The numerical renormalization group method for quantum impurity systems

In the beginning of the 1970's, Wilson developed the concept of a fully non-perturbative renormalization group transformation. Applied to the Kondo problem, this numerical renormalization group method (NRG) gave for the first time the full crossover from the high-temperature phase of a free spin to the low-temperature phase of a completely screened spin. The NRG has been later generalized to a variety of quantum impurity problems. The purpose of this review is to give a brief introduction to the NRG method including some guidelines of how to calculate physical quantities, and to survey the development of the NRG method and its various applications over the last 30 years. These applications include variants of the original Kondo problem such as the non-Fermi liquid behavior in the two-channel Kondo model, dissipative quantum systems such as the spin-boson model, and lattice systems in the framework of the dynamical mean field theory.Comment: 55 pages, 27 figures, submitted to Rev. Mod. Phy

Renormalization Group Approach to Non-equilibrium Green Functions in Correlated Impurity Systems

We present a technique for calculating non-equilibrium Green functions for impurity systems with local interactions. We use an analogy to the calculation of response functions in the x-ray problem.The initial state and the final state problems, which correspond to the situations before and after the disturbance (an electric or magnetic field, for example) is suddenly switched on, are solved with the aid of Wilson's momentum shell renormalization group. The method is illustrated by calculating the non-equilibrium dynamics of the ohmic two-state problem.Comment: 7 pages, 2 figure

Scaling and universality in the anisotropic Kondo model and the dissipative two-state system

Scaling and universality in the Ohmic two-state system is investigated by exploiting the equivalence of this model to the anisotropic Kondo model. For the Ohmic two-state system, we find universal scaling functions for the specific heat, $C_{\alpha}(T)$, static susceptibility, $\chi_{\alpha}(T)$, and spin relaxation function $S_{\alpha}(\omega)$ depending on the reduced temperature $T/\Delta_{r}$ (frequency $\omega/\Delta_{r}$), with $\Delta_{r}$ the renormalized tunneling frequency, and uniquely specified by the dissipation strength $\alpha$ ($0<\alpha<1$). The scaling functions can be used to extract $\alpha$ and $\Delta_{r}$ in experimental realizations.Comment: 5 pages (LaTeX), 4 EPS figures. Minor changes, typos corrected, journal reference adde

Out of equilibrium transport through an Anderson impurity: Probing scaling laws within the equation of motion approach

We study non-equilibrium electron transport through a quantum impurity coupled to metallic leads using the equation of motion technique at finite temperature T. Assuming that the interactions are taking place solely in the impurity and focusing in the infinite Hubbard limit, we compute the out of equilibrium density of states and the differential conductance G_2(T,V) to test several scaling laws. We find that G_2(T,V)/G_2(T,0) is a universal function of both eV/T_K and T/T_K, being T_K the Kondo temperature. The effect of an in plane magnetic field on the splitting of the zero bias anomaly in the differential conductance is also analyzed. For a Zeeman splitting \Delta, the computed differential conductance peak splitting depends only on \Delta/T_K, and for large fields approaches the value of 2\Delta . Besides the traditional two leads setup, we also consider other configurations that mimics recent experiments, namely, an impurity embedded in a mesoscopic wire and the presence of a third weakly coupled lead. In these cases, a double peak structure of the Kondo resonance is clearly obtained in the differential conductance while the amplitude of the highest peak is shown to decrease as \ln(eV/T_K). Several features of these results are in qualitative agreement with recent experimental observations reported on quantum dots.Comment: 9 pages, 7 figure

Evidence of destabilization of the human thymidylate synthase (hTS) dimeric structure induced by the interface mutation Q62R

In human cells, thymidylate synthase (TS) provides the only source of 2\u2019-deoxythymidyne-5\u2019-monophosphate (dTMP), which is required for DNA biosynthesis. Because of its pivotal role, human TS (hTS) represents a validated target for anticancer chemotherapy. Nonetheless, the efficacy of drugs blocking the hTS active site has limitations due to the onset of resistance in cancer cells, requiring the identification of new strategies to effectively inhibit this enzyme. Human TS works as an obligate homodimer, making the inter-subunit interface an attractive targetable area. Here, we report the design and investigation of a new hTS variant, in which Gln62, located at the dimer interface, has been replaced by arginine in order to destabilize the enzyme quaternary assembly. The hTS Q62R variant has been characterized though kinetic assay, thermal denaturation analysis and X-ray crystallography. Our results provide evidence that hTS Q62R has a reduced melting temperature. The effective destabilization of the TS quaternary structure is also confirmed by structural analysis, showing that the introduced mutation induces a slight aperture of the hTS dimer. The generation of hTS variants having a more accessible interface area can facilitate the screening of interface-targeting molecules, providing key information for the rational design of innovative hTS interface inhibitors

THU0454 SOMATIC SYMPTOMS IN FIBROMYALGIA AND THEIR CORRELATION WITH DRUG TREATMENT

Background:Drug treatment in fibromyalgia (FM) is often disappointingly ineffective, and there are currently very few data to support therapeutic choices towards a personalized medicine approach.Objectives:To evaluate the prevalence of selected somatic symptoms in FM, and to study their relationship with drug treatments.Methods:The study population consisted of 526 patients (471 F 55 M, mean age 47.31±11.33 yrs) affected by FM not associated with other rheumatic diseases. All patients were required to compile a questionnaire reporting the presence of 42 somatic symptoms -as suggested (1) – in the last 7 days. Drug usage was assessed by interview.Results:On average, patients reported the presence of 17.04±6.68 symptoms (range 4-35), with ample variations in the prevalence of different symptoms (Fig. 1), ranging from over 95% (fatigue and muscle pain) to less than 10 %, seizures being reported by only 2 patients (0.4%). 31.1% of patients were not taking any drug for their FM. The most frequently used drugs were analgesics (ANA, 41.7%) followed by benzodiazepines (BD, 29.1%), SSRIs (16%), gabapentinoids (GABA, 14,4%), and NSRI (14.3%) (Fig. 2). Different drugs were associated with a different spectrum of somatic symptoms: as compared to non users, BD users reported a significantly higher (p< 0.05 by chi-square test) prevalence of irritable bowel (65.4% vs 52.3%), fatigue (98.7% vs 94.9%), thinking difficulties (78.4% vs 68.5%), muscle weakness (94.1% vs 81.7%), abdominal pain (55.6% vs 43.9%), insomnia (73.9% vs 56.6%), depression (63.4 % vs 37.2%), constipation (60.1% vs 42.9%), pain in upper abdomen (50.3% vs 40.2%), nausea (53.6% vs 38.3%), nervousness (71.9% vs 61.5%), chest pain (49.0 vs 37.75), blurred vision (65.4% vs 53.6%), dry mouth (72.5% vs 52.3%), itching (56.2% vs 44.5%), vomiting (13.7% vs 7.8%), taste change (22.2% vs 12.7%), dry eyes (55.6% vs 41.0%), breath shortness (56.9% vs 47.7%), appetite loss (33.3% vs 19.7%), painful urination (15.0% vs 8.4%), and bladder spasms (18.3% vs 8.6%). NRSI users reported a significantly higher prevalence of thinking difficulties, constipation, blurred vision, dry mouth, wheezing, dry eyes, easy bruising. Among GABA users, there was a higher prevalence of thinking difficulties, numbness, insomnia, constipation, nausea, dry mouth, dry eyes, appetite loss, sun sensitivity, easy bruising, and bladder spasms. In no cases a higher prevalence of symptoms was recorded in drug non users vs users.Conclusion:The usage of different drugs in FM is associated with different somatic symptoms. The higher prevalence of symptoms in drug users as compared to non users raises serious questions concerning the opportunity or the appropriateness of drug selection in FM.References:[1]Wolfe F., et al. Arthritis Care Res (Hoboken). 2010 May;62(5):600-10Disclosure of Interests: :None declare

Sesquiterpene lactones with dual inhibitory activity against the trypanosoma brucei pteridine reductase 1 and dihydrofolate reductase

The parasite Trypanosoma brucei (T. brucei) is responsible for human African trypanosomi-asis (HAT) and the cattle disease “Nagana” which to this day cause severe medical and socio-eco-nomic issues for the affected areas in Africa. So far, most of the available treatment options are ac-companied by harmful side effects and are constantly challenged by newly emerging drug re-sistances. Since trypanosomatids are auxotrophic for folate, their pteridine metabolism provides a promising target for an innovative chemotherapeutic treatment. They are equipped with a unique corresponding enzyme system consisting of the bifunctional dihydrofolate reductase-thymidylate synthase (TbDHFR-TS) and the pteridine reductase 1 (TbPTR1). Previously, gene knockout experi-ments with PTR1 null mutants have underlined the importance of these enzymes for parasite sur-vival. In a search for new chemical entities with a dual inhibitory activity against the TbPTR1 and TbDHFR, a multi-step in silico procedure was employed to pre-select promising candidates against the targeted enzymes from a natural product database. Among others, the sesquiterpene lactones (STLs) cynaropicrin and cnicin were identified as in silico hits. Consequently, an in-house database of 118 STLs was submitted to an in silico screening yielding 29 further virtual hits. Ten STLs were subsequently tested against the target enzymes in vitro in a spectrophotometric inhibition assay. Five compounds displayed an inhibition over 50% against TbPTR1 as well as three compounds against TbDHFR. Cynaropicrin turned out to be the most interesting hit since it inhibited both TbPTR1 and TbDHFR, reaching IC50 values of 12.4 µM and 7.1 µM, respectively

Numerical Renormalization Group Study of Pseudo-Fermion and Slave-Boson Spectral Functions in the Single Impurity Anderson Model

We use the numerical renormalization group to calculate the auxiliary spectral functions of the $U=\infty$ Anderson impurity model. The slave--boson and pseudo--fermion spectral functions diverge at the threshold with exponents $\alpha_{b}$ and $\alpha_{f}$ given in terms of the conduction electron phase shifts by the X--ray photoemission and the X--ray absorption exponents respectively. The exact exponents obtained here depend on the impurity occupation number, in contrast to the NCA results. Vertex corrections in the convolution formulae for physical Green's functions are singular at the threshold and may not be neglected in the Fermi liquid regime.Comment: 12 pages, RevTeX 3.0, 2 PS figures appende

Coherence correlations in the dissipative two-state system

We study the dynamical equilibrium correlation function of the polaron-dressed tunneling operator in the dissipative two-state system. Unlike the position operator, this coherence operator acts in the full system-plus-reservoir space. We calculate the relevant modified influence functional and present the exact formal expression for the coherence correlations in the form of a series in the number of tunneling events. For an Ohmic spectral density with the particular damping strength $K=1/2$, the series is summed in analytic form for all times and for arbitrary values of temperature and bias. Using a diagrammatic approach, we find the long-time dynamics in the regime $K<1$. In general, the coherence correlations decay algebraically as $t^{-2K}$ at T=0. This implies that the linear static susceptibility diverges for $K\le 1/2$ as $T\to 0$, whereas it stays finite for $K>1/2$ in this limit. The qualitative differences with respect to the asymptotic behavior of the position correlations are explained.Comment: 19 pages, 4 figures, to be published in Phys. Rev.

How did italian adolescents with disability and parents deal with the covid-19 emergency?

The COVID-19 emergency has imposed distanced education and has interrupted most rehabilitation services. Adolescents with disabilities have been isolated, and the burden on their families has been exacerbated. A cross-sectional survey was administered to adolescents with disability and to parents of disabled children to describe their experience during lockdown and their concerns or expectations about rehabilitation. A sample of 53 adolescents and 239 parents completed the survey. Adolescents were ages 13\u201318 years old (45.3% female). Most parents were between 35 and 55 years old (84.9% female). While 53.6% of the parents reported no positive effects of the lockdown, 92.5% of the adolescents expressed favorable consequences. The increased time spent with family members was judged positively by 27.2% of parents and by 64.2% of adolescents. Concern for their child\u2019s disability was expressed by 47.3% of parents, while 73.6% of adolescents expressed concerns regarding the ban on meeting friends. In both groups, anxiety symptoms were correlated with the fear of contracting COVID-19 and with financial problems. Parents would have liked even more remote support from school and healthcare professionals, which was available for most participants. Thus, socioeconomic support, assistive technology and telerehabilitation strategies might help families with disabilities during a lockdown