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Rashid Nazmitdinov • Quantum dot: basics • Models: mean-field results • Mechanisms of symmetry breaking : RPA analysis • Summary QDs created in thin film semiconductor heterostructures With the use of epitaxial deposition techniques (like molecular beam epitaxy) it is possible to grow semiconductor crystals in coherent layers, a few lattice constant thick, and create multilayer semiconductor heterostructures. AlGaAs between GaAs (insulator) layers, one confines electrons in a AlGaAs 'quantum well'. The electrons in the 2DEG result from Si donors in the n- AlGaAs layer. (The thickness of the diferent layers is not to scale.) By sandwiching a Reduction of the remaining 2D ‘infinite’ extension of the quantum well, i.e. lateral confinement, leads to carrier confinement in all three dimensions and creation of a QD. Electrostatic DQs M.A.Reed et al, PRL 60, 535 (1988) A scanning electron micrograph of various size GaAs nanostructures containing quantum dots. The dark regions on top of the column is the electron-beam defined Ohmic contact and etch mask. The horizontal bars are 0.5 μm. One can consider the QD as a tiny laboratories in which quantum mechanics and the effects of electron-electron interaction can be studied. If the carrier motion in a solid is limited in a layer of a thickness of the order of the carrier de Broglie wavelength (λ), one will observe effects of size quantization. h h 1.22 nm p 3meff kT Ekin /[ eV] Quantum dots (QD) are small boxes (2 – 10 nm on a side, corresponding to 10 to 50 atoms in diameter), contained in semiconductor, and holding a number of electrons. At 10 nm in diameter, nearly 100000 quantum dots can be fit within the width of a human thumb. • By sandwiching a 10 nm thickness of GaAs between AlGaAs (insulator) layers, one confines electrons in a GaAs 'quantum well'. By placing electrostatic gates on the surface of the wafer, we can laterally confine this 2D electron gas and create a quantum dot • For the typical voltage 1V applied to the gate (top plate), the confining potential is some eV deep which is large compared to the few meV of the confining frequency. Hence, the electron wave function is localized close to the minimum of the well which always can be approximated by a parabolic potential. FIR spectroscopy Sikorski and Merkt, PRL 62 (1989) 2164 - The first direct observation of resonance transitions between discrete states of QDs on InSb. Left: Scanning electron micrograph of arrays of QDs on InSb and the schematic sketch of the band structure across the dots. Right: Experimental resonance positions (bullets) together with theoretical curves calculated from ( 02 L2 )1 / 2 L . FIR spectroscopy Sikorski and Merkt, PRL 62 (1989) 2164 - The first direct observation of resonance transitions between discrete states of QDs on InSb. The problem was solved more than 70 years ago (Fock 1928, Darwin 1930). The so-called FockDarwin energy levels are H 1 p eA 2 1 m *02 x 2 y 2 . 2m * 2 A 1 B r, B 0,0, B , 2 p2 l z2 1 H m * 2 2 Ll z , 2 2m * 2m * 2 where ωL = eB/2m* is the Larmor frequency and Enm 2n m 1 L m , 2 02 L2 . ( 02 L2 )1 / 2 L . The Kohn theorem The Kohn theorem - In a parabolic confining potential the centre-ofmass (CM) and relative (rel) motion decouple. For a N-electron QD: H 1 P QA 2 1 M02 R 2 H rel , 2M 2 N N i 1 i 1 where P p i , R ri N , Q N e, M N m * . Since Q/M = e/m*, the CM energy is identical to the single-electron energy Enm. The generalized Kohn theorem – The far-infrared (FIR) absorption spectra are independent on the number of electrons. d nm,nm nm r e i nm , m 1, n 0,1. E L , S.Tarucha et al, Phys. Rev. Lett. 77, 3613 (1996). The standard theoretical model is based on a number of approximations: • • • The underlying lattice structure is taken into account in effective mass approximation The confining potential is parabolic The electrons interact via a pure Coulomb interaction The Hamiltonian for N electrons interacting in a QD in a magnetic field B, perpendicular to the dot plane reads: 1 e2 1 2 2 2 p i eA i m *0 ri H 2 40 r n 1 2m * N N 1, N 1 i 1 j i ij r g * B B S z , where e, m*, ε0 and εr and the unit charge, effective electron mass, vacuum and relative dielectric constant of a semiconductor, respectively. Oosterkamp et al, PRL 82,2931(1999) Reimann&Manninen, RMP 74,1283 (2002) Serra, Nazmitdinov, Puente, PRB68 (2003) 035341;PRB69(2004)125315 • Hamiltonian • Units The length The energy The strength • Result Thouless (60,61): Stability Symmetry Orthognality The generators of symmetries broken on the mean field level create eigenstates with zero energy in RPA Rotational symmetry Equations of motion lead the generalized eigenvalue problem Conditions to construct the RPA ground state One seeks solution in the form (Thouless theorem) The matrix Zmi,ni is complex and symmetric in the boson indexes, i.e, Zmi,ni = Zni, mi Canonical operators Lz and an angle operator (Thouless, 61; Marshalek and Weneser, 69) Two additional RPA vectors For the C operator one needs to solve the linear system of equations • Once these two sets of coefficients are determined,we can calculate the Thouless-Valatin moment of inertia And determine the complete set Serra, Nazmitdinov, Puente, PRB68 (2003) 035341, PRB69 (2004) 125315 The particle density The RPA ground state density where RPA UHF RPA Nazmitdinov&Simonovic, PRB76 (2007)193306 RPA Rotational behavior of the experimental, kinematical ¶(1)=I/Ω and dynamical ¶(2) ≈ 4/∆Eγ moments of inertia Nazmitdinov,Almehed,Donau, PRC65,041307(2002) Do triaxial nuclei exist ? Bohr&Mottelson,1975 Marshalek (1979), Kvasil&Nazmitdinov (2007). S jz jy E2 2 Kvasil&Nazmitdinov, Phys.Lett. B650 (2007) 33. Nazmitdinov&Kvasil, JETP105 (2007) 962 Nazmitdinov, Kvasil,Tcvetkov, Phys.Lett.B657 (2007) 159 J , O KO z K K H H J Z • We suggest to consider the vanishing of one of the vibrational modes in the rotating frame as an indicator of the possible shape transitions in fast rotating nuclei. • We propose the selection rules for electromagnetic transitions from excited vibrational states of the negative signature that can be used to identify wobbling excitations and the sign of -deformation. • • • • • • Jan Kvasil , Charles University, Prague, Czech Republic Daniel Almehed, Notre Dame University, USA Fritz Donau , Rossendorf, Dresden, Germany Llorens Serra , University Illes Balears, Palma, Spain Nenad Simonovic , UInstitute of Physics, Belgrade, Serbia Antonio Puente , University Illes Balears, Palma, Spain • Shell effects play important role in small quantum dots. At specific values of the magnetic field the interplay between the Coulomb interaction and shell structure may lead to degeneracy of the quantum spectrum. • We suggest to consider the ratio as an indicator of the possible shape transitions in fast rotating nuclei. • We propose the selection rules for electromagnetic transitions from excited vibrational states of the negative signature that can be used to identify wobbling excitations and the sign of -deformation. • oscillator basis • An arbitrary single-particle orbital |i is expanded as The oscillator states |a are characterized by radial (na) and angular momentum (ma) quantum numbers ( Fock-Darwin states; Fock, 28) • We assume a good spin : each orbital i has non zero components only for a given spin orientation i=or i=, i.e., B(i)a=iB(i) a. Heiss&Nazmitdinov, Phys.Lett.A222 (1996) 309 E H x y 0 Heiss&Nazmitdinov, Phys.Rev.B55 (1997) 16310 • B = 0 the magic numbers (including spin) turn out to to be the usual sequence of the two-dimensional isotropic oscillator, ωx=ωy, that is 2, 6, 12, 20, . . .. • B ≈ 1.23 we find a new shell structure as if the confining potential would be a deformed harmonic oscillator without magnetic field. The magic numbers are 2, 4, 8, 12, 18, 24, . . . which are just the numbers obtained from the two-dimensional oscillator with ω+ = 2 ω-. • B ≈ 2.01 the magic numbers 2, 4, 6, 10, 14, 18, 24, . . . which corresponds to ω+ = 3 ω- . E Nazmitdinov&Simonovic, PRB76 (2007)193306 Hartree-Bogoliubov equations: h(1) kl ( H 0 ) kl jklx h(2) kl ( H 0 ) kl jklx Kvasil&Nazmitdinov, PRC73,014312(2006) Kvasil&Nazmitdinov, PRC73 (2006) 014312 J.A.Krumhansl and R.J. Gooding, Phys.Rev.B39 (1989) 3047 1/ 2 Analog of Landau phase transition of 2 order ! Serra, Nazmitdinov, Puente, PRB68 (2003) 035341 • It is assumed that the system's excitations are created by operators of the type whose action on the as yet unknown ground state |0 yields the excited vibrational states • The quasi-boson approximation (QBA) amounts to treat each particle-hole pair (mi) as an elementary boson