arXiv subjects

C. Pepin

Publications and source records attributed to C. Pepin.

At least 19 recordsLinked to original sources

arXiv2025

Dynamic compression of glassy GeO$2$ up to the TPa range and first observation of shock induced crystallization

In this work we present an extensive study of glassy GeO$_2$ under laser induced dynamic compression. New VISAR and SOP data provide the extension of Hugoniot EoS up to the TPa range for this material including temperature measurements. Reflectivity data at both 532 and 1064 nm wavelenght are also reported. In the low compression range we observe changes of the optical properties from transparent, to opaque, to metallic state. The second part of this work describes a further laser shock experiment combined with in-situ X-ray diffraction. Here we observe, for the first time, the laser shock-induced crystallization of glassy GeO$_2$ to a structure compatible with the rutile phase at pressure higher than 20 GPa and melting occurring at around 75 GPa.

cond-mat.mtrl-sci↗

arXiv2010

The modulated spin liquid: a new paradigm for URu$_2$Si$_2$

We argue that near a Kondo breakdown critical point, a spin liquid with spatial modulations can form. Unlike its uniform counterpart, we find that this occurs via a second order phase transition. The amount of entropy quenched when ordering is of the same magnitude as for an antiferromagnet. Moreover, the two states are competitive, and at low temperatures are separated by a first order phase transition. The modulated spin liquid we find breaks $Z_4$ symmetry, as recently seen in the hidden order phase of URu$_2$Si$_2$. Based on this, we suggest that the modulated spin liquid is a viable candidate for this unique phase of matter.

cond-mat.str-el↗

arXiv2010

The thermopower as a fingerprint of the Kondo breakdown quantum critical point

We propose that the thermoelectric power distinguishes two competing scenarios for quantum phase transitions in heavy fermions : the spin-density-wave (SDW) theory and breakdown of the Kondo effect. In the Kondo breakdown scenario, the Seebeck coefficient turns out to collapse from the temperature scale $E^{*}$, associated with quantum fluctuations of the Fermi surface reconfiguration. This feature differs radically from the physics of the SDW theory, where no reconstruction of the Fermi surface occurs, and can be considered as the hallmark of the Kondo breakdown theory. We test these ideas, upon experimental results for YbRh$_2$Si$_2$.

cond-mat.str-el↗

arXiv2010

The thermopower as a signature of quantum criticality in heavy fermions

We present a series of arguments showing that the Seebeck coefficient can be used as a decisive experiment to characterize the nature of the quantum critical point (QCP) in heavy fermion compounds. Being reactive almost exclusively to the presence of delocalized entropic carriers, the Seebeck coefficient shows a drastic collapse at the Kondo breakdown QCP, as the reconstruction of the Fermi surface takes place. In contrast, around a Spin Density Wave (SDW) QCP, the Seebeck coefficient is broadly symmetric. We discuss the possibility of a change of sign at the QCP, the characteristic variation of $| S / T | $ with temperature and external parameter, as well as the capacity of the Seebeck coefficient to distinguish between localized and itinerant anti-ferromagnetism (AF). Suggestions of experiments are given in the case of four non conventional compounds : YbRh$_2$Si$_2$, Ce(Mn)In$_5$, CeCu$_{6-x}$Au$_x$ and URu$_2$Si$_2$.

cond-mat.str-el↗

arXiv2009

Exact bosonization for an interacting Fermi gas in arbitrary dimensions

We present an exact mapping of models of interacting fermions onto boson models. The bosons correspond to collective excitations in the initial fermionic models. This bosonization is applicable in any dimension and for any interaction between fermions. We show schematically how the mapping can be used for Monte Carlo calculations and argue that it should be free from the sign problem. Introducing superfields we derive a field theory that may serve as a new way of analytical study.

cond-mat.str-el↗

arXiv2009

Quantum Boltzman equation study for the Kondo breakdown quantum critical point

We develop the quantum Boltzman equation approach for the Kondo breakdown quantum critical point, involved with two bands for conduction electrons and localized fermions. Particularly, the role of vertex corrections in transport is addressed, crucial for non-Fermi liquid transport of temperature linear dependence. Only one band of spinons may be considered for scattering with gauge fluctuations, and their associated vertex corrections are introduced in the usual way, where divergence of self-energy corrections is cancelled by that of vertex corrections, giving rise to the physically meaningful result in the gauge invariant expression for conductivity. On the other hand, two bands should be taken into account for scattering with hybridization excitations, giving rise to coupled quantum Boltzman equations. We find that vertex corrections associated with hybridization fluctuations turn out to be irrelevant due to heavy mass of spinons in the so called decoupling limit, consistent with the diagrammatic approach showing the non-Fermi liquid transport.

cond-mat.str-el↗

arXiv2008

Selective Mott transition and heavy fermions

Starting with an extended version of the Anderson lattice where the f-electrons are allowed a weak dispersion, we examine the possibility of a Mott localization of the f-electrons, for a finite value of the hybridization $V$. We study the fluctuations at the quantum critical point (QCP) where the f-electrons localize. We find they are in the same universality class as for the Kondo breakdown QCP, with the following notable features. The quantum critical regime sees the appearance of an additional energy scale separating two universality classes. In the low energy regime, the fluctuations are dominated by massless gauge modes, while in the intermediate energy regime, the fluctuations of the modulus of the order parameter are the most relevant ones. In the latter regime, electric transport simplifies drastically, leading to a quasi-linear resistivity in 3D and anomalous exponents lower than T in 2 D. This rather unique feature of the quantum critical regime enables us to make experimentally testable predictions.

cond-mat.str-el↗

arXiv2008

Multi-scale fluctuations near a Kondo Breakdown Quantum Critical Point

We study the Kondo-Heisenberg model using a fermionic representation for the localized spins. The mean-field phase diagram exhibits a zero temperature quantum critical point separating a spin liquid phase where the f-conduction hybridization vanishes, and a Kondo phase where it does not. Two solutions can be stabilized in the Kondo phase, namely a uniform hybridization when the band masses of the conduction electrons and the f spinons have the same sign, and a modulated one when they have opposite sign. For the uniform case, we show that above a very small Fermi liquid temperature scale (~1 mK), the critical fluctuations associated with the vanishing hybridization have dynamical exponent z=3, giving rise to a specific heat coefficient that diverges logarithmically in temperature, as well as a conduction electron inverse lifetime that has a T log T behavior. Because the f spinons do not carry current, but act as an effective bath for the relaxation of the current carried by the conduction electrons, the latter result also gives rise to a T log T behavior in the resistivity. This behavior is consistent with observations in a number of heavy fermion metals.

cond-mat.str-el↗

arXiv2007

Kondo Breakdown as a Selective Mott Transition in the Anderson Lattice

We show within the slave boson technique, that the Anderson lattice model exhibits a Kondo breakdown quantum critical point (KB-QCP) where the hybridization goes to zero at zero temper- ature. At this fixed point, the f-electrons experience as well a selective Mott transition separating a local-moment phase from a Kondo-screened phase. The presence of a multi-scale QCP in the An- derson lattice in the absence of magnetism is discussed in the context of heavy fermion compounds. This study is the first evidence for a selective Mott transition in the Anderson lattice.

cond-mat.str-el↗

arXiv2006

Kondo Breakdown and Hybridization Fluctuations in the Kondo-Heisenberg Lattice

We study the deconfined quantum critical point of the Kondo-Heisenberg lattice in three dimensions using a fermionic representation for the localized spins. The mean-field phase diagram exhibits a zero temperature quantum critical point separating a spin liquid phase where the hybridization vanishes and a Kondo phase where it does not. Two solutions can be stabilized in the Kondo phase, namely a uniform hybridization when the band masses of the conduction electrons and the spinons have the same sign, and a modulated one when they have opposite sign. For the uniform case, we show that above a very small temperature scale, the critical fluctuations associated with the vanishing hybridization have dynamical exponent z=3, giving rise to a resistivity that has a T log T behavior. We also find that the specific heat coefficient diverges logarithmically in temperature, as observed in a number of heavy fermion metals.

cond-mat.str-el↗

arXiv2006

Quantum critical end point of the Kondo volume collapse

The Kondo volume collapse describes valence transitions in f-electron metals, and is characterized by a line of first order transitions in the pressure-temperature phase plane terminated at critical end points. We analyze the quantum critical end point, when the lower end point is tuned to T=0, and determine the specific heat, thermal expansion, and compressibility. We find that the inclusion of quantum critical fluctuations leads to a novel bifurcation of the first order phase line. Finally, we show that critical strain fluctuations can cause both, superconductivity and non-Fermi liquid behavior near the critical point.

cond-mat.str-el↗

arXiv2006

Quantum critical behavior in itinerant electron systems -- Eliashberg theory and instability of a ferromagnetic quantum-critical point

We consider the problem of fermions interacting with gapless long-wavelength collective bosonic modes. The theory describes, among other cases, a ferromagnetic quantum-critical point (QCP) and a QCP towards nematic ordering. We construct a controllable expansion at the QCP in two steps: we first create a new, non Fermi-liquid ``zero-order'' Eliashberg-type theory, and then demonstrate that the residual interaction effects are small. We prove that this approach is justified under two conditions: the interaction should be smaller than the fermionic bandwidth, and either the band mass $m_B$ should be much smaller than $m = p_F/v_F$, or the number of fermionic flavors $N$ should be large. For an SU(2) symmetric ferromagnetic QCP, we find that the Eliashberg theory itself includes a set of singular renormalizations which can be understood as a consequence of an effective long-range dynamic interaction between quasi-particles, generated by the Landau damping term. These singular renormalizations give rise to a negative non-analytic $q^{3/2}$ correction to the static spin susceptibility, and destroy a ferromagnetic QCP. We demonstrate that this effect can be understood in the framework of the $ϕ^4$ theory of quantum criticality. We also show that the non-analytic $q^{3/2}$ correction to the bosonic propagator is specific to the SU(2) symmetric case. For systems with a scalar order parameter, the $q^{3/2}$ contributions from individual diagrams cancel out in the full expression of the susceptibility, and the QCP remains stable.

cond-mat.str-el↗

arXiv2003

Scaling approach to itinerant quantum critical points

Based on phase space arguments, we develop a simple approach to metallic quantum critical points, designed to study the problem without integrating the fermions out of the partition function. The method is applied to the spin-fermion model of a T=0 ferromagnetic transition. Stability criteria for the conduction and the spin fluids are derived by scaling at the tree level. We conclude that anomalous exponents may be generated for the fermion self-energy and the spin-spin correlation functions below $d=3$, in spite of the spin fluid being above its upper critical dimension.

cond-mat.str-el↗

arXiv2003

Pseudogaps in Nested Antiferromagnets

We analyze the fluctuation corrections to magnetic ordering in the case of a 3D antiferromagnet with flat Fermi surfaces, as physically realized in the case of chromium, and find that they are insufficient to produce a quantum critical point. This implies that the critical point observed in vandium doped chromium is due to a loss of nesting. We also derive the fermion self-energy in the paramagnetic phase and find that a pseudogap exists, though its magnitude is significantly reduced as compared to the spectral gap in the ordered state in the limit where the latter is small in comparison to the Fermi energy.

cond-mat.str-el↗

arXiv2003

The break up of heavy electrons at a quantum critical point

The point at absolute zero where matter becomes unstable to new forms of order is called a quantum critical point (QCP). The quantum fluctuations between order and disorder that develop at this point induce profound transformations in the finite temperature electronic properties of the material. Magnetic fields are ideal for tuning a material as close as possible to a QCP, where the most intense effects of criticality can be studied. A previous study on theheavy-electron material $YbRh_2Si_2$ found that near a field-induced quantum critical point electrons move ever more slowly and scatter off one-another with ever increasing probability, as indicated by a divergence to infinity of the electron effective mass and cross-section. These studies could not shed light on whether these properties were an artifact of the applied field, or a more general feature of field-free QCPs. Here we report that when Germanium-doped $YbRh_2Si_2$ is tuned away from a chemically induced quantum critical point by magnetic fields there is a universal behavior in the temperature dependence of the specific heat and resistivity: the characteristic kinetic energy of electrons is directly proportional to the strength of the applied field. We infer that all ballistic motion of electrons vanishes at a QCP, forming a new class of conductor in which individual electrons decay into collective current carrying motions of the electron fluid.

cond-mat.str-el↗

arXiv2003

The Electronic Nature of High Temperature Cuprate Superconductors

We review the field of high temperature cuprate superconductors, with an emphasis on the nature of their electronic properties. After a general overview of experiment and theory, we concentrate on recent results obtained by angle resolved photoemission, inelastic neutron scattering, and optical conductivity, along with various proposed explanations for these results. We conclude by reviewing efforts which attempt to identify the energy savings involved in the formation of the superconducting ground state.

cond-mat.supr-con↗

arXiv2003

Non Fermi Liquid behavior in the under-screened Kondo model

Using the Schwinger boson spin representation, we reveal a new aspect to the physics of a partially screened magnetic moment in a metal, as described by the spin $S$ Kondo model. We show that the residual ferromagnetic interaction between a partially screened spin and the electron sea destabilizes the Landau Fermi liquid, forming a singular Fermi liquid with a $1/ (T \ln ^{4} (T_{K}/T))$ divergence in the low temperature specific heat coefficient $C_{V}/T$. A magnetic field $B$ tunes this system back into Landau Fermi liquid with a Fermi temperature proportional to $B \ln^2 (T_K/B)$. We discuss a possible link with field-tuned quantum criticality in heavy electron materials.

cond-mat↗

arXiv2002

Heavy Electron Quantum Criticality

Although the concept of a quantum phase transition has been known since the nineteen seventies, their importance as a source of radical transformation in metallic properties has only recently been appreciated. A quantum critical point forms an essential singularity in the phase diagram of correlated matter. We discus new insights into the nature of this phenomenon recently gained from experiments in heavy electron materials.

cond-mat.str-el↗