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Two dimensional electron gases in narrow GaAs quantum wells show huge longitudinal resistance (HLR) values at certain fractional filling factors. Applying an RF field with frequencies corresponding to the nuclear spin splittings of 69 Ga, 71 Ga and 75 As leads to a substantial decreases of the HLR establishing a novel type of resistively detected NMR. These resonances are split into four sub lines each. Neither the number of sub lines nor the size of the splitting can be explained by established interaction mechanisms.Two-dimensional electron gases (2DEGs) with very high mobilities of the electrons can be formed in quantum wells and heterostructures based on the GaAs/Al x Ga 1−x As system. If such a 2DEG is subjected to an intense perpendicular magnetic field at very low temperatures, it shows the integer [1] and the fractional [2] quantum Hall effects at integer and fractional filling factors of one or more Landau levels. The signature of both types of quantum Hall effects is the quantization of the Hall resistance and the vanishing of the longitudinal resistance. Recently, however, huge longitudinal resistance maxima (HLR) have been observed at fractional filling factors between 1 2 and 1 [3]. The HLR is only found in samples which have a reduced well thickness (15 nm, [4]) as compared to the conventional ones. As an example, figure 1 shows longitudinal resistance measurements on a sample similar to the one used in [3] for two different carrier densities (dotted and dashed line) at a temperature of 0.35 K. Here, the magnetic field is swept at a rate of 0.7 T/min and the applied source drain current is 100 nA. The width of the sample is 80 µm and the voltage probes are 80 µm apart. For both carrier densities a very regular behavior is seen. At integer filling factors the resistance vanishes completely and at filling factor ν = 2 3 one finds a clear minimum. However, if the sweep rate of the magnetic field is drastically reduced to 0.002 T/min, a huge maximum in the longitudinal resistance (solid lines) is observed at ν = 2 3 for both carrier densities. The size of the HLR is maximal at a current density of approximately 0.6 mA/m. The HLR vanishes in tilted magnetic fields, indicating that the electron spin polarization plays an important role for the HLR. Similar maxima are also reported at other fractional filling factors [3], but in this paper we want to concentrate on the HLR at ν = 2 3 at 0.35K. The HLR develops with a time constant of about 15 min. These very long times are typical for relaxation effects of the nuclear spin system [5,6]. The only direct way to demonstrate an involvement of the nuclear spins in the HLR is a nuclear magnetic resonance (NMR) [7][8][9][10] experiment, because it allows direct modification of the nuclear polarization. In this Letter we report on experiments where radio frequency is irradiated on a sample in the HLR state and a drastic reduction of the resistance values is observed whenever the nuclei are in resonance. This is to our knowledge the clearest form of a resistively ...

The dinuclear radical anion complexes [(mu-L)[Re(CO)(3)Cl](2)](*)(-), L = 2,2'-azobispyridine (abpy) and 2,2'-azobis(5-chloropyrimidine) (abcp), were investigated by EPR at 9.5, 94, 230, and 285 GHz (abpy complex) and at 9.5 and 285 GHz (abcp complex). Whereas the X-band measurements yielded only the isotropic metal hyperfine coupling of the (185,187)Re isotopes, the high-frequency EPR experiments in glassy frozen CH(2)Cl(2)/toluene solution revealed the g components. Both the a((185,187)Re) value and the g anisotropy, g(1) - g(3), are larger for the abcp complex, which contains the better pi-accepting bridging ligand. Confirmation for this comes also from IR and UV/vis spectroscopy of the new [(mu-abcp)[Re(CO)(3)Cl](2)](o/)(*)(-)(/2)(-) redox system. The g values are reproduced reasonably well by density functional calculations which confirm higher metal participation at the singly occupied MO and therefore larger contributions from the metal atoms to the g anisotropy in abcp systems compared to abpy complexes. Additional calculations for a series of systems [(mu-abcp)[M(CO)(3)X](2)](*)(-) (M = Tc or Re and X = Cl, and X = F, Cl, or Br with M = Re) provided further insight into the relationship between spin density distribution and g anisotropy.

Direct electron spin resonance (ESR) on a high mobility two dimensional electron gas in a single AlAs quantum well reveals an electronic g-factor of 1.991 at 9.35 GHz and 1.989 at 34 GHz with a minimum linewidth of 7 Gauss. The ESR amplitude and its temperature dependence suggest that the signal originates from the effective magnetic field caused by the spin orbit-interaction and a modulation of the electron wavevector caused by the microwave electric field. This contrasts markedly to conventional ESR that detects through the microwave magnetic field. [5,6], without the need for Ohmic contacts to the samples. Moreover, from the dependence of the g-factor anisotropy on Fermi wavevector and from the dependence of the g-factor on angle between microwave field and static magnetic field, recently the (tiny) Bychkov-Rashba spin-orbit interaction of 2D electrons in Si/SiGe samples could be determined [7,8]. In this paper we show that in high mobility 2D samples, this spin-orbit interaction allows to resonantly manipulate the electron spin by means of GHz electric fields.Direct ESR on a two dimensional electron gas (2DEG) has proved difficult because of the typically small number of spins in the 2DEG. So far it has been restricted to Si (either in Si/SiC or in Si/SiGe samples) because of its favourable physical properties. As the sensitivity of ESR is proportional to the inverse of the linewidth squared, narrow linewidths are a prerequisite. In Si linewidths down to 3 µT are observed [8], as little T 1 -broadening occurs. This is because Si has a rather small spin-orbit (SO) interaction. Also it has only one isotope with nuclear spin ( 29 Si) which additionally has only a small natural abundance (4.7 %). This contrasts markedly to the III-V semiconductors where there are many isotopes with nuclear spin ( 69 Ga, 71 Ga, 27 Al, 75 As, 115 In, 31 P etc.) with large natural abundance, many of which have a strong SO coupling. This leads to considerable line broadening and at low temperatures, where ESR usually has the best sensitivity, to large hyperfine fields that vary slowly with time. Consequently direct ESR has never been demonstrated on 2D electrons in III-V semiconductors.Here, we present the first direct ESR on a 2DEG in a III-V semiconductor. We study ESR of high mobility 2D electrons in a single AlAs quantum well. At 9.35 GHz and at 34 GHz g-factors of 1.991 and 1.989 were determined respectively. By rotating the sample in the cavity we demonstrate that our ESR originates from the microwave electric field (E 1 -field) and not from the microwave magnetic field (B 1 -field). For small power (P ) of the E 1 -field, the ESR follows a P 0.5 -law, but for larger powers, the exponent increases to ∼1. The temperature dependence of the ESR is much stronger than the 2D magnetisation expected for such a system [2]. Our observations can be explained by assuming that the spin transitions occur through the effective magnetic field caused by SO interaction and the modulation of the electron wavevector around k F induced by the mi...

The complexes [(mu-bmtz(*-))[Cu(PPh(3))(2)](2)](BF(4)) (1) and [(mu-H(2)bmtz)[Cu(PPh(3))(2)](2)](BF(4))(2) (2) (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine and H(2)bmtz = 1,4-dihydro-3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine) were obtained as stable materials that could be crystallized for structure determination. 1.2 CH(2)Cl(2): C(84)H(70)BCl(4)Cu(2)F(4)N(8)P(4); monoclinic, C2/c; a = 26.215(7) A, b = 22.122(6) A, c = 18.114(5) A, beta = 133.51(1) degrees; Z = 4. 2.CH(2)Cl(2): C(83)H(70)B(2)Cl(2)Cu(2)F(8)N(8)P(4); triclinic, P1; a = 10.948(2) A, b = 12.067(2) A, c = 30.287(6) A, alpha = 93.82(3) degrees, beta = 94.46(3) degrees, gamma = 101.60(3) degrees; Z = 2. Bmtz itself was also structurally characterized (C(10)H(6)N(8); monoclinic, P2(1)/c; a = 3.8234(8) A, b = 10.147(2) A, c = 13.195(3) A, beta = 94.92(3) degrees; Z = 2). Whereas the radical complex ion contains a planar tetrazine ring in the center, the 1,4-dihydrotetrazine heterocycle in the corresponding complex of H(2)bmtz is considerably folded. Both systems exhibit slight twists between the tetrazine and the pyrimidine rings. The intra-tetrazine distances are characteristically affected by the electron transfer, as is also evident from a comparison with the new structure of free bmtz; the bonding to copper(I) changes accordingly. Spectroscopy including X- and W-band EPR of the radical species confirms that the electron addition is mainly to the tetrazine ring.

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