Topological Superconductivity and Majorana Fermions in RKKY Systems

We consider quasi one-dimensional RKKY systems in proximity to an s-wave superconductor. We show that a $2k_F$-peak in the spin susceptibility of the superconductor in the one-dimensional limit supports helical order of localized magnetic moments via RKKY interaction, where $k_F$ is the Fermi wavevector. The magnetic helix is equivalent to a uniform magnetic field and very strong spin-orbit interaction (SOI) with an effective SOI length $1/2k_F$. We find the conditions to establish such a magnetic state in atomic chains and semiconducting nanowires with magnetic atoms or nuclear spins. Generically, these systems are in a topological phase with Majorana fermions. The inherent self-tuning of the helix to $2k_F$ eliminates the need to tune the chemical potential

Cd3As2 is Centrosymmetric

This is a revised version of a manuscript that was originally posted here in February of 2014. It has been accepted at the journal Inorganic Chemistry after reviews that included those of two crystallographers who made sure all the t's were crossed and the i's were dotted. The old work (from 1968) that said that Cd3As2 was noncentrosymmetric was mistaken, with the authors of that study making a type of error that in the 1980s became infamous in crystallography. As a result of the increased scrutiny of the issue of centrosymmetricity of the 1980's, there are now much better analysis tools to resolve the issue fully, and its important to understand that not just our crystals are centrosymmetric, even the old guy's crystals were centrosymmetric (and by implication everyone's are). There is no shame in having made that error back in the day and those authors would not find the current centrosymmetric result controversial; their paper is excellent in all other aspects. This manuscript describes how the structure is determined, explains the structure schematically, calculates the electronic structure based on the correct centrosymmetric crystal structure, and gives the structural details that should be used for future analysis and modeling.Comment: Accepted by ACS Inorganic Chemistr

Mapping the wavefunction of transition metal acceptor states in the GaAs surface

We utilize a single atom substitution technique with spectroscopic imaging in a scanning tunneling microscope (STM) to visualize the anisotropic spatial structure of magnetic and non-magnetic transition metal acceptor states in the GaAs (110) surface. The character of the defect states play a critical role in the properties of the semiconductor, the localization of the states influencing such things as the onset of the metal-insulator transition, and in dilute magnetic semiconductors the mechanism and strength of magnetic interactions that lead to the emergence of ferromagnetism. We study these states in the GaAs surface finding remarkable similarities between the shape of the acceptor state wavefunction for Mn, Fe, Co and Zn dopants, which is determined by the GaAs host and is generally reproduced by tight binding calculations of Mn in bulk GaAs [Tang, J.M. & Flatte, M.E., Phys. Rev. Lett. 92, 047201 (2004)]. The similarities originate from the antibonding nature of the acceptor states that arise from the hybridization of the impurity d-levels with the host. A second deeper in-gap state is also observed for Fe and Co that can be explained by the symmetry breaking of the surface.Comment: 19 pages, 6 figure

The Majorana spin in magnetic atomic chain systems

In this paper, we establish that Majorana zero modes emerging from a topological band structure of a chain of magnetic atoms embedded in a superconductor can be distinguished from trivial localized zero energy states that may accidentally form in this system using spin resolved measurements. To demonstrate this key Majorana diagnostics, we study the spin composition of magnetic impurity induced in-gap Shiba states in a superconductor using a quantum impurity model (at the mean-field level). By examining the spin and spectral densities in the context of the Bogoliubov-de Gennes (BdG) particle-hole symmetry, we derive a sum rule that relates the spin densities of localized Shiba states with those in the normal state without superconductivity. Extending our investigations to ferromagnetic chain of magnetic impurities, we identify key features of the spin properties of the extended Shiba state bands, as well as those associated with a localized Majorana end mode when the effect of spin-orbit interaction is included. We then formulate a phenomenological theory for the measurement of the local spin densities with spin-polarized scanning tunneling microscopy (STM) techniques. By combining the calculated spin densities and the measurement theory, we show that spin-polarized STM measurements can reveal a sharp contrast in spin polarization between an accidentally-zero-energy trivial Shiba state and a Majorana zero mode in a topological superconducting phase in atomic chains. We further confirm our results with numerical simulations that address generic parameter settings.Comment: 22 pages, 12 figures (references updated

Pair Density Wave in the Pseudogap State of High Temperature Superconductors

Recent scanning tunneling microscopy (STM) experiments of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ have shown evidence of real-space organization of electronic states at low energies in the pseudogap state. We argue based on symmetry considerations as well as model calculations that the experimentally observed modulations are due to a density wave of d-wave Cooper-pairs without global phase coherence. We show that STM measurements can distinguish a pair-density-wave from more typical electronic modulations such as those due to charge density wave ordering or scattering from an onsite periodic potential.Comment: 4 pages, 4 figures. Final version. PRL 93, 187002 (2004

Impurity-Induced Bound Excitations on the Surface of Bi2Sr2CaCu2O8

We have probed the effects of atomic-scale impurities on superconductivity in Bi_{2}Sr_{2}CaCu_{2}O_{8} by performing low-temperature tunneling spectroscopy measurements with a scanning tunneling microscope. Our results show that non-magnetic defect structures at the surface create localized low-energy excitations in their immediate vicinity. The impurity-induced excitations occur over a range of energies including the middle of the superconducting gap, at the Fermi level. Such a zero bias state is a predicted feature for strong non-magnetic scattering in a d-wave superconductor.Comment: 4 pages, revtex, 4 figures. To appear in Physical Review Letter

Iran’s poultry meat export status in the Middle East region

Agriculture is literally a matter of life and death in the developing world. Ninety-six per cent of the world’s farmers – approximately 1.3 billion people – live in developing countries. In the rural areas of the developing world, close to 900 million people live on less than $1 a day.The Doha mandate encouraged the hope that dumping of subsidised exports from rich countries would finally be brought to an end, that market access for developing countries would be improved,and that developing countries would continue to be permitted to use tariffs and other border measures as part of their rural development strategies. In the Middle East,agriculture is the largest contributor to the Gross Domestic Product and foreign exchange,and is the second largest employer.Yet despite its economic importance it is largely overlooked in the development of new technologies and in the allocation of trained human resources. Agriculture in the Middle East focuses on new ways to improve food production, the challenges of a largely arid land, and managing limited agricultural resources.Agriculture is one of the oldest income sources in Iran. Close to one-third of the working population are engaged in agriculture and related industries such as poultry farming.Agriculture plays a significant role in the Iranian economy. At now agriculture account for an average of 27.8 percent of the GDP, and employ 26.1 percent of the labor force.Also agricultural products account for 47 percent of non-oil exports.Poultry species play important roles in the livelihood of human beings throughout the world both in ways that are quantifiable economically and in others that cannot and are therefore not captured in economic indices such as national GDP estimates.Poultry meat contribute directly to food security because there are no taboos against their consumption.With regard to F.A.O annual reports(2004) ,Iran has been rated 1st producer and also 1st exporter of poultry meat in the Middle East region. Capital stock in this industry consists of Breeding Farms (lines) ,Grand Parents(GP), Parent stock (PS), Broilers, Slaughter-Houses and dependant industries of governmental and private sectors is 4000 Billions Rials (5 Billions$). The current production is related to 70% of production units potential ,so with regard to remained considerable potential,it is necessary to make of them in an optimum rate. Therefore poultry products exports particularly poultry meat can be used for increasing of income and development for poultry industry. The objective of this paper was to study the dynamics on comparative advantage of Iran’s poultry meat export in the Middle East market.Poultry meat,export,Revealed comparative advantage,iran,middle east

Association between Behavioral Patterns of Parents with Depression and Suicidal Thoughts among Adolescents.

Depression, one of the most common psychiatric disorders in the world, is common among adolescents. Social and psychology scientists believe that if the family plays its roles correctly in social, educative and cultural areas , they could make healthy and normal members for the society. Otherwise psychological and personality disorders will appear in the society especially among children and adolescents. So, the role of parents and interpersonal relationships with adolescents seem to be important in development and continuation of their depression, hopelessness and suicide. The object of this study is to determine association between patterns of parental behaviors with depression and suicidal thoughts among adolescents. This study is a cross-sectional, correlative research performed in 15 high schools in Isfahan- Iran. In this research, data have been collected with a questionnaire. Then, the patterns of parent-adolescent relationships have been categorized into 7 categories by the vie wpoint of the adolescents regarding content of care and protection. The final number of samples in this study was 323 adolescents at ages between 14-17 years. levels of depression and frequency of suicidal thoughts among adolescents in each category have been surveyed, and tested by One-Way ANOVA and Chi-square statistical tests by SPSS statistical software. Prevalence of depression and suicidal thoughts among adolescent students were about 43.4 and 32.7 percent respectively. The highest mean depression score(27.7) and the highest frequency of suicidal thoughts(58.3 percent) have been seen among adolescents who described their parents’ behavior as “Autocratic” and the less mean depression score(10.4) and the less frequency of suicidal thoughts(11.6 percent) have been seen among those who described to have “Democratic” parents. Also, we have found that with decreasing “control” and increasing “care” in parent-adolescent relationships, the mean depression score and relative frequency of suicidal thoughts have been decreased. These findings were significant with one-way ANOVA and Chi-square statistical tests with p=0.00 in both cases. Parent-adolescent relationship has an important role in mental health of adolescents. According to the findings of this study, the best behavior from parents is to give the adolescents more freedom and having such a relationship with them that they feel to be in love and at support, but not in continuous contro