Random Field effects in field-driven quantum critical points

We investigate the role of disorder for field-driven quantum phase transitions of metallic antiferromagnets. For systems with sufficiently low symmetry, the combination of a uniform external field and non-magnetic impurities leads effectively to a random magnetic field which strongly modifies the behavior close to the critical point. Using perturbative renormalization group, we investigate in which regime of the phase diagram the disorder affects critical properties. In heavy fermion systems where even weak disorder can lead to strong fluctuations of the local Kondo temperature, the random field effects are especially pronounced. We study possible manifestation of random field effects in experiments and discuss in this light neutron scattering results for the field riven quantum phase transition in CeCu_5.8Au_0.2.Comment: 8 page

Knight shifts around vacancies in the 2D Heisenberg model

The local response to a uniform field around vacancies in the two-dimensional (2D) spin-1/2 Heisenberg antiferromagnet is determined by numerical quantum Monte Carlo simulations as a function of temperature. It is possible to separate the Knight shifts into uniform and staggered contributions on the lattice which are analyzed and understood in detail. The contributions show interesting long and short range behavior that may be of relevance in NMR and susceptibility measurements. For more than one impurity remarkable non-linear enhancement and cancellation effects take place. We predict that the Curie impurity susceptibility will be observable for a random impurity concentration even in the thermodynamic limit.Comment: 5 pages, 4 figures, submitted to Phys. Rev. Lett. The latest pdf version can be found at http://www.physik.uni-kl.de/eggert/papers/knight.pd

A Luttinger Liquid in Box

We study a Luttinger Liquid in a finite one-dimensional wire with box-like boundary conditions by considering the local distribution of the single particle spectral weight. This corresponds to the experimental probability of extracting a single electron at a given place and energy, which can be interpreted as the square of an electron wave-function. For the non-interacting case, this is given by a standing wave at the Fermi wave-vector. In the presence of interactions, however, the wave-functions obtain additional structure with a sharp depletion near the edges and modulations throughout the wire. In the spinful case, these modulations correspond to the separate spin- and charge-like excitations in the system.Comment: 5 pages in revtex4 format including 3 epsf-embedded figures. The latest version in PDF format is available from http://fy.chalmers.se/~eggert/papers/LLbox.pd

Universal alternating order around impurities in antiferromagnets

The study of impurities in antiferromagnets is of considerable interest in condensed matter physics. In this paper we address the elementary question of the effect of vacancies on the orientation of the surrounding magnetic moments in an antiferromagnet. In the presence of a magnetic field, alternating magnetic moments are induced, which can be described by a universal expression that is valid in any ordered antiferromagnet and turns out to be independent of temperature over a large range. The universality is not destroyed by quantum fluctuation, which is demonstrated by quantum Monte Carlo simulations in the two-dimensional Heisenberg antiferromagnet. Physical predictions for finite doping are made, which are relevant for experiments probing Knight shifts and the order parameter.Comment: 5 pages, 2 figures. The most recent version in PDF format can be found at http://www.physik.uni-kl.de/eggert/papers

Doping effects in low dimensional antiferromagnets

The study of impurities in low dimensional antiferromagnets has been a very active field in magnetism ever since the discovery of high temperature superconductivity. One of the most dramatic effects is the appearance of large Knight shifts in a long range around non-magnetic impurities in an antiferromagnetic background. The dependence of the Knight shifts on distance and temperature visualizes the correlations in the system. In this work we present the results for Knight shifts around a single vacancy in the one and two dimensional Heisenberg model.Comment: 4 pages, 7 figures. The latest version can be found at http://www.physik.uni-kl.de/eggert/papers/law3m.pd

Effect of Lipoic Acid on the Biochemical Mechanisms of Resistance to Bortezomib in SH-SY5Y Neuroblastoma Cells

Neuroblastoma (NB) is an extracranial solid cancer and the most common cancer in infancy. Despite the standard treatment for NB is based on the combination of chemotherapeutic drugs such as doxorubicin, vincristine, cyclophosphamide, and cisplatin, chemoresistance occurs over the time. The aim of the present research was to evaluate the effect of bortezomib (BTZ) (50 nM) on NB cell viability and how lipoic acid (ALA) (100 μM) modifies pharmacological response to this chemotherapeutic agent. Cell viability was assessed by ATP luminescence assay whereas expression of oxidative stress marker (i.e., heme oxygenase-1) and endoplasmic reticulum stress proteins was performed by real-time PCR, western blot, and immunofluorescence. Our data showed that BTZ treatment significantly reduced cell viability when compared to untreated cultures (about 40%). Interestingly, ALA significantly reduced the efficacy of BTZ (about 30%). Furthermore, BTZ significantly induced heme oxygenase-1 as a result of increased oxidative stress and such overexpression was prevented by concomitant treatment with ALA. Similarly, ALA significantly reduced BTZ-mediated endoplasmic reticulum stress as measured by reduction in BiP1 and IRE1α, ERO1α, and PDI expression. In conclusion, our data suggest that BTZ efficacy is dependent on cellular redox status and such mechanisms may be responsible of chemoresistance to this chemotherapeutic agent

Strongly Interacting Electrons in One and Two Dimensions

In this thesis I present a theoretical study of three differentstrongly correlated systems: Luttinger liquids, spin-1 chains andthe two-dimensional spin-1/2 Heisenberg model.For every sub-field of our research I summarize the background andprovide further details to the work presented in the appendedpapers. Chapter 1 corresponds to paper I, Chapter 2 to paper II,Chapter 3 and 4 to paper III and IV.In paper I, we calculate with bosonization techniques the localdensity of states of a finite segment of Luttinger liquid. Besides ofbeing of experimental relevance for Scanning Tunneling Microscopyexperiments on carbon nanotubes and cleaved overgrowth wires, thewave functions of a "Luttinger liquid in a box" very nicelyvisualize the fundamental bosonic nature of the low energyexcitations.In paper II, we analyze with field theory methods how the presenceof a single-ion anisotropy affects the behavior of a spin-1 chain.We determine in the framework of the Non Linear Sigma Model how the gapsof the anisotropic chain depend on an applied staggered magneticfield. Our theoretical analysis is meant to model compounds like$\rm R_2BaNiO_5$ that show coexistence of Haldane phase andlong-range order.In paper III, we compute with quantum Monte Carlo simulations thelocal response to a uniform magnetic field around vacancies in thetwo-dimensional Heisenberg model. From the full understanding of thenumerical results, we can make quantitative predictions of relevancefor Nuclear Magnetic Resonance and susceptibility experiments. Inparticular, we expect, even in the thermodynamic limit, a finiteCurie contribution to the total susceptibility arising from theimpurities.Finally in paper IV, we consider interaction effects amongimpurities. We compute the potential between static vacancies inseveral low-dimensional spin-1/2 antiferromagnets comparingthe numerics with exact results from conformal field theory andlinear spin wave theory. The potential is directly related to thelocal valence bond order and its magnitude is a loyal measure of theamount of quantum correlations present into the system

Strongly Interacting Electrons in One and Two Dimensions

In this thesis I present a theoretical study of three differentstrongly correlated systems: Luttinger liquids, spin-1 chains andthe two-dimensional spin-1/2 Heisenberg model.For every sub-field of our research I summarize the background andprovide further details to the work presented in the appendedpapers. Chapter 1 corresponds to paper I, Chapter 2 to paper II,Chapter 3 and 4 to paper III and IV.In paper I, we calculate with bosonization techniques the localdensity of states of a finite segment of Luttinger liquid. Besides ofbeing of experimental relevance for Scanning Tunneling Microscopyexperiments on carbon nanotubes and cleaved overgrowth wires, thewave functions of a "Luttinger liquid in a box" very nicelyvisualize the fundamental bosonic nature of the low energyexcitations.In paper II, we analyze with field theory methods how the presenceof a single-ion anisotropy affects the behavior of a spin-1 chain.We determine in the framework of the Non Linear Sigma Model how the gapsof the anisotropic chain depend on an applied staggered magneticfield. Our theoretical analysis is meant to model compounds like$\rm R_2BaNiO_5$ that show coexistence of Haldane phase andlong-range order.In paper III, we compute with quantum Monte Carlo simulations thelocal response to a uniform magnetic field around vacancies in thetwo-dimensional Heisenberg model. From the full understanding of thenumerical results, we can make quantitative predictions of relevancefor Nuclear Magnetic Resonance and susceptibility experiments. Inparticular, we expect, even in the thermodynamic limit, a finiteCurie contribution to the total susceptibility arising from theimpurities.Finally in paper IV, we consider interaction effects amongimpurities. We compute the potential between static vacancies inseveral low-dimensional spin-1/2 antiferromagnets comparingthe numerics with exact results from conformal field theory andlinear spin wave theory. The potential is directly related to thelocal valence bond order and its magnitude is a loyal measure of theamount of quantum correlations present into the system