Quantum interference measurement of spin interactions in a bio-organic/semiconductor device structure

OPEN SUBJECT AREAS: PHYSICS CONDENSED-MATTER PHYSICS Quantum interference measurement of spin interactions in a bio-organic/ semiconductor device structure Vincent Deo1,2, Yao Zhang2, Victoria Soghomonian2 & Jean J. Heremans2 Received 12 November 2014 Accepted 4 March 2015 Published 30 March 2015 Correspondence and requests for materials should be addressed to J.J.H. (heremans@vt. edu) 1 Physics Department, Ecole Polytechnique, 91128 Palaiseau, France, 2Department of Physics, Virginia Tech, Blacksburg VA 24061, USA. Quantum interference is used to measure the spin interactions between an InAs surface electron system and the iron center in the biomolecule hemin in nanometer proximity in a bio-organic/semiconductor device structure. The interference quantifies the influence of hemin on the spin decoherence properties of the surface electrons. The decoherence times of the electrons serve to characterize the biomolecule, in an electronic complement to the use of spin decoherence times in magnetic resonance. Hemin, prototypical for the heme group in hemoglobin, is used to demonstrate the method, as a representative biomolecule where the spin state of a metal ion affects biological functions. The electronic determination of spin decoherence properties relies on the quantum correction of antilocalization, a result of quantum interference in the electron system. Spin-flip scattering is found to increase with temperature due to hemin, signifying a spin exchange between the iron center and the electrons, thus implying interactions between a biomolecule and a solid-state system in the hemin/InAs hybrid structure. The results also indicate the feasibility of artificial bioinspired materials using tunable carrier systems to mediate interactions between biological entities. I n a hybrid bio-organic/semiconductor lithographic structure, quantum interference experiments are used to study spin interactions between the iron center in hemin and a proximate two-dimensional electron system (2DES) at the surface of InAs. Hemin (Fig. 1a) is an iron porphyrin similar to the prosthetic heme group in hemoglobin, where the iron center impacts biological functions. In the hemin/semiconductor structure a magnetic characterization method is employed deriving its sensitivity from electrically-measured quantum interference, evidenced as electron antilocalization1, and from the engineered nanoscale proximity between the 2DES to the local spin moments in hemin. Hemin influences the spin environment of the electrons, resulting in a temperature-dependence of the electron spin-flip scattering. Here we demonstrate that the influence of the spin environment on the decoherence times of low-dimensional electrons can be electronically measured in a bioorganic/semiconductor device to characterize a biomolecule. The approach is in principle similar to magnetic resonance techniques in its use of decoherence times, but is implemented electronically. The 2DES in this study is the electron accumulation layer at the surface of (001) InAs, schematically represented in Fig. 2a2,3. The Rashba spin-orbit interaction (SOI)4,5 in this 2DES enables our method. Metalloporphyrins, cyclic p-conjugated molecules with a centrally hosted metal ion, are actively studied in fields from optoelectronics to spin electronics and sensing, due to the richness of phenomena arising from the interaction of the metal ion and the p-system. Of special biological interest are metalloporphyrins containing iron, such as hemoglobin, myoglobin, and cytochromes, all containing a heme moiety. The spin and local magnetic moment of the iron centers in heme imbue the functional group with many of its properties6, and can be used to determine the state of the ion7–10. The biophysics interest in the iron centers’ spin states and their impact on biological functions is substantial, long-standing11–12 and ongoing. Hemin-functionalized InAs and InP surfaces e.g. act as sensitive sensors for NO13. The electronic structure of iron in hemoglobin and hemin in solution indicate a high-spin ferric state14. First-principles atomistic calculations15 of myoglobin bound with different ligands suggests fluctuating magnetic moments in the heme portion, correlated with high- and low-spin states of the iron center. In this work, the labile state of the biologically relevant spin in hemin opens avenues for measuring the spin exchange with nearby electrons in a nanoscale solid-state hybrid device, thereby quantifying the spin state. Like studies may moreover provide the insight to construct bioinspired nanostructured materials using tunable electron systems as intermediary to mediate an exchange between biological entities. SCIENTIFIC REPORTS | 5 : 9487 | DOI: 10.1038/srep09487 1 www.nature.com/scientificreports Figure 1 | Chemical structures of the surface species used in this work. (a), Structure of hemin chloride (hemin), with the Fe center in the 13 oxidation state. (b), Structure of Protoporphyrin IX (PP-IX). The spin interactions measurably modify the quantum corrections to the 2DES electrical conductivity at low temperatures (, 10 K). The corrections in electronic transport stem from quantum interference of electron partial waves on time-reversed pairs of backscattered trajectories. Constructive interference results in increased backscattering and hence increased resistance (weak localization, WL). Under SOI a closed diffusive path is accompanied by a spin rotation of 2p, resulting in a change of sign of the wavefunction, destructive interference, reduced backscattering and hence decreased resistance (antilocalization, AL)16. AL thus originates in spin-dependent interference of electrons and is sensitive to spin decoherence1,16–19. The spin decoherence of the surface electrons in turn is a sensitive gauge of their spin interactions with magnetic impurities, exceeding direct magnetic measurements in sensitivity20 and capable of quantifying spin interactions in our low-dimensional spin system. Parallels with magnetic resonance methods (EPR and NMR) can be found in the method of characterizing the local spin environment by measuring the spin decoherence time (a T2 time). In AL, a magnetic field B applied normally to the 2DES breaks the time reversal symmetry and reduces the interference effect, leading to a characteristic magnetoresistance (MR)1,17–19 determined by four characteristic decoherence or scattering rates1,19,21. The scattering rates (inverse scattering times) are the elastic scattering rate t021 as independently deduced from the areal electron density ns and from the electron mobility, the inelastic scattering rate ti21, the SOI scattering rate tSO21, and the magnetic spin-flip scattering rate ts21. The total electron decoherence rate, tw21, is obtained as tw21 5 ti21 1 2 ts21. The spin-flip rate ts21 is here of particular value because it conveys information about the interactions between the surface hemin and the 2DES1,19. The experiments me (...truncated)