rashba spin-orbit interaction effects on thermal and magnetic properties of parabolic GaAs quantum dot in the presence of donor impurity under external electric and magnetic fields

Based on the effective mass approximation, the magnetic and thermal properties of parabolic GaAs quantum dot have been investigated in the presence of Rashba Spin-Orbit interaction (RSOI), donor impurity and applied magnetic and electric fields. The exact diagonalization method has been used to solve the Hamiltonian of an electron confined in a quantum dot (QD) and obtain the eigenenergies and the binding energy of the donor impurity as a function of various QD physical parameters. We have shown the dependence of the average statistical energy, magnetization, magnetic susceptibility and heat capacity of the donor impurity in the QD on: the Rashba interaction parameter, the magnetic and electric fields, confining frequency, and temperature. The results reveal that these parameters can tune the magnetic properties of the GaAs quantum dot and flip the sign of magnetic susceptibility from negative (diamagnetic) to positive (paramagnetic) type material.     

Spin waves and quantum criticality in the frustrated XY pyrochlore antiferromagnet Er2Ti2O7

We report detailed measurements of the low temperature magnetic phase diagram of Er2Ti2O7. Heat capacity and time-of-flight neutron scattering studies of single crystals reveal unconventional low-energy states. Er3+ magnetic ions reside on a pyrochlore lattice in Er2Ti2O7, where local XY anisotropy and antiferromagnetic interactions give rise to a unique frustrated system. In zero field, the ground state exhibits coexisting short and long-range order, accompanied by soft collective spin excitations previously believed to be absent. The application of finite magnetic fields tunes the ground state continuously through a landscape of noncollinear phases, divided by a zero temperature phase transition at micro{0}H{c} approximately 1.5 T. The characteristic energy scale for spin fluctuations is seen to vanish at the critical point, as expected for a second order quantum phase transition driven by quantum fluctuations.     

Magneto-conductance fingerprints of purely quantum states in the open quantum dot limit

We present quantum magneto-conductance simulations, at the quantum low energy condition, to study the open quantum dot limit. The longitudinal conductance G(E,B) of spinless and non-interacting electrons is mapped as a function of the magnetic field B and the energy E of the electrons. The quantum dot linked to the semi-infinite leads is tuned by quantum point contacts of variable width w. We analyze the transition from a quantum wire to an open quantum dot and then to an effective closed system. The transition, as a function of w, occurs in the following sequence: evolution of quasi-Landau levels to Fano resonances and quasi-bound states between the quasi-Landau levels, followed by the formation of crossings that evolve to anti-crossings inside the quasi-Landau level region. After that, Fano resonances are created between the quasi-Landau states with the final generation of resonant tunneling peaks. By comparing the G(E,B) maps, we identify the closed and open-like limits of the system as a function of the applied magnetic field. These results were used to build quantum openness diagrams G(w,B). Also, these maps allow us to determine the w-limit value from which we can qualitatively relate the closed system properties to the open one. The above analysis can be used to identify single spinless particle effects in experimental measurements of the open quantum dot limit.     

Micromagnetic properties and magnetization switching of single domain Co dots studied by magnetic force microscopy

Magnetic force microscopy was applied to study the magnetic properties of Co dot microstructures. The high density magnetic dot arrays were fabricated using nanolithographic techniques on GaAs substrates. The ferromagnetic Co dots were found to be in a single domain state for Co film thicknesses of 7 nm and 17 nm. The magnetization of the as-prepared Co dot array was found to be in a non-uniform state. After applying a magnetic field the Co dots are in a uniform magnetization state. Induced switching of the magnetization of single Co dots by the stray field of the probing tip using an additionally applied in-situ magnetic field has been demonstrated.     

Spin degeneracy and conductance fluctuations in open quantum dots

The dependence of conductance fluctuations on parallel magnetic field is used as a probe of spin degeneracy in open GaAs quantum dots. The variance of fluctuations at high parallel field is reduced from the low-field variance (with broken time-reversal symmetry) by factors ranging from roughly 2 in a 1 microm (2) dot to greater than 4 in 8 microm (2) dots. The factor of 2 is expected for Zeeman splitting of spin-degenerate channels. A possible explanation for the larger suppression based on field-dependent spin-orbit scattering is proposed.     

Geometrical effects on the optical properties of quantum dots doped with a single magnetic atom

The emission spectra of individual self-assembled quantum dots containing a single magnetic Mn atom differ strongly from dot to dot. The differences are explained by the influence of the system geometry, specifically the in-plane asymmetry of the quantum dot and the position of the Mn atom. Depending on both these parameters, one has different characteristic emission features which either reveal or hide the spin state of the magnetic atom. The observed behavior in both zero field and under magnetic field can be explained quantitatively by the interplay between the exciton-manganese exchange interaction (dependent on the Mn position) and the anisotropic part of the electron-hole exchange interaction (related to the asymmetry of the quantum dot).     

Coherent versus sequential electron tunneling in quantum dots

Manifestations of quantum coherence in the electronic conductance through nearly closed quantum dots in the Coulomb-blockade regime are addressed. We show that quantum coherent tunneling processes explain some puzzling statistical features of the conductance peak heights observed in recent experiments at low temperatures. We employ the constant interaction model and the random matrix theory to model the quantum dot electronic interactions and its single-particle statistical fluctuations, taking full account of the finite decay width of the quantum dot levels.     

Spin fractionalization of an even number of electrons in a quantum Dot

We find that Kondo resonant conductance can occur in a quantum dot in the Coulomb blockade regime with an even number of electrons N. The contacts are attached to the dot in a pillar configuration, and a magnetic field B( perpendicular) along the axis is applied. B( perpendicular) lifts the spin degeneracy of the dot energies. Usually, this prevents the system from developing the Kondo effect. Tuning B( perpendicular) to the value B(*) where levels with different total spin cross restores both the degeneracy and the Kondo effect. We analyze a dot charged with N = 2 electrons. Coupling to the contacts is antiferromagnetic due to a spin selection rule and, in the Kondo state, the charge is unchanged while the total spin on the dot is S = 1/2.     

Coherent optical manipulations of the fundamental state in a single quantum dot

By means of an original all-optical experimental technique using microphotoluminescence in a waveguiding geometry, resonant coherent manipulation of quantum states in a single quantum dot becomes possible now. Resonant Rabi oscillation of the fundamental exciton state in a single quantum dot has been realized. We present the results obtained on two different kinds of samples: InAs/GaAs self-assembled quantum dots and naturally formed GaAs quantum dots by thickness fluctuations.     

Models of Semiconductor Quantum Dots Blinking based on Spectral Diffusion

Optics and Spectroscopy - Three models of single colloidal quantum dot emission fluctuations (blinking) based on spectral diffusion were considered analytically and numerically. It was shown that...     

Multi-exciton spectroscopy on a self-assembled InGaAs/GaAs quantum dot

We report about spatially resolved magneto-optical experiments on a self-assembled InGaAs quantum dot. Using electron beam lithograpy for patterning a metal shadow mask we can isolate a single dot. This allows us to study the optical response of a single dot as a function of excitation power and magnetic field. We investigate the influence of many body interaction in the emission spectra for different exciton occupation numbers of the dot. The diamagnetic/orbital shift as well as Zeeman splitting in a magnetic field can be fully resolved and are used to identify the observed emission lines. Further we report on absorption properties of the quantum dot as a function of magnetic field. We analyse in detail the phonon-assisted absorption process connected with the GaAs LO-phonon 36 meV above the single-exciton ground state.     

The multilayered spherical quantum dot under a magnetic field

The binding energy of an impurity located at the center of multilayered spherical quantum dot (MSQD) is reported as a function of the dot and barrier thickness for different alloy compositions under the influence of a magnetic field. Within the effective mass approximation, the binding energy has been calculated using the fourth order Runge-Kutta method without magnetic field. A variational approach has been employed if a magnetic field is present. The binding energy in MSQD with equal dot and barrier thickness is calculated. It is shown that the binding energy in MSQD differs from that of a single quantum dot. Also, the geometry is dominant on the binding energy for thin MSQDs, but the magnetic field becomes more effective for thick MSQDs.     

Ferromagnetism mediated by few electrons in a semimagnetic quantum dot

A (II,Mn)VI diluted magnetic semiconductor quantum dot with an integer number of electrons controlled with a gate voltage is considered. We show that a single electron is able to induce a collective spontaneous magnetization of the Mn spins, overcoming the short range antiferromagnetic interactions, at a temperature order of 1 K, 2 orders of magnitude above the ordering temperature in bulk. The magnetic behavior of the dot depends dramatically on the parity of the number of electrons in the dot.     

Shot noise of the spin inelastic tunneling through a quantum dot with single molecule-magnet

We have studied the quantum fluctuations of inelastic spin-electron scattering in quantum dot with an embedded biaxial single molecule-magnet and particularly investigated the zero-frequency shot noise and Fano factor in different magnetic fields. It is found that the shot noise and Fano factor exhibit a stepwise behaviour as bias increases in the presence of interaction between the electron and molecule-magnet for a weak magnetic field. As magnetic field becomes strong, a dip is displayed in the shot-noise-bias curve due to the suppression of inelastic shot noise caused by the quantum tunneling of magnetisation. Because of the spontaneous inelastic tunneling at zero bias, a small shot noise occurs, which results in the case of Fano factor F >> 1. Moreover, our results show that the sweeping speed can also influence the shot noise and Fano factor obviously.     

Optically detected resonance spectroscopy of interface fluctuation quantum dots

We have performed optically detected resonance (ODR) spectroscopy on modulation-doped GaAs/AlGaAs quantum wells of different widths in which lateral fluctuations of the well width were purposely introduced by growth interruption at the interfaces. These monolayer fluctuations form quantum dots for which confinement and Coulomb correlation energies are comparable. By monitoring resonant changes of the dot ensemble photoluminescence induced by far-infrared (FIR) radiation in a magnetic field, we have observed cyclotron resonance (CR) of free electrons in the widest wells, as well as internal transitions of mobile and localized charged excitons. The latter, which are forbidden by magnetic translational invariance, have previously not been observed. For the narrower wells the effects of non-parabolicity and carrier localization on the CR and CR-like transitions have to be included for a proper interpretation of the measurements.