Instability of the quantum-critical point of itinerant ferromagnets

Structural and physical properties of Ni1-xVxalloys around and away from quantum critical point

We investigate the room temperature structure (global and local), temperature dependent magnetic and transport behaviour of Ni1-xVx(0⩽x⩽0.13) alloys. Our Energy Dispersive Analysis of x-rays results show that the prepared compositions are stoichiometric. With increase in V doping, the compounds exhibit a quantum phase transition aroundxc= 0.12, where the ferromagnetic phase is suppressed. Our results show that all the compounds stabilize in face centred cubic structure at RT and the lattice parameter shows unusual behaviour close toxc. The magnetic and heat capacity studies show signature of Griffiths phase on either side ofxc. From 25 K to the lowest collected temperature, we observe a linear T dependence of resistivity atx = 0.1 and aroundxc, which is separated by a Fermi-liquid region aroundx = 0.106. This suggests that the origin of the transport behaviour is different around the quantum critical point and away from it. Our Ni K-edge x-ray Absorption Spectroscopy results show that there is a significant reduction in the first coordination number around Ni central atom on doping. Further, with doping, there is distortion in the first coordination shell around Ni. This suggests, with V doping, the local structure around Ni is different from the global structure as obtained from the x-ray Diffraction results. Interestingly, with doping, we observe a direct connection between the extent of distortion at RT and the magnetic disorder obtained at 2 K. We believe our results will motivate the scientific community to further study the interplay between the structural disorder and quantum fluctuations with temperature at the local level.

Pressure driven magnetic order in Sr[Formula: see text]Ca[Formula: see text]Co[Formula: see text]P[Formula: see text]

The magnetic phase diagram of Sr[Formula: see text]Ca[Formula: see text]Co[Formula: see text]P[Formula: see text] as a function of hydrostatic pressure and temperature is investigated by means of high pressure muon spin rotation, relaxation and resonance ([Formula: see text]SR). The weak pressure dependence for the [Formula: see text] compounds suggests that the rich phase diagram of Sr[Formula: see text]Ca[Formula: see text]Co[Formula: see text]P[Formula: see text] as a function of x at ambient pressure may not solely be attributed to chemical pressure effects. The [Formula: see text] compound on the other hand reveals a high pressure dependence, where the long range magnetic order is fully suppressed at [Formula: see text] kbar, which seem to be a first order transition. In addition, an intermediate phase consisting of magnetic domains is formed above [Formula: see text] kbar where they co-exist with a magnetically disordered state. These domains are likely to be ferromagnetic islands (FMI) and consist of an high- (FMI-[Formula: see text]) and low-temperature (FMI-[Formula: see text]) region, respectively, separated by a phase boundary at [Formula: see text] K. This kind of co-existence is unusual and is originating from a coupling between lattice and magnetic degrees of freedoms.

Localized 4f-electrons in the quantum critical heavy fermion ferromagnet CeRh6Ge4

Ferromagnetic quantum critical points were predicted to be prohibited in clean itinerant ferromagnetic systems, yet such a phenomenon was recently revealed in CeRh₆Ge₄, where the Curie temperature can be continuously suppressed to zero under a moderate hydrostatic pressure. Here we report the observation of quantum oscillations in CeRh₆Ge₄ from measurements using the cantilever and tunnel-diode oscillator methods in fields up to 45 T, clearly demonstrating that the ferromagnetic quantum criticality occurs in a clean system. In order to map the Fermi surface of CeRh₆Ge₄, we performed angle-dependent measurements of quantum oscillations at ambient pressure, and compared the results to density functional theory calculations. The results are consistent with the Ce 4f electrons remaining localized and not contributing to the Fermi surface, suggesting that localized ferromagnetism is a key factor for the occurrence of a ferromagnetic quantum critical point in CeRh₆Ge₄.

Kondo Holes in the Two-Dimensional Itinerant Ising Ferromagnet Fe3GeTe2

Heavy Fermion (HF) states emerge in correlated quantum materials due to the intriguing interplay between localized magnetic moments and itinerant electrons but rarely appear in 3d-electron systems due to high itinerancy of d-electrons. Here, an anomalous enhancement of Kondo screening is observed at the Kondo hole of local Fe vacancies in Fe₃GeTe₂ which is a recently discovered 3d-HF system featuring Kondo lattice and two-dimensional itinerant ferromagnetism. An itinerant Kondo-Ising model is established to reproduce the experimental results and provides insight into the competition between Ising ferromagnetism and Kondo screening. Our work explains the microscopic origin of the d-electron HF states in Fe₃GeTe₂ and inspires future studies of the enriched quantum many-body effects with Kondo holes.

Anisotropic c-f Hybridization in the Ferromagnetic Quantum Critical Metal CeRh_{6}Ge_{4}

Heavy fermion compounds exhibiting a ferromagnetic quantum critical point have attracted considerable interest. Common to two known cases, i.e., CeRh_{6}Ge_{4} and YbNi_{4}P_{2}, is that the 4f moments reside along chains with a large interchain distance, exhibiting strong magnetic anisotropy that was proposed to be vital for the ferromagnetic quantum criticality. Here, we report an angle-resolved photoemission study on CeRh_{6}Ge_{4} in which we observe sharp momentum-dependent 4f bands and clear bending of the conduction bands near the Fermi level, indicating considerable hybridization between conduction and 4f electrons. The extracted hybridization strength is anisotropic in momentum space and is obviously stronger along the Ce chain direction.The hybridized 4f bands persist up to high temperatures, and the evolution of their intensity shows clear band dependence. Our results provide spectroscopic evidence for anisotropic hybridization between conduction and 4f electrons in CeRh_{6}Ge_{4}, which could be important for understanding the electronic origin of the ferromagnetic quantum criticality.

Field-Induced Modulated State in the Ferromagnet PrPtAl

The theory of quantum order-by-disorder (QOBD) explains the formation of modulated magnetic states at the boundary between ferromagnetism and paramagnetism in zero field. PrPtAl has been argued to provide an archetype for this. Here, we report the phase diagram in magnetic field, applied along both the easy a axis and hard b axis. For field aligned to the b axis, we find that the magnetic transition temperatures are suppressed and at low temperature there is a single modulated fan state, separating an easy a axis ferromagnetic state from a field polarized state. This fan state is well explained with the QOBD theory in the presence of anisotropy and field. Experimental evidence supporting the QOBD explanation is provided by the large increase in the T^{2} coefficient of the resistivity and direct detection of enhanced magnetic fluctuations with inelastic neutron scattering, across the field range spanned by the fan state. This shows that the QOBD mechanism can explain field induced modulated states that persist to very low temperature.

First-Order Magnetic Phase Transition of Mobile Electrons in Monolayer MoS_{2}

Evidence is presented for a first-order magnetic phase transition in a gated two-dimensional semiconductor, monolayer-MoS_{2}. The phase boundary separates a ferromagnetic phase at low electron density and a paramagnetic phase at high electron density. Abrupt changes in the optical response signal an abrupt change in the magnetism. The magnetic order is thereby controlled via the voltage applied to the gate electrode of the device. Accompanying the change in magnetism is a large change in the electron effective mass.

Mechanism for transitions between ferromagnetic and antiferromagnetic orders in d-electron metallic magnets

We propose mechanism for pressure-induced transitions between ferromagnetic and antiferromagnetic phases that relies on a competition between characteristic energy scales ubiquitous among d-electron metallic magnetic compounds. Principles behind the mechanism are demonstrated on the example of the minimal two-orbital p-d lattice model. We suggest that LaCrGe3, where pressure-induced ferromagnetic-to-antiferromagnetic phase transition has been recently observed, is a promising candidate to realize discussed mechanism.

Ultrasmall Moment Incommensurate Spin Density Wave Order Masking a Ferromagnetic Quantum Critical Point in NbFe_{2}

In the metallic magnet Nb_{1-y}Fe_{2+y}, the low temperature threshold of ferromagnetism can be investigated by varying the Fe excess y within a narrow homogeneity range. We use elastic neutron scattering to track the evolution of magnetic order from Fe-rich, ferromagnetic Nb_{0.981}Fe_{2.019} to approximately stoichiometric NbFe_{2}, in which we can, for the first time, characterize a long-wavelength spin density wave state burying a ferromagnetic quantum critical point. The associated ordering wave vector q_{SDW}=(0,0,l_{SDW}) is found to depend significantly on y and T, staying finite but decreasing as the ferromagnetic state is approached. The phase diagram follows a two-order-parameter Landau theory, for which all of the coefficients can now be determined. Our findings suggest that the emergence of spin density wave order cannot be attributed to band structure effects alone. They indicate a common microscopic origin of both types of magnetic order and provide strong constraints on related theoretical scenarios based on, e.g., quantum order by disorder.

Revealing fermionic quantum criticality from new Monte Carlo techniques

This review summarizes recent developments in the study of fermionic quantum criticality, focusing on new progress in numerical methodologies, especially quantum Monte Carlo methods, and insights that emerged from recently large-scale numerical simulations. Quantum critical phenomena in fermionic systems have attracted decades of extensive research efforts, partially lured by their exotic properties and potential technology applications, and partially awakened by the profound and universal fundamental principles that govern these quantum critical systems. Due to the complex and non-perturbative nature, these systems face the most difficult and challenging problems in the study of modern condensed matter physics, and many important fundamental problems remain open. Recently, new developments in model design and algorithm improvements enabled unbiased large-scale numerical solutions to be achieved in the close vicinity of these quantum critical points, which paves a new pathway towards achieving controlled conclusions through combined efforts of theoretical and numerical studies, as well as possible theoretical guidance for experiments in heavy-fermion compounds, Cu-based and Fe-based superconductors, ultra-cold fermionic atomic gas, twisted graphene layers, etc, where signatures of fermionic quantum criticality exist.

Itinerant quantum critical point with fermion pockets and hotspots

The present work summarizes major progress in research on the itinerant quantum critical point (QCP). The authors designed a model and developed quantum Monte Carlo simulation to examine itinerant QCPs generated by antiferromagnetic fluctuations. The model has immediate relevance to a wide range of strongly correlated systems, such as cuprate superconductors. Large system size and low temperature are comfortably accessed and quantum critical scaling relations are revealed with high accuracy. At the QCP, a finite anomalous dimension is observed, and fermions at hotspots evolve into a non-Fermi liquid. These results are being observed in an unbiased manner and they could bridge future developments both in analytical theory and in numerical simulation of itinerant QCPs.

Metallic quantum criticality is among the central themes in the understanding of correlated electronic systems, and converging results between analytical and numerical approaches are still under review. In this work, we develop a state-of-the-art large-scale quantum Monte Carlo simulation technique and systematically investigate the itinerant quantum critical point on a 2D square lattice with antiferromagnetic spin fluctuations at wavevector Q=(π,π)—a problem that resembles the Fermi surface setup and low-energy antiferromagnetic fluctuations in high-Tc cuprates and other critical metals, which might be relevant to their non–Fermi-liquid behaviors. System sizes of 60×60×320 (L×L×Lτ) are comfortably accessed, and the quantum critical scaling behaviors are revealed with unprecedented high precision. We found that the antiferromagnetic spin fluctuations introduce effective interactions among fermions and the fermions in return render the bare bosonic critical point into a different universality, different from both the bare Ising universality class and the Hertz–Mills–Moriya RPA prediction. At the quantum critical point, a finite anomalous dimension η∼0.125 is observed in the bosonic propagator, and fermions at hotspots evolve into a non-Fermi liquid. In the antiferromagnetically ordered metallic phase, fermion pockets are observed as the energy gap opens up at the hotspots. These results bridge the recent theoretical and numerical developments in metallic quantum criticality and can serve as the stepping stone toward final understanding of the 2D correlated fermions interacting with gapless critical excitations.

Chiral Tricritical Point: A New Universality Class in Dirac Systems

Tricriticality, as a sister of criticality, is a fundamental and absorbing issue in condensed-matter physics. It has been verified that the bosonic Wilson-Fisher universality class can be changed by gapless fermionic modes at criticality. However, the counterpart phenomena at tricriticality have rarely been explored. In this Letter, we study a model in which a tricritical Ising model is coupled to massless Dirac fermions. We find that the massless Dirac fermions result in the emergence of a new tricritical point, which we refer to as the chiral tricritical point (CTP), at the phase boundary between the Dirac semimetal and the charge-density wave insulator. From functional renormalization group analysis of the effective action, we obtain the critical behaviors of the CTP, which are qualitatively distinct from both the tricritical Ising universality and the chiral Ising universality. We further extend the calculations of the chiral tricritical behaviors of Ising spins to the case of Heisenberg spins. The experimental relevance of the CTP in two-dimensional Dirac semimetals is also discussed.

Tricritical wings and modulated magnetic phases in LaCrGe3 under pressure

Experimental and theoretical investigations on itinerant ferromagnetic systems under pressure have shown that ferromagnetic quantum criticality is avoided either by a change of the transition order, becoming of the first order at a tricritical point, or by the appearance of modulated magnetic phases. In the first case, the application of a magnetic field reveals a wing-structure phase diagram as seen in itinerant ferromagnets such as ZrZn2 and UGe2. In the second case, no tricritical wings have been observed so far. Here, we report on the discovery of wing-structure as well as the appearance of modulated magnetic phases in the temperature-pressure-magnetic field phase diagram of LaCrGe3. Our investigation of LaCrGe3 reveals a double-wing structure indicating strong similarities with ZrZn2 and UGe2. But, unlike these simpler systems, LaCrGe3 also shows modulated magnetic phases similar to CeRuPO. This finding provides an example of an additional possibility for the phase diagram of metallic quantum ferromagnets.The study of phase transitions in quantum ferromagnets has shown that the approach to a continuous quantum ferromagnetic transition is typically interrupted by either a tricritical point or a new magnetic phase. Here the authors show that LaCrGe3 exhibits both these features in its phase diagram.

Collapse of Ferromagnetism and Fermi Surface Instability near Reentrant Superconductivity of URhGe

We present thermoelectric power and resistivity measurements in the ferromagnetic superconductor URhGe for a magnetic field applied along the hard magnetization b axis of the orthorhombic crystal. Reentrant superconductivity is observed near the spin reorientation transition at H_{R}=12.75  T, where a first order transition from the ferromagnetic to the polarized paramagnetic state occurs. Special focus is given to the longitudinal configuration, where both the electric and heat current are parallel to the applied field. The validity of the Fermi-liquid T^{2} dependence of the resistivity through H_{R} demonstrates clearly that no quantum critical point occurs at H_{R}. Thus, the ferromagnetic transition line at H_{R} becomes first order implying the existence of a tricritical point at finite temperature. The enhancement of magnetic fluctuations in the vicinity of the tricritical point stimulates the reentrance of superconductivity. The abrupt sign change observed in the thermoelectric power with the thermal gradient applied along the b axis together with the strong anomalies in the other directions is definitive macroscopic evidence that in addition a significant change of the Fermi surface appears through H_{R}.

Ferromagnetic Quantum Critical Point Avoided by the Appearance of Another Magnetic Phase in LaCrGe_{3} under Pressure

The temperature-pressure phase diagram of the ferromagnet LaCrGe_{3} is determined for the first time from a combination of magnetization, muon-spin-rotation, and electrical resistivity measurements. The ferromagnetic phase is suppressed near 2.1 GPa, but quantum criticality is avoided by the appearance of a magnetic phase, likely modulated, AFM_{Q}. Our density functional theory total energy calculations suggest a near degeneracy of antiferromagnetic states with small magnetic wave vectors Q allowing for the potential of an ordering wave vector evolving from Q=0 to finite Q, as expected from the most recent theories on ferromagnetic quantum criticality. Our findings show that LaCrGe_{3} is a very simple example to study this scenario of avoided ferromagnetic quantum criticality and will inspire further study on this material and other itinerant ferromagnets.

Doping-Induced Ferromagnetism and Possible Triplet Pairing in d(4) Mott Insulators

We study the effects of electron doping in Mott insulators containing d(4) ions such as Ru(4+), Os(4+), Rh(5+), and Ir(5+) with J=0 singlet ground state. Depending on the strength of the spin-orbit coupling, the undoped systems are either nonmagnetic or host an unusual, excitonic magnetism arising from a condensation of the excited J=1 triplet states of t(2g)(4). We find that the interaction between J excitons and doped carriers strongly supports ferromagnetism, converting both the nonmagnetic and antiferromagnetic phases of the parent insulator into a ferromagnetic metal, and further to a nonmagnetic metal. Close to the ferromagnetic phase, the low-energy spin response is dominated by intense paramagnon excitations that may act as mediators of a triplet pairing.

Magnetic fluctuations and specific heat in Na(x)CoO2 near a Lifshitz transition

We analyze the temperature and doping dependence of the specific heat C(T) in Na(x)CoO(2). This material was conjectured to undergo a Lifshitz-type topological transition at x=x(c)=0.62, in which a new electron Fermi pocket emerges at the Γ point, in addition to the existing hole pocket with large k(F). The data show that near x=x(c), the temperature dependence of C(T)/T at low T gets stronger as x approaches x(c) from below and then reverses the trend and changes sign at x≥x(c). We argue that this behavior can be quantitatively explained within the spin-fluctuation theory. We show that magnetic fluctuations are enhanced near x(c) at momenta around k(F), and their dynamics changes between x≤x(c) and x>x(c), when the new pocket forms. We demonstrate that this explains the temperature dependence of C(T)/T. We show that at larger x (x>0.65) the system enters a magnetic quantum critical regime where C(T)/T roughly scales as logT. This behavior extends to progressively lower T as x increases towards a magnetic instability at x≈0.75.

Disorder dependence of the ferromagnetic quantum phase transition

We quantitatively discuss the influence of quenched disorder on the ferromagnetic quantum phase transition in metals, using a theory that describes the coupling of the magnetization to gapless fermionic excitations. In clean systems, the transition is first order below a tricritical temperature T_{tc}. Quenched disorder is predicted to suppress T_{tc} until it vanishes for residual resistivities ρ_{0} on the order of several μΩ cm for typical quantum ferromagnets. We discuss experiments that allow us to distinguish the mechanism considered from other possible realizations of a first-order transition.

Fluctuation-driven magnetic hard-axis ordering in metallic ferromagnets

We demonstrate that the interplay between soft electronic particle-hole fluctuations and magnetic anisotropies can drive ferromagnetic moments to point along a magnetic hard axis. As a proof of concept, we show this behavior explicitly for a generic two-band model with local Coulomb and Hund's interactions and a spin-orbit-induced easy plane anisotropy. The phase diagram is calculated within the fermionic quantum order-by-disorder approach, which is based on a self-consistent free-energy expansion around a magnetically ordered state with unspecified orientation. Quantum fluctuations render the transition of the easy-plane ferromagnet first order below a tricritical point. At even lower temperatures, directionally dependent transverse fluctuations dominate the magnetic anisotropy, and the moments flip to lie along the magnetic hard axis. We discuss our findings in the context of recent experiments that show this unusual ordering along the magnetic hard direction.

Intense low-energy ferromagnetic fluctuations in the antiferromagnetic heavy-fermion metal CeB6

Heavy-fermion metals exhibit a plethora of low-temperature ordering phenomena . Among these are the so-called hidden-order phases that, in contrast to conventional magnetic order, are invisible to standard neutron diffraction experiments. One of the structurally most simple hidden-order compounds, CeB6, has been intensively studied for an elusive phase that was attributed to the antiferroquadrupolar ordering of cerium-4f moments . As the ground state of CeB6 is characterized by a more conventional antiferromagnetic (AFM) order , the low-temperature physics of this system has generally been assumed to be governed solely by AFM interactions between the dipolar and multipolar Ce moments . Here we overturn this established picture by observing an intense ferromagnetic (FM) low-energy collective mode that dominates the magnetic excitation spectrum of CeB6. Inelastic neutron-scattering data reveal that the intensity of this FM excitation significantly exceeds that of conventional spin-wave magnons emanating from the AFM wavevectors, thus placing CeB6 much closer to a FM instability than previously anticipated. This propensity for ferromagnetism may account for much of the unexplained behaviour of CeB6, and should lead to a re-examination of existing theories that have so far largely neglected the role of FM interactions.

Non-Landau damping of magnetic excitations in systems with localized and itinerant electrons

We discuss the form of the damping of magnetic excitations in a metal near a ferromagnetic instability. The paramagnon theory predicts that the damping term should have the form γ(q,Ω)∝Ω/Γ(q), with Γ(q)∝q (the Landau damping). However, the experiments on uranium metallic compounds UGe2 and UCoGe showed that Γ(q) is essentially independent of q. A nonzero γ(q=0,Ω) is impossible in systems with one type of carrier (either localized or itinerant) because it would violate the spin conservation. It has been conjectured recently that a near-constant Γ(q) in UGe2 and UCoGe may be due to the presence of both localized and itinerant electrons in these materials, with ferromagnetism involving predominantly localized spins. We present the microscopic analysis of the damping of near-critical localized excitations due to interaction with itinerant carriers. We show explicitly how the presence of two types of electrons breaks the cancellation between the contributions to Γ(0) from the self-energy and vertex correction insertions into the spin polarization bubble. We compare our theory with the available experimental data.

RKKY interaction and intrinsic frustration in non-Fermi-liquid metals

We study the RKKY interaction in non-Fermi-liquid metals. We find that the RKKY interaction mediated by some non-Fermi-liquid metals can be of much longer range than for a Fermi liquid. The oscillatory nature of the RKKY interaction thus becomes more important in such non-Fermi liquids, and gives rise to enhanced frustration when the spins form a lattice. Frustration suppresses the magnetic ordering temperature of the lattice spin system. Furthermore, we find that the spin system with a longer range RKKY interaction can be described by the Brazovskii model, where the ordering wave vector lies on a higher dimensional manifold. Strong fluctuations in such a model lead to a first-order phase transition and/or glassy phase. This may explain some recent experiments where glassy behavior was observed in stoichiometric heavy fermion material close to a ferromagnetic quantum critical point.

Avoided ferromagnetic quantum critical point: unusual short-range ordered state in CeFePO

Cerium 4f electronic spin dynamics in single crystals of the heavy-fermion system CeFePO is studied by means of ac susceptibility, specific heat, and muon-spin relaxation (μSR). Short-range static magnetism occurs below the freezing temperature T(g) ≈ 0.7 K, which prevents the system from accessing a putative ferromagnetic quantum critical point. In the μSR, the sample-averaged muon asymmetry function is dominated by strongly inhomogeneous spin fluctuations below 10 K and exhibits a characteristic time-field scaling relation expected from glassy spin dynamics, strongly evidencing cooperative and critical spin fluctuations. The overall behavior can be ascribed neither to canonical spin glasses nor other disorder-driven mechanisms.

Eliashberg theory of excitonic insulating transition in graphene

A sufficiently strong Coulomb interaction may open an excitonic fermion gap and thus drive a semi-metal-insulator transition in graphene. In this paper, we study the Eliashberg theory of excitonic transition by coupling the fermion gap equation self-consistently to the equation of the vacuum polarization function. Including the fermion gap into the polarization function increases the effective strength of the Coulomb interaction because it reduces the screening effects due to the collective particle-hole excitations. Although this procedure does not change the critical point, it leads to a significant enhancement of the dynamical fermion gap in the excitonic insulating phase. The validity of the Eliashberg theory is justified by showing that the vertex corrections are suppressed at the large N limit.

Evolution of critical scaling behavior near a ferromagnetic quantum phase transition

Magnetic critical scaling in URu(2-x)Re(x)Si(2) single crystals continuously evolves as the ferromagnetic critical temperature is tuned towards zero via chemical substitution. As the quantum phase transition is approached, the critical exponents gamma and (delta-1) decrease to zero in tandem with the critical temperature and ordered moment, while the exponent beta remains constant. This novel trend distinguishes URu(2-x)Re(x)Si(2) from stoichiometric quantum critical ferromagnets and appears to reflect an underlying competition between Kondo and ferromagnetic interactions.

Kondo-cluster-glass state near a ferromagnetic quantum phase transition

We report on a comprehensive study of CePd(1-x)Rh(x) (0.6 <or= x <or= 0.95) poly- and single crystals close to the ferromagnetic instability by means of low-temperature ac susceptibility, magnetization, and volume thermal expansion. The signature of ferromagnetism in this heavy-fermion system can be traced from 6.6 K in CePd down to 25 mK for x = 0.87. Despite pronounced non-Fermi-liquid effects in both specific heat and thermal expansion, the Grüneisen ratio does not diverge as T --> 0, providing evidence for the absence of a quantum critical point. Instead, a peculiar "Kondo-cluster-glass" state is found for x >or= 0.65, and the non-Fermi-liquid effects in the specific heat, ac susceptibility, and magnetization are compatible with the quantum Griffiths phase scenario.

Logarithmic Fermi-liquid breakdown in NbFe2

The d-electron low temperature magnet NbFe2 is poised near the threshold of magnetism at ambient pressure, and can be tuned across the associated quantum critical point by adjusting the precise stoichiometry within the Nb1-yFe2+y homogeneity range. In a nearly critical single crystal (y= -0.01), we observe a T3/2 power-law dependence of the resistivity rho on temperature T and a logarithmic temperature dependence of the Sommerfeld coefficient gamma=C/T of the specific heat capacity C over nearly 2 orders of magnitude in temperature, extending down to 0.1 K.

Effect of fermi surface curvature on low-energy properties of fermions with singular interactions

We discuss the effect of Fermi surface curvature on long-distance or time asymptotic behaviors of two-dimensional fermions interacting via a gapless mode described by an effective gauge-field-like propagator. By comparing the predictions based on the idea of multidimensional bosonization with those of the strong-coupling Eliashberg approach, we demonstrate that an agreement between the two requires a further extension of the former technique.

Quantum correction to conductivity close to a ferromagnetic quantum critical point in two dimensions

We study the temperature dependence of the conductivity due to quantum interference processes for a two-dimensional disordered itinerant electron system close to a ferromagnetic quantum critical point. Near the quantum critical point, the crossover between diffusive and ballistic regimes of quantum interference effects occurs at a temperature T*=1/taugamma(E(F)tau)2, where gamma is the parameter associated with the Landau damping of the spin fluctuations, tau is the impurity scattering time, and E(F) is the Fermi energy. For a generic choice of parameters, T* is smaller than the nominal crossover scale 1/tau. In the ballistic quantum critical regime, the conductivity behaves as T1/3.