International Journal of Modern Physics B

The widely used Thomas Fermi model always produces pressure which is less than or equal to that of the ideal Fermi gas. On the other hand the spherical cellular model, in certain regions of the temperature-density plane, can produce pressures which are greater than that of the ideal gas. This phenomenon is investigated.

We use the equations–of–motion approach for time–dependent pair correlations in strongly interacting Fermi liquids to develop a theory of the excitation spectrum and the single–particle self energy in such systems. We present here the fully correlated equations and their approximate solutions for ³He. Our theory has the following properties: It reduces to both, i) the "correlated" random–phase approximation (RPA) for strongly interacting fermions if the two–particle–two–hole correlations are ignored, and, ii) to the correlated Brillouin–Wigner perturbation theory for boson quantum fluids in the appropriate limit. iii) It preserves the two first energy–weighted sum rules, and systematically improves upon higher ones. iv) A familiar problem of the standard RPA is that it predicts a roton energy that lies more than a factor of two higher than what is found in experiments. A popular cure for this is to introduce an effective mass in the Lindhard function. No such ad–hoc assumption is invoked in our work. We demonstrate that the inclusion of correlated pair–excitations improves the dispersion relation significantly. Finally, a novel form of the density response function is derived that arises from vertex corrections in the proper polarization.

We investigate the adsorption of helium in alkali wedges at zero temperature within a fully unrestricted density–functional theory. We have found a sequence of shapes that includes one filling transition on the nonwettable Cs and one emptying transition for every alkali, as well as a threshold for metastable configurations where helium does not populate the wedge apex.

Non-Fermi liquid behavior of strongly correlated Fermi systems is derived within the Landau approach. We attribute this behavior to a phase transition associated with a rearrangement of the Landau state that leads to flattening of a portion of the single-particle spectrum ∊(p) in the vicinity of the Fermi surface. We demonstrate that the quasiparticle subsystem responsible for the flat spectrum possesses the same thermodynamic properties as a gas of localized spins. Theoretical results compare favorably with available experimental data. While departing radically from prevalent views on the origin of non-Fermi-liquid behavior, the theory advanced here is nevertheless a conservative one of in continuing to operate within the general framework of Landau theory.

In this paper, we summarize the results of recent studies of third sound in thin, superfluid ³He-⁴He mixture films and the relation of the third sound spectrum to the question of the films' thermodynamic stability. We have considered films on several representative substrates: Nuclepore, glass, Li and Na. Our approach utilizes the variational, hypernetted chain/Euler-Lagrange (HNC–EL) theory as applied to inhomogeneous boson systems to calculate chemical potentials for both the ⁴He superfluid film and the physisorbed ³He. Numerical density derivatives of the chemical potentials lead to the sought-after third sound speeds. On all substrates, the third sound speeds show a series of oscillations as a function of film coverage that is driven by the layered structure of the ⁴He film. We find that the effect on the third sound response of adding a small amount of ³He to the ⁴He film can depend sensitively on the particular ⁴He film coverage. The third sound speed can either increase or decrease. In fact, in some regimes, the added ³He destabilizes the film and can drive "layering transitions" leading to quite complicated geometric structures of the film in which the outermost layer consists of phase–separated regimes of ³He and ⁴He. Finally, we examine the range of applicability of the usual film–averaged hydrodynamic description. We find that at least up to film thicknesses of six liquid layers, there is no regime in which this hydrodynamic description is applicable.

We study the mobility of electrons adsorbed to thin ⁴He films. Utilizing the time-dependent version of the Euler-Lagrange, hypernetted chain variational theory, we compute the inelastic scattering rate of an electron due to collisions with film excitations (third sound). We obtain an analytic result valid in the long wavelength limit. In agreement with experiment, the mobility shows oscillations due to the underlying transverse film structure. The oscillations are explicitly due to the appearance of the third sound speed in the scattering rate since the third sound speed itself oscillates in conjunction with the ⁴He film structure. The mobilities tend to be higher than reported mobilities on thin films, which we attribute to substrate structure. We interpret our results as generic mobilities that are valid in the limit of perfectly smooth, structureless substrates.

We consider inter- and intra-species pairing interactions in an asymmetrical Fermi system. Using equation of motion method, we obtain coupled mean-field equations for superfluid gap functions and population densities. We construct a phase diagram across BCS-BEC regimes. Due to intra-species correlations, the BCS singlet superfluid state can sustain finite polarizations, P. For larger P, we find phase separations in BCS and BEC regimes. A superfluid phase exists for all P deep in BEC regime. Our results may apply to pairing in ultracold fermions, nuclear and quark matter physics.

We present recent results from a diffusion Monte Carlo study of small ³He drops in the limit of zero temperature. Using the same methodology than in previous calculations carried out for bulk ³He, we have obtained accurate results for their energy and structure properties. A relevant concern of this analysis has been the determination of the stability threshold for self binding. Our results show that the smallest self-bound drop is formed by N = 30 atoms, with preferred orbitals for open shells corresponding to maximum value of the spin. The quality of the upper bound energies provided by the fixed node approximation is analyzed by performing released-node estimations for short released times. Possible improvements of the nodal surface by including BCS-like correlation functions are also discussed.

Two-dimensional (2D) electron systems in the presence of disorder are of interest in connection with the observed metal-insulator transition in such systems. We use density functional theory in its local-spin density approximation (LSDA) to calculate the ground-state energy of a 2D electron system in the presence of remote charged impurities which up on averaging provides disorder. The inverse compressibility calculated from the ground-state energy exhibits a minimum at a critical density controlled by the disorder strength. Our findings are in agreement with experimental results.

The problem of a trion (charged exciton) immersed in an incompressible quantum liquid formed by two-dimensional electrons in a high magnetic field is studied theoretically by combining the composite fermion model and exact numerical diagonalization. Laughlin correlation of the trion with the surrounding electrons is shown to lead to the formation of a family of novel many-body excitations called "quasiexcitons" (QXs). The residual charge of the most stable QX can be either zero or the fractional quantum ±e/3, depending on the presence of negative or positive Laughlin quasiparticles in the electron liquid (controlled by the Landau level filling factor ν), but also on the width of the quantum well, Zeeman spin splitting, etc. The prediction of the magnetic-field driven transition from negative to positive QXs is argued to explain the recently observed minute discontinuity in the photoluminescence spectra of sufficiently wide (w ≥ 20 nm) asymmetric GaAs quantum wells at ν = 1/3. Conversely, the observed discontinuity is hence indirectly linked to the emergence of a fractional charge quantum in the electron liquid.

We study homogeneous normal systems of bosons under the influence of interparticle forces with a strongly repulsive component at short relative particle-particle distances. The repulsion prevents short-ranged exchange between the bosonic constituents in the quantum fluid. Consequently, the bosons remain distinguishable at temperatures far below the classical high-temperature regime. At these low temperatures such fluids and liquids display nevertheless distinct quantum effects due to quantum-mechanical phase-phase correlations. Typical examples are liquid para-hydrogen and fluid ⁴He under certain thermodynamic conditions. The study employs Correlated Density-Matrix theory and Path-Integral Monte-Carlo simulations.

We present results on the behavior of the dynamic structure function in the short wave length limit using the equation of motion method. The one-body continuity equation defines the self-energy, which becomes a functional of the fluctuating two-body correlation function. We evaluate the self-energy in this limit and show that sum rules up to the second moment, which requires the self-energy in the short wave length limit and zero frequency to be proportional to the kinetic energy per particle, are exactly satisfied. We compare our results with the impulse approximation and calculate the condensate fraction. An analytic expression for the momentum distribution is also derived.

Nanotube structures are unprecedented in their stability and current-carrying capacity at intense driving fields. A comprehensive understanding of electron conduction from equilibrium through to the high-driving-field regime is needed. We present a microscopically conserving quantum-kinetic description of transport for ohmically contacted carbon nanotubes. The approach is computationally straightforward and can describe nonequilibrium response over a wide range of parameters. We have analyzed the interplay of degeneracy and scattering dynamics on gate-controlled conduction in the one-dimensional channel, and have determined transconductances.

We consider the simplest example of a double quantum dot system: two equivalent, capacitively coupled single-level dots, each coupled to its own lead. Focussing on a regime of two electrons on the double dot, a rich range of behaviour is found on increasing the interdot coupling: first a crossover from spin- to charge-Kondo physics, via an intermediate SU(4) state with entangled spin and charge degrees of freedom; followed by a quantum phase transition of Kosterlitz-Thouless type to a non-Fermi liquid 'charge-ordered' phase with finite residual entropy and anomalous transport properties. The numerical renormalization group method, coupled with simple physical arguments, is employed to obtain a detailed understanding of the problem.

The method of increments is a wavefunction-based ab initio correlation method for solids, which explicitly calculates the many-body wavefunction of the system. After a Hartree-Fock treatment of the infinite system the correlation energy of the solid is expanded in terms of localised orbitals or of a group of localised orbitals. The method of increments has been applied to a great variety of materials with a band gap, but in this paper the extension to metals is described. The application to solid mercury is presented, where we achieve very good agreement of the calculated ground-state properties with the experimental data.

Systems of strongly correlated fermions on certain geometrically frustrated lattices at particular filling factors support excitations with fractional charges ±e/2. We calculate quantum mechanical ground states, low–lying excitations and spectral functions of finite lattices by means of numerical diagonalization. The ground state of the most thorough-fully studied case, the criss-crossed checkerboard lattice, is degenerate and shows long–range order. Static fractional charges are confined by a weak linear force, most probably leading to bound states of large spatial extent. Consequently, the quasi-particle weight is reduced, which reflects the internal dynamics of the fractionally charged excitations. By using an additional parameter, we fine–tune the system to a special point at which fractional charges are manifestly deconfined—the so–called Rokhsar–Kivelson point. For a deeper understanding of the low–energy physics of these models and for numerical advantages, several conserved quantum numbers are identified.

Increasing experimental evidence points towards a two-fluid model for the low-temperature behavior of 5f systems. Assuming some 5f electrons to be localized atomic-like and others to be band-like itinerant can explain various unusual low-energy properties of U compounds including the formation of heavy fermion and their unconventional superconductivity. The dual model, however, provides an effective Hamiltonian which is valid only for the low-energy regime. It has been shown that intra-atomic correlations among the 5f electrons are a possible mechanism for orbital-selective (partial) localization. In this paper we theoretically derive criteria and specify properties which experimentally characterize correlation-induced partial localization in actinide compounds. Of particular interest are the 5f spectral function which can be related to photoemission data. Results for the U-based heavy fermion superconductor UPd₂Al₃ are presented emphasizing the consequences for the interpretation of experimental data.

We analyze the effects of site-dilution disorder on the thermodynamic properties of the two-dimensional Kondo necklace using finite-temperature stochastic series expansion. Results will be discussed for the dependence on dilution concentration, temperature, and Kondo exchange-coupling strength of the uniform susceptibility, the staggered structure factor, and the Chakravarty-Halperin-Nelson ratio. Dilution is shown to induce effective free-spin clusters in the gapped phase of the clean system with a low-temperature Curie constant renormalized below 1/4. Furthermore, dilution is demonstrated to suppress the quantum critical point of the clean system and to generate antiferromagnetic order in the quantum disordered phase of the clean system, i.e. order-from-disorder.

The statistical weight W as a function of energy E for quasielectrons with mass m* subject to a fixed magnetic field B is , where A is the sample area, and ω_c ≡ eB/m* the cyclotron frequency. Significantly, there is no Landau term proportional to B² in 2D. This leads to the conclusion that the 2D electron system is always paramagnetic, but shows a magnetic oscillation.

We study the one-dimensional S = 1/2 Heisenberg model with an uniform and a staggered magnetic fields, using the dynamical density-matrix renormalization group (DDMRG) technique. The DDMRG enables us to investigate the dynamical properties of chain with lengths up to a few hundreds, and the results are numerically exact in the same sense as 'exact diagonalization' results are. Thus, we can analyze the low-energy spectrum almost in the thermodynamic limit. In this work, we calculate the dynamical spin structure factor and demonstrate the performance of the DDMRG method applying the open-end boundary conditions as well as the periodic boundary conditions.

We report on recent results for strongly frustrated quantum J₁ - J₂ antiferromagnets in dimensionality d = 1, 2, 3 obtained by the coupled cluster method (CCM). We demonstrate that the CCM in high orders of approximation allows us to investigate quantum phase transitions driven by frustration and to discuss novel quantum ground states. In detail we consider the ground-state properties of (i) the Heisenberg spin-1/2 antiferromagnet on the cubic lattice in d = 1, 2, 3, and use the results for the energy, the sublattice magnetization and the spin stiffness as a benchmark test for the precision of the method; (ii) coupled frustrated spin chains (the quasi-one-dimensional J₁ - J₂ model) and discuss the influence of the quantum fluctuations and the interchain coupling on the incommensurate spiral state present in the classical model; (iii) the Shastry-Sutherland antiferromagnet on the square lattice; and (iv) a stacked frustrated square-lattice Heisenberg antiferromagnet (the quasi-two-dimensional J₁ - J₂ model), and discuss the influence of the interlayer coupling on the quantum paramagnetic ground-state phase that is present for the strictly two-dimensional model.

The existence of stable or metastable vortex and antivortex states in submicron ferromagnetic films is reviewed. Using standard simulation methods, the dynamics of an antivortex in a permalloy asteroid geometry is explored. As in the case of the vortex in similarly sized permalloy disks and squares, there is a sub-GHz frequency gyrotropic mode of the core of the antivortices, and a rich spin wave spectrum in the multi-GHz range. The spin waves are generated in the simulations by using magnetic pulses similar to those available experimentally, which have been applied to circular and square particles of similar sizes in previous research.

Strong finite-range electron-phonon interaction (EPI) bounds carriers into small mobile bipolarons. The Bogoliubov-de Gennes (BdG) equations are formulated and solved in the strong-coupling limit, when small bipolarons bose-condense with finite center-of-mass momentum, K ≠ 0. There are two energy scales in this regime, a temperature independent incoherent gap Δ_p and a temperature dependent coherent gap Δ_c(T, R) modulated in the real space, R. The order parameter has d-wave symmetry in R-space and the single-particle density of states (DOS) reveals checkerboard modulations similar to the checkerboard tunnelling DOS observed in cuprates.

Five commonly held premises considered questionable assumptions in the microscopic theory of superconductivity are discussed as possible reasons why the search appears to be stalled for a theoretical framework, admittedly ambitious, capable of predicting materials with critical temperatures T_c higher than the 1993 record of 164K in HgTlBaCaCuO (under pressure). We focus the dilemma as a whole in terms of a generalized Bose-Einstein condensation (GBEC) interpretation that includes and further extends BCS theory, as well as substantially enhancing its predicted T_cs within the electron-phonon mechanism producing pairing. The new GBEC model is an extension of the Friedberg-T.D. Lee 1989 boson-fermion BEC theory of high-T_c superconductors in that it includes hole pairs as well as electron pairs.

A regular ring of Josephson junctions, connected in parallel, is studied analytically and numerically. We show that, depending on the strength of the r-well cosine potential the energy landscape of the Hamiltonian can have of the order of r^N/N locally stable minima separated by large barriers specified by unstable saddle points. The counting problem for the degeneracy of the total energy is equivalent to a wellknown necklace problem in combinatorial mathematics. We also demonstrate that the distribution of the phase differences as well as the energy spectrum is fractal provided that the strength of the cosine potential is sufficiently strong.

Applying two-time Green-function techniques to the Friedberg-T.D. Lee phenomenological Hamiltonian of a many-fermion system, it is shown that positive-energy resonant bosonic pairs associated with four-fermion excitations above the Fermi sea are energetically lower in a ground-state that is a mixture of two coexisting and dynamically interacting many-particle subsystems: a) unpaired fermions and b) composite bosons. It is argued that an interaction between free fermions and bosons excited above the Fermi sea in the mixture, namely, the continuous processes of pair-formation from, and disintegration into, two unpaired electrons, results in a substantially lowering the total system energy. The positive-energy composite bosons begin to appear incoherently below a depairing temperature T* as their coupling- and temperature-dependent number density gradually increases from zero. This leads quite naturally to the pseudogap phenomenon observed in high-T_c cuprates.

The correlated density appears in many physical systems ranging from dense interacting gases up to Fermi liquids which develop a coherent state at low temperatures, the superconductivity. One consequence of the correlated density is the Bernoulli potential in superconductors which compensates forces from dielectric currents. This Bernoulli potential allows to access material parameters. Though within the surface potential these contributions are largely canceled, the bulk measurements with NMR can access this potential. Recent experiments are explained and new ones suggested. The underlying quantum statistical theory in nonequilibrium is the nonlocal kinetic theory developed earlier.

We consider systems where the amplitude and the phase of a superfluid order parameter are controlled independently by local, respectively non-local phase correlations of the internal degrees of freedom of quasi-particles which characterize the different phases of such fermionic systems. A possible realization of such a situation is when pairing into the bosonic entities occurs via resonant scattering of itinerant fermionic particles. Examples for that are the ultracold fermionic gases in optical traps and lattices, the correlated electronic systems and strongly coupled electron-phonon systems, such as seen in the high T_c cuprates. We demonstrate the occurrence of a quantum phase transition from a superfluid to a phase uncorrelated boson liquid state in such systems, which is controlled by the competing effects of long and short range phase correlations of the constituents of composite quasi-particles.

The extended Hubbard model with nearest neighbor hopping and intersite interaction for a 2D square lattice has been studied within the frame work of a simple mean-field formalism. The self-consistent solutions of superconducting order parameters have been obtained for various fillings. The results indicate that superconducting pairing occurs on doping for the repulsive on-site U and attractive nearest neighbor V.

The significant features of exchange symmetry are displayed by simple systems such as two identical, spinless fermions in a one-dimensional well with infinite walls. The conclusion is that the maxima of probability of the antisymmetrized wave function of these two fermions lie at the same positions as if a repulsive force (of unknown nature) was applied between these two fermions. This conclusion is combined with the solution of a mathematical problem dealing with the equilibrium of identical repulsive particles (of one or two kinds) on one or more spheres like neutrons and protons on nuclear shells. Such particles are at equilibrium only for specific numbers of particles and, in addition, if these particles lie on the vertices of regular polyhedra or their derivative polyhedra. Finally, this result leads to a pictorial representation of the structure of all closed shell nuclei. This representation could be used as a laboratory for determining nuclear properties and corresponding wave functions.

Pairing properties of infinite matter are surveyed that are obtained from self-consistent Green's function calculations. A complete treatment of off-shell propagation that incorporates the effects of short-range correlations is included. Mean-field calculations based on the BCS approach are superseded by the present results, which resolve a long-standing puzzle associated with ³S₁-³D₁ (proton-neutron) pairing in symmetric nuclear matter. Results for ¹S₀ pairing in pure neutron matter are in agreement with recent Monte-Carlo calculations for this system. The possibility of proton pairing in finite nuclei, as a result of increasing proton correlations with increasing nucleon asymmetry, is pointed out.

Short range correlations are introduced using the unitary operator method in a relativistic theory of nuclear matter based on the Hartree-Fock approximation. Pion exchange, including the tensor contribution, is taken into account. It is found that both the tensor contribution of pion exchange and short range correlations soften the equation of state. The neutron matter equation of state is also discussed.

The famous Hoyle state (0_2⁺ at 7.654 MeV in ¹²C) is identified as being an almost ideal condensate of three α-particles, hold together only by the Coulomb barrier. It, therefore, has a ⁸Be-α structure of low density. Transition probability and inelastic form factor together with position and other physical quantities are correctly reproduced without any adjustable parameter from our two parameter wave function of α-particle condensate type. The possibility of the existence of α-particle condensed states in heavier nα nuclei is also discussed.

The real parts of the alpha-alpha and alpha-nucleus potential, particularly for ^40,44,48Ca and ⁵⁸Ni targets are determined from a realistic two-nucleon potential using an energy-density functional approach and are found to be non-monotonic with a short range repulsion in their functional forms that are similar to those needed in accounting for elastic scattering data. The deduced alpha-alpha potential has no bound states and the calculated decay width of ⁸Be of about 6.4 eV at about 200 keV energy is in agreement with the measurement.

We investigate the dynamics of Na clusters embedded in Ar matrices. We use a hierarchical approach, accounting microscopically for the cluster's degrees of freedom and more coarsely for the matrix. The dynamical polarizability of the Ar atoms and the strong Pauli-repulsion exerted by the Ar-electrons are taken into account. We discuss the impact of the matrix on the cluster gross properties and on its optical response. We then consider a realistic case of irradiation by a moderately intense laser and discuss the impact of the matrix on the hindrance of the explosion, as well as a possible pump probe scenario for analyzing dynamical responses.

A recently developed formalism in which Kohn-Sham calculations are combined with an "average pair density functional theory" is reviewed, and some new properties of the effective electron-electron interaction entering in this formalism are derived. A preliminary construction of a fully self-consitent scheme is also presented in this framework.

Within linear response theory, the dielectric and optical properties of a charged particle system are related to equilibrium correlation functions. In particular, the dynamical conductivity and the dynamical collision frequency are expressed in terms of the current-current or force-force correlation function, which can be evaluated analytically using perturbation expansions or numerically by MD simulations. Results are given for bulk material. Furthermore, finite systems such as laser excited clusters are considered. It is shown that the collision frequency is reduced in finite systems. The interaction with the laser field is discussed with respect to the current-current correlation function which changes with time due to the expansion of the laser-irradiated cluster.

In this contribution biomolecular systems are analyzed in a framework of differential geometry in order to derive important condensed matter physics information. In the first section lipid bi-layer membranes are examined with respect to statistical properties and topology, e.g. a relation between vesicle formation and the proliferation of genus number. In the second section differential geometric methods are used for analyzing the surface structure of proteins and thereby understanding catalytic properties of larger proteins.

Attractive possibilities exist for the brain to solve important problems of visual processing in Fourier space. First, we describe a wave-interference model for computing motion contrast directly from the moving intensity distribution, without the need for directionally selective motion sensors. We then propose a global method for motion-based image segmentation based on unsupervised clustering in a three-dimensional Fourier space. The Fourier components of coherently moving entities are segregated from the remainder by means of a simple velocity proximity measure. This is accomplished without altering the spatial frequency components encoding the object, thereby ensuring that the spatiotemporal features of the segregated object can be reconstructed.

Proposals for scalable quantum computing devices suffer not only from decoherence due to their interaction with the environment, but also from severe engineering constraints. For example, our ability to implement quantum gates is determined, in part, by the experimentally available interactions with which quantum information may be processed. Here we review a practical solution to some of the major concerns, control and error prevention, addressing solid state proposals for quantum computing devices. Some noise is eliminated by encoding a logical qubit into two qubits, other noise is reduced by an efficient set of decoupling pulse sequences. The same encoding removes the need for single-qubit operations which pose a difficult design constraint. We also discuss several generalizations which follow from this work.

We consider the Hubbard-like Hamiltonian used in Physica B 352 (2004) 269-279, using the MaxEnt approach and a CSCO, as the set of relevant operators which, obviously, close a partial Lie algebra with the Hamiltonian. So, it is possible to define a positive definite metric on the space spanned by the mean values of the CSCO. Particularly, the selected operators represent the occupation number, the aligned and antialigned pairs per site, respectively. The interesting question to be examined here is whether the above mentioned Hamiltonian have the property that the expectation values of a suitable operator are indeed zero and one (or a constant) qubits, and zero and +1 and -1, i.e., qutrits.

We introduce coherent mixed states (or thermal coherent states) associated with the displaced harmonic oscillator at nonzero temperature (T ≠ 0), as a "random" (or "thermal" or "noisy") basis in Hilbert space. A resolution of the identity for these states is proven and is used to generalize the usual pure (T = 0) coherent state formalism to the mixed (T ≠ 0) case. This new formalism for thermal coherent states is then further generalized to a broader class of so-called negative-binomial mixed states. It is known that the negative-binomial distribution is itself intimately related to the discrete series of SU(1,1) representations. We consider the pure SU(1,1) coherent states in the two-mode harmonic oscillator space, and show how our negative-binomial mixed states arise from taking the partial trace with respect to one of these two modes. This observation is then used to show how the formalism of thermo-field dynamics may be generalized to a correspondingly much broader negative-binomial-field dynamics, which we expect to have many uses.

A condition for ergodicity is derived, applicable to a Hermitian many body model in both the classical and quantum domains. Using this ergodic condition, the validity of the ergodic hypothesis is examined in certain solvable 1d magnetic models. A simple but general picture has emerged which shows why the hypothesis can be valid and why it can also fail.

The maximum entropy principle (MaxEnt) is one of the most powerful approaches in the theorist arsenal. It has been used in many areas of science, and, in particular, in both statistical mechanics and the quantum many body problem. Here we show how to derive MaxEnt from the laws of thermodynamics.

Over the last decade, the density-matrix renormalization group (DMRG) has emerged as the most powerful method for the simulation of strongly correlated one-dimensional (1D) quantum systems. Input from quantum information has allowed to trace the method's performance to the entanglement properties of quantum states, revealing why it works so well in 1D and not so well in 2D; it has allowed to devise algorithms for time-dependent quantum systems and, by clarifying the link between DMRG and Wilson's numerical renormalization group (NRG), for quantum impurity systems.