arXiv subjects

Luis O. Manuel

Publications and source records attributed to Luis O. Manuel.

7 recordsLinked to original sources

arXiv2026

Lindbladian approach for many-qubit thermal machines: enhancing the performance with geometric heat pumping by interaction

We present a detailed analysis of slowly driven quantum thermal machines based on interacting qubits within the framework of the Lindblad master equation. By implementing a systematic expansion in the driving rate, we derive explicit expressions for the rate of work of the driving forces, the heat currents exchanged with the reservoirs, and the entropy production up to second order, ensuring full thermodynamic consistency in the linear-response regime. The formalism naturally separates geometric and dissipative contributions, identified by a Berry curvature and a metric in parameter space, respectively. Analytical results show that the geometric heat pumped per cycle is bounded by $k_B T N_q \ln 2$ for $N_q$ non-interacting qubits, in direct analogy with the Landauer limit for entropy change. This bound can be surpassed when qubit interactions and asymmetric couplings to the baths are introduced. Numerical results for the interacting two-qubit system reveal a non-trivial role of the interaction between qubits and the coupling between the qubits and the baths in the behavior of the dissipated power. The approach provides a general platform for studying dissipation, pumping, and performance optimization in driven quantum devices operating as heat engines.

quant-ph↗

arXiv2025

Nontrivial entanglement passively mediated by a quenched magnetic impurity

We investigate the entanglement properties of a Kondo system undergoing a transition to a state with a quenched magnetic impurity, using the density matrix renormalization group (DMRG) method. We focus on a two-channel spin-1 Kondo impurity with single-ion anisotropy, where a quantum phase transition occurs between two topologically distinct local Fermi liquids. In the fully screened Kondo phase, realized at lower anisotropies, the entangled region surrounding the magnetic impurity mimics the Kondo screening cloud, although its length does not follow the conventional behavior. In contrast, beyond the transition, the system enters a non-Landau Fermi liquid phase with a markedly different entanglement structure: as the impurity is quenched and disentangled from the rest of the system due to the breakdown of the Kondo effect, the two conduction channels coupled only through the impurity develop a significant degree of entanglement with one another. Our findings demonstrate that a quenched magnetic impurity can passively and efficiently mediate entanglement between spatially separated conduction bands.

cond-mat.str-el↗

arXiv2024

Anisotropy-driven topological quantum phase transition in magnetic impurities

A few years ago, a topological quantum phase transition (TQPT) has been found in Anderson and Kondo 2-channel spin-1 impurity models that include a hard-axis anisotropy term $DS_z^2$ with $D > 0$. The most remarkable manifestation of the TQPT is a jump in the spectral density of localized electrons, at the Fermi level, from very high to very low values as $D$ is increased. If the two conduction channels are equivalent, the transition takes place at the critical anisotropy $D_c \sim 2.5\; T_K$, where $T_K$ is the Kondo temperature for $D=0$. This jump might be important to develop a molecular transistor. The jump is due to a corresponding one in the Luttinger integral, which has a topological non-trivial value $π/2$ for $D > D_c$. Here, we review the main results for the spectral density and highlight the significance of the theory for the interpretation of measurements conducted on magnetic atoms or molecules on metallic surfaces. In these experiments, where $D$ is held constant, the energy scale $T_K$ is manipulated by some parameters. The resulting variation gives rise to a differential conductance $dI/dV$, measured by scanning-tunneling spectroscopy, which is consistent with a TQPT at an intermediate value of $T_K$. We also show that the theory can be extended to integer spin $S>1$ and two-impurity systems. This is also probably true for half-integer spin and non-equivalent channels in some cases.

cond-mat.str-el↗

arXiv2022

The fate of pairing and spin-charge separation in the presence of long-range antiferromagnetism

We present a numerical study of competing orders in the 1D $t$-$J$ model with long-range RKKY-like staggered spin interactions. By circumventing the constraints imposed by Mermin-Wagner's theorem, this Hamiltonian can realize long-range Néel order at half-filling. We determine the full phase diagram as a function of the exchange and particle density using the density matrix renormalization group (DMRG) method. We show that pairing is disfavored and the AFM insulator and metallic phases are separated by a broad regime with phase segregation, before spin-charge separation re-emerges at low densities. Upon doping, interactions induce a confining potential that binds holons and spinons into full fledged fermionic quasi-particles in a range of parameters and densities. We numerically calculate the photoemission spectrum of the model, showing the appearance of a coherent quasi-particle band splitting away from the holon-spinon continuum with a width determined by $J$ that survives at finite doping. Comparison with analytical results using the self-consistent Born approximation (SCBA) and by solving the spinon-holon problem offer insight into the internal structure of the quasi-particles and help us explain the different features in the spectrum. We discuss how this simple toy-model can teach us about the phenomenology of its higher dimensional counterpart.

cond-mat.str-el↗

arXiv2018

Spin excitations of half-doped bilayer manganites: intermediate phase

The ground state of half-doped manganites involves intricate spin, charge and orbital orderings, which are difficult to discern experimentally. In this work, we resort to the theoretical analysis of the spin fluctuation spectrum of the half-doped bilayer Pr(Ca$_{0.9}$Sr$_{0.1}$)$_2$ Mn$_2$O$_7$ in order to get an insight of its electronic ground state. By means of the linear spin wave approximation, we compute the magnon dispersion for a family of localized spin models, which can describe several phases proposed for the ground state of half-doped manganites, like the intermediate one proposed by Efremov et al. [Nat.Mats. 3, 853, (2004)] along with its particular cases corresponding to Goodenough's CE phase [Phys. Rev. 100, 564, (1955)] and Zener polaron or dimer phases. We obtain an excellent agreement between theory and experiment when the ground state is assumed to be a generalized Goodenough's CE phase, with a Mn-charge disproportionation inside the experimentally expected range. As essential ingredients for our improved fit of the upper and lower magnon branches measured around the gap, we identified two next-nearest neighbour exchange interactions between the planar Mn zig-zag chains, one for each type of Mn ion present. In connection with this finding, we revisited the magnetic excitations of the laminar related compounds, focusing on the upper magnon branches. Here we prove that their measurement would provide the key to identify unambiguously the ground state present in the layered half-doped manganites.

cond-mat.str-el↗

arXiv2013

Quantum magnons of the intermediate phase of half-doped manganite oxides

At half doping, the ground state of three-dimensional manganite perovskite oxides like R$_{1-x}$Ca$_x$MnO$_3$, where R is a trivalent ion such as La, Pr, etc, is still unclear. Many experimental findings agree better with the combined magnetic, charge, and orbital order characteristic of the "intermediate phase", introduced by Efremov et al. in 2004 [Nature Mats. 3, 853]. This phase consists of spin dimers (thus incorporating aspects of the Zener polaron phase (ZP) proposed in 2002 by Daoud-Aladine et al. [Phys. Rev. Lett. 89, 097205]), though formed by a pair of parallel Mn spins of different magnitude, in principle (thereby allowing for a degree of Mn charge disproportionation: not necessarily as large as that of Mn$^{3+}$-Mn$^{4+}$ in Goodenough's original CE phase [Phys. Rev. 100, 564 (1955)]). In the intermediate phase, consecutive spin dimers localed along the planar zig-zag chains are oriented at a constant relative angle $Φ$ between them. Varying Mn-charge disproportionation and $Φ$, the intermediate phase should allow to continuously interpolate between the two limiting cases of the CE phase and the dimer phase denoted as "orthogonal intermediate $π/2-$phase". It is not easy to find a microscopic model able to describe the phenomenological intermediate phase adequately for the spin, charge, and orbital degrees of freedom simultaneously. Here, we study the quantum spin excitations of a planar model of interacting localized spins, which we found can stabilize the intermediate phase classically. We compare the quantum magnons of the intermediate phase with those of the CE and orthogonal $π/2$ phases, in the context of recent experimental results.

cond-mat.str-el↗

arXiv2010

Effects of vertex corrections on diagrammatic approximations applied to the study of transport through a quantum dot

In the present work, we calculate the conductance through a single quantum dot weakly coupled to metallic contacts. We use the spin-1/2 Anderson model to describe the quantum dot, while considering a finite Coulomb repulsion. We solve the interacting system using the non-crossing-approximation (NCA) and the one-crossing approximation (OCA). We obtain the linear response conductance as a function of temperature and energy position of the localized level. From the comparison of both approximations we extract the role of the vertex corrections, which are introduced in the OCA calculations and neglected in the NCA scheme. As a function of the energy position, we observe that the diagrams omitted within NCA are really important for appropriately describing transport phenomena in Kondo systems as well as in the mixed valence regime. On the other hand, as a function of temperature, the corrections introduced by OCA partly recover the universal scaling properties known from numerical approaches such as the Numerical Renormalization Group(NRG).

cond-mat.str-el↗