Diffusion and ballistic transport in one-dimensional quantum systems.

It has been conjectured that transport in integrable one-dimensional systems is necessarily ballistic. The large diffusive response seen experimentally in nearly ideal realizations of the S=1/2 1D Heisenberg model is therefore puzzling and has not been explained so far. Here, we show that, contrary to common belief, diffusion is universally present in interacting 1D systems subject to a periodic lattice potential. We present a parameter-free formula for the spin-lattice relaxation rate which is in excellent agreement with experiment. Furthermore, we calculate the current decay directly in the thermodynamic limit using a time-dependent density matrix renormalization group algorithm and show that an anomalously large time scale exists even at high temperatures.

Pairing symmetry and Josephson current in doped 2-leg t-J ladders.

We perform the numerical equivalent of a phase sensitive experiment on doped 2-leg t-J ladders. We apply proximity effect fields with different complex phases at both ends of an open system and we study the transport of Cooper pairs. Measuring the response of the system and the induced Josephson current, density matrix renormalization group calculations show how, depending on the doping fraction, the rung-leg parity of the pair field changes from minus to plus as the density of holes is increased. The Josephson current exhibits a phase transition as a function of J/t.

Chain breaks and the susceptibility of Sr2Cu1-xPdxO3+delta and other doped quasi-one-dimensional antiferromagnets.

We study the magnetic susceptibility of one-dimensional S=1/2 antiferromagnets containing nonmagnetic impurities which cut the chain into finite segments. For the susceptibility of long anisotropic Heisenberg chain segments with open boundaries we derive a parameter-free result at low temperatures using field-theory methods and the Bethe ansatz. The analytical result is verified by comparing with quantum Monte Carlo calculations. We then show that the partitioning of the chain into finite segments can explain the Curie-like contribution observed in recent experiments on Sr2Cu(1-x)PdxO(3+delta). Possible additional paramagnetic impurities seem to play only a minor role.

Dynamical spin structure factor for the anisotropic spin-1/2 Heisenberg chain.

The longitudinal spin structure factor for the XXZ-chain at small wave vector q is obtained using Bethe ansatz, field theory methods, and the density matrix renormalization group. It consists of a peak with a peculiar, non-Lorentzian shape and a high-frequency tail. We show that the width of the peak is proportional to q2 for finite magnetic field compared to q3 for a zero field. For the tail we derive an analytic formula without any adjustable parameters and demonstrate that the integrability of the model directly affects the line shape.

Quantum magnetic impurities in magnetically ordered systems.

We discuss the problem of a spin 1/2 impurity immersed in a spin S magnetically ordered background. We show that the problem maps onto a generalization of the dissipative two level system with two independent heat baths, associated with the Goldstone modes of the magnet, that couple to different components of the impurity spin operator. Using analytical perturbative renormalization group methods and accurate numerical renormalization group we show that contrary to other dissipative models there is quantum frustration of decoherence and quasiscaling even in the strong coupling regime. We make predictions for the behavior of the impurity magnetic susceptibility. Our results may also have relevance to quantum computation.

Local magnetic susceptibility of the positive muon in the quasi-one-dimensional S=1/2 antiferromagnet dichlorobis (pyridine) copper (II).

We report muon spin rotation measurements of the local magnetic susceptibility around a positive muon in the paramagnetic state of the quasi-one-dimensional spin 1/2 antiferromagnet dichlorobis (pyridine) copper (II). Signals from three distinct sites are resolved and have a temperature dependent frequency shift which is significantly different than the magnetic susceptibility. This difference is attributed to a muon induced perturbation of the spin 1/2 chain. The obtained frequency shifts are compared with transfer matrix density-matrix renormalization-group numerical simulations.

Dimerization induced by the RKKY interaction.

We report the presence of spin dimerization in the ground state of the one-dimensional Kondo lattice model at quarter filling. The emergence of this new phase of the Kondo lattice can be traced to the form of the RKKY interaction between the localized moments and provides the first example of dimerization induced indirectly by itinerant electrons. We propose this dimer ordering as the driving mechanism of the spin-Peierls phase observed in the quasi-one-dimensional organic compounds (Per)2M(mnt)(2) (M=Pt, Pd). Moreover, this suggests that a richer phase diagram than the Doniach paradigm may be needed to accommodate the physics of heavy fermion materials.

Detecting the Kondo screening cloud around a quantum dot.

A fundamental prediction of scaling theories of the Kondo effect is the screening of an impurity spin by a cloud of electrons spread out over a mesoscopic distance. This cloud has never been observed experimentally. Recently, aspects of the Kondo effect have been observed in experiments on quantum dots embedded in quantum wires. Since the length of the wire may be of order the size of the screening cloud, such systems provide an ideal opportunity to observe it. We point out that persistent current measurements in a closed ring provide a conceptually simple way of detecting this fundamental length scale.

Effective "penetration depth" in the vortex state of a d-wave superconductor.

The temperature and the field dependence of the effective magnetic penetration depth (lambdaeff) in the vortex state of a d-wave superconductor, as measured by muon spin rotation (muSR) experiments, is calculated using a nonlocal London model. We show that at temperatures below [EQUATION: SEE TEXT], the linear T dependence of lambda-2eff crosses over to a T3 dependence. This could provide an explanation for the low temperature flattening of the lambda-2eff curve observed in a recent muSR experiment.

Breast screening: the psychological sequelae of false-positive recall in women with and without a family history of breast cancer.

The psychological effects of false-positive mammography were evaluated in 124 women who had taken part in the U.K. National Health Service Breast Screening Programme. In addition, the effects of recall on women with and without a family history were compared. These women were asked to complete the Hospital Anxiety and Depression Scale (HADS) before being invited to attend for screening, at recall and 5 weeks and 4 months after recall. At screening and at recall, the women were asked to complete the Health Questionnaire (HQ) which measures stress-related behaviour changes in the previous week. In the week before screening, compared with women who did not have a family history of breast cancer, women with a family history had lower scores on HADS depression and reported fewer stress-related behaviour changes. At recall, regardless of family history, the women were more likely to have borderline or clinically significant anxiety than at baseline or screening. Nevertheless, for most women, recall-induced anxiety was relatively transient (less than 5 weeks). Compared with women without a family history, women with a family history were more anxious 4 months after recall, although their anxiety scores tended to be lower (P < 0.06) than at baseline. A strength of the present study is that the initial baseline measure was uncontaminated by the screening process. Women who did not complete questionnaires at one or more of the subsequent time points scored higher on HADS depression at baseline, indicating that the results are likely to have underestimated the effects of recall. Screening appears to be less stressful for women with a family history than for those without a history. However, for both groups recall causes short term distress. Breast screening programmes should ensure that steps are taken to minimise the number of women who are recalled for unnecessary investigations.