Unusual spin dynamics in topological insulators

The dynamic spin susceptibility (DSS) has a ubiquitous Lorentzian form in conventional materials with weak spin orbit coupling, whose spectral width characterizes the spin relaxation rate. We show that DSS has an unusual non-Lorentzian form in topological insulators, which are characterized by strong SOC. At zero temperature, the high frequency part of DSS is universal and increases in certain directions as $\omega^{d-1}$ with $d=2$ and 3 for surface states and Weyl semimetals, respectively, while for helical edge states, the interactions renormalize the exponent as $d=2K-1$ with $K$ the Luttinger-liquid parameter. As a result, spin relaxation rate cannot be deduced from the DSS in contrast to the case of usual metals, which follows from the strongly entangled spin and charge degrees of freedom in these systems. These parallel with the optical conductivity of neutral graphene.Comment: 5 pages, 3 figure

Comment on "Low Temperature Magnetic Instabilities in Triply Charged Fulleride Polymers" by D. Arcon et al., PRL 84, 562 (2000)

Recently, Arcon et al. reported ESR studies of the polymer phase (PP) of Na_{2}Rb_{0.3}Cs_{0.7}C_{60} fulleride. It was claimed that this phase is a quasi-one-dimensional metal above 45 K with a spin-gap below this temperature and has antiferromagnetic(AF) order below 15 K, that is evidenced by antiferromagnetic resonance(AFMR). For the understanding of the rich physics of fullerides it is important to identify the different ground states. ESR has proven to be a useful technique for this purpose. However, since it is a very sensitive probe, it can detect a multitude of spin species and it is not straightforward to identify their origin, especially in a system like Na_{2}Rb_{x}Cs_{1-x}C_{60} with three dopants, when one part of the sample polymerizes but the majority does not. The observation of a low dimensional instability in the single bonded PP would be a novel and important result. Nevertheless, in this Comment we argue that Na_{2}Rb_{0.3}Cs_{0.7}C_{60} is not a good choice for this purpose since, as we show, the samples used by Arcon et al. are inhomogeneous. We point out that recent results on the PP of Na_{2}CsC_{60} contradicts the observation of low dimensional instabilities in Na_{2}Rb_{0.3}Cs_{0.7}C_{60}.Comment: 1 pags, no figure

Új nanoszerkezetek vizsgálata mágneses rezonancia módszerrel = Magnetic resonance studies of novel nanostructures

A szénalapú nanoszerkezetek korunk ígéretes anyagai. Alkalmazási területük az űrtechnológiától az orvostudományig terjed. Az alkalmazási potenciáljuk kiaknázásához elengedhetetlen az alapvető tulajdonságaik ismerete, amihez ezen pályázat eredményei is hozzájárultak. Egyfalú szén nanocsöveket és módosulatait vizsgáltuk szilárdtest spektroszkópia alkalmazásával. Előállítottunk és elektron spin rezonancia spektroszkópiával vizsgáltuk szén nanocsövekbe bevitt spin nyomjelzőket. Segítségükkel lehetőség nyílt a nanocsövek elektronikus tulajdonságainak jobb megértésére. Kiderült, hogy a nanocsövek elektronikus szerkezete egyedi korrellált viselkedést mutat, az ún. Luttinger folyadék állapotot. A kísérleti eredményeket élvonalbeli elméleti módszerekkel értelmeztük. Megmutattuk, hogy a nanocsövekbe bevitt spin-nyomjelzők termikusan stabilabbak mint egyéb előfordulási formájukban. Ezen anyagok várható alkalmazási területe a kvantum információ elmélet és az ún. spintronika (elektronikával analóg számítások elektron spinje felhasználásával). Az eredményeket a terület vezető folyóirataiban jelentettük meg (Physical Review Letters, Physical Review B, Chemical Physics Letters) illetve egy könyvfejezet formájában. A kutatás céljainak megvalósításába 40.000 euró Európai Uniós forrást is bevontunk. A BME Fizika Intézetében megtalálható nagyfrekvenciás elektron spin rezonancia spektrométert korszerűsítettük, aminek eredményekeppen az érzékenysége tekintetében újra a világ élvonalához tartozik. | Carbon based nanostructures are promising materials with applications ranging from space research till medical sciences. However, exploitation of this enormous application potential requires a thorough understanding of their fundamental properties. This project contributed to such fundamental research aspects. We studied single wall carbon nanotubes using solid state spectroscopy. We synthesized and studied spin labels encapsulated inside the carbon nanotubes. This unraveled the unusual electronic properties of these materials namely strongly correlated electronic behavior, with the so called Luttinger-liquid ground state. Cutting-edge theoretical studies supported the experiments. We showed that the encapsulated spin labels are more stable than in their pristine form. The expected applications of these materials is in the field of quantum information processing and in spintronics (a method analogous to electronics but using the electron spin as information carrier). We published the results in leading international journals (Physical Review Letters, Physical Review B, and Chemical Physics Letters) and in the form of a book chapter. The project goals were financed by the European Union with 40.000 Euro. This allowed to modernize the high frequency electron spin resonance spectrometer at the host institute. With the development the instrument has a high sensitivity, which puts it among the top instruments in the world

Heating causes non-linear microwave absorption anomaly in single wall carbon nanotubes

Microwave impedance measurements indicate a non-linear absorption anomaly in single wall carbon nanotubes at low temperatures (below $20$ K). We investigate the nature of the anomaly using a time resolved microwave impedance measurement technique. It proves that the anomaly has an extremely slow, a few hundred second long dynamics. This strongly suggests that the anomaly is not caused by an intrinsic electronic effect and that it is rather due to a slow heat exchange between the sample and the environment

Comment on ``Magnetoresistance Anomalies in Antiferromagnetic YBa2_2Cu3_3O6+x_{6+x}: Fingerprints of Charged Stripes''

In a recent Letter Ando et al (cond-mat/9905071) discovered an anomalous magnetoresistance(MR) in hole doped antiferromagnetic YBa$_2$Cu$_3$O$_{6+x}$, which they attributed to charged stripes, i.e., to segregation of holes into lines. In this Comment we show that the experiments, albeit being interesting, do not prove the existence of stripes. In our view the anomalous behavior is due to an (a,b) plane anisotropy of the resistivity in the bulk and to a magnetic field dependent antiferromagnetic (AF) domain structure. It is unlikely that domain walls are charged stripes.Comment: 1 page, Accepted to PRL, Reply exists by authors of original pape

Unusual hyperfine interaction of Dirac electrons and NMR spectroscopy in graphene

Theory of nuclear magnetic resonance (NMR) in graphene is presented. The canonical form of the electron-nucleus hyperfine interaction is strongly modified by the linear electronic dispersion. The NMR shift and spin-lattice relaxation time are calculated as function of temperature, chemical potential, and magnetic field and three distinct regimes are identified: Fermi-, Dirac-gas, and extreme quantum limit behaviors. A critical spectrometer assessment shows that NMR is within reach for fully 13C enriched graphene of reasonable size.Comment: 5 pages, 3 figure

Another barrel-lined well a road section and late Roman graves from Brigetio

As in 1929 and 2007, an excavation was conducted in the close south-eastern neighbour-hood of the legionary fortress of Brigetio unearthing similar archaeological features, graves of the late Roman cemetery and a barrel-lined well. The trial excavation also uncovered a pair of ditches, most likely belonging to a dirt road