627 research outputs found
Break up of heavy fermions at an antiferromagnetic instability
We present results of high-resolution, low-temperature measurements of the
Hall coefficient, thermopower, and specific heat on stoichiometric YbRh2Si2.
They support earlier conclusions of an electronic (Kondo-breakdown) quantum
critical point concurring with a field induced antiferromagnetic one. We also
discuss the detachment of the two instabilities under chemical pressure. Volume
compression/expansion (via substituting Rh by Co/Ir) results in a
stabilization/weakening of magnetic order. Moderate Ir substitution leads to a
non-Fermi-liquid phase, in which the magnetic moments are neither ordered nor
screened by the Kondo effect. The so-derived zero-temperature global phase
diagram promises future studies to explore the nature of the Kondo breakdown
quantum critical point without any interfering magnetism.Comment: minor changes, accepted for publication in JPS
Incoherent transport across the strange metal regime of highly overdoped cuprates
Strange metals possess highly unconventional transport characteristics, such
as a linear-in-temperature () resistivity, an inverse Hall angle that varies
as and a linear-in-field () magnetoresistance. Identifying the origin
of these collective anomalies has proved profoundly challenging, even in
materials such as the hole-doped cuprates that possess a simple band structure.
The prevailing dogma is that strange metallicity in the cuprates is tied to a
quantum critical point at a doping inside the superconducting dome. Here,
we study the high-field in-plane magnetoresistance of two superconducting
cuprate families at doping levels beyond . At all dopings, the
magnetoresistance exhibits quadrature scaling and becomes linear at high
ratios. Moreover, its magnitude is found to be much larger than predicted by
conventional theory and insensitive to both impurity scattering and magnetic
field orientation. These observations, coupled with analysis of the zero-field
and Hall resistivities, suggest that despite having a single band, the cuprate
strange metal phase hosts two charge sectors, one containing coherent
quasiparticles, the other scale-invariant `Planckian' dissipators.Comment: 15 pages plus 7 figures (including Supplementary Information
Investigation of superstorm Sandy 2012 in a multi-disciplinary approach
At the end of October 2012, Hurricane Sandy moved from the Caribbean Sea into the Atlantic Ocean and entered the United States not far from New York. Along its track, Sandy caused more than 200 fatalities and severe losses in Jamaica, The Bahamas, Haiti, Cuba, and the US. This paper demonstrates the capability and potential for near-real-time analysis of catastrophes. It is shown that the impact of Sandy was driven by the superposition of different extremes (high wind speeds, storm surge, heavy precipitation) and by cascading effects. In particular the interaction between Sandy and an extra-tropical weather system created a huge storm that affected large areas in the US. It is examined how Sandy compares to historic hurricane events, both from a hydro-meteorological and impact perspective. The distribution of losses to different sectors of the economy is calculated with simple input-output models as well as government estimates. Direct economic losses are estimated about USD 4.2 billion in the Caribbean and between USD 78 and 97 billion in the US. Indirect economic losses from power outages is estimated in the order of USD 16.3 billion. Modelling sector-specific dependencies quantifies total business interruption losses between USD 10.8 and 15.5 billion. Thus, seven years after the record impact of Hurricane Katrina in 2005, Hurricane Sandy is the second costliest hurricane in the history of the United States
Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCuSi versus YbRhSi
In this paper the low-temperature properties of two isostructural canonical
heavy-fermion compounds are contrasted with regards to the interplay between
antiferromagnetic (AF) quantum criticality and superconductivity. For
CeCuSi, fully-gapped d-wave superconductivity forms in the vicinity of
an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum
critical point (QCP). Inelastic neutron scattering results highlight that both
quantum critical SDW fluctuations as well as Mott-type fluctuations of local
magnetic moments contribute to the formation of Cooper pairs in CeCuSi.
In YbRhSi, superconductivity appears to be suppressed at
mK by AF order ( = 70 mK). Ultra-low temperature measurements reveal a
hybrid order between nuclear and 4f-electronic spins, which is dominated by the
Yb-derived nuclear spins, to develop at slightly above 2 mK. The hybrid
order turns out to strongly compete with the primary 4f-electronic order and to
push the material towards its QCP. Apparently, this paves the way for
heavy-fermion superconductivity to form at = 2 mK. Like the pressure -
induced QCP in CeRhIn, the magnetic field - induced one in YbRhSi
is of the local Kondo-destroying variety which corresponds to a Mott-type
transition at zero temperature. Therefore, these materials form the link
between the large family of about fifty low- unconventional heavy - fermion
superconductors and other families of unconventional superconductors with
higher s, notably the doped Mott insulators of the cuprates, organic
charge-transfer salts and some of the Fe-based superconductors. Our study
suggests that heavy-fermion superconductivity near an AF QCP is a robust
phenomenon.Comment: 30 pages, 7 Figures, Accepted for publication in Philosophical
Magazin
Frustration and the Kondo effect in heavy fermion materials
The observation of a separation between the antiferromagnetic phase boundary
and the small-large Fermi surface transition in recent experiments has led to
the proposal that frustration is an important additional tuning parameter in
the Kondo lattice model of heavy fermion materials. The introduction of a Kondo
(K) and a frustration (Q) axis into the phase diagram permits us to discuss the
physics of heavy fermion materials in a broader perspective. The current
experimental situation is analysed in the context of this combined "QK" phase
diagram. We discuss various theoretical models for the frustrated Kondo
lattice, using general arguments to characterize the nature of the -electron
localization transition that occurs between the spin liquid and heavy Fermi
liquid ground-states. We concentrate in particular on the Shastry--Sutherland
Kondo lattice model, for which we establish the qualitative phase diagram using
strong coupling arguments and the large- expansion. The paper closes with
some brief remarks on promising future theoretical directions.Comment: To appear in a special issue of JLT
Behavior of the Quantum Critical Point and the Fermi-liquid Domain in the Heavy Fermion Superconductor CeCoIn5 studied by resistivity
We report detailed very low temperature resistivity measurements on the heavy
fermion compounds Ce_{1-x}La_{x}CoIn5 (x=0 and x=0.01), with current applied in
two crystallographic directions [100] (basal plane) and [001] (perpendicular to
the basal plane) under magnetic field applied in the [001] or [011] direction.
We found a Fermi liquid (\rho \propto T^{2}) ground state, in all cases, for
fields above the superconducting upper critical field. We discuss the possible
location of a field induced quantum critical point with respect to Hc2(0), and
compare our measurements with the previous reports in order to give a clear
picture of the experimental status on this long debated issue.Comment: 17 pages, 7 figures accepted for publication in JPS
Global Phase Diagram of the Kondo Lattice: From Heavy Fermion Metals to Kondo Insulators
We discuss the general theoretical arguments advanced earlier for the T=0
global phase diagram of antiferromagnetic Kondo lattice systems, distinguishing
between the established and the conjectured. In addition to the well-known
phase of a paramagnetic metal with a "large" Fermi surface (P_L), there is also
an antiferromagnetic phase with a "small" Fermi surface (AF_S). We provide the
details of the derivation of a quantum non-linear sigma-model (QNLsM)
representation of the Kondo lattice Hamiltonian, which leads to an effective
field theory containing both low-energy fermions in the vicinity of a Fermi
surface and low-energy bosons near zero momentum. An asymptotically exact
analysis of this effective field theory is made possible through the
development of a renormalization group procedure for mixed fermion-boson
systems. Considerations on how to connect the AF_S and P_L phases lead to a
global phase diagram, which not only puts into perspective the theory of local
quantum criticality for antiferromagnetic heavy fermion metals, but also
provides the basis to understand the surprising recent experiments in
chemically-doped as well as pressurized YbRh2Si2. We point out that the AF_S
phase still occurs for the case of an equal number of spin-1/2 local moments
and conduction electrons. This observation raises the prospect for a global
phase diagram of heavy fermion systems in the Kondo-insulator regime. Finally,
we discuss the connection between the Kondo breakdown physics discussed here
for the Kondo lattice systems and the non-Fermi liquid behavior recently
studied from a holographic perspective.Comment: (v3) leftover typos corrected. (v2) Published version. 32 pages, 4
figures. Section 7, on the connection between the Kondo lattice systems and
the holographic models of non-Fermi liquid, is expanded. (v1) special issue
of JLTP on quantum criticalit
Separation of energy scales in undoped YbRhSi under hydrostatic pressure
The temperature ()-magnetic field () phase diagram of YbRhSi in
the vicinity of its quantum critical point is investigated by low-
magnetization measurements. Our analysis reveals that the energy scale
, previously related to the Kondo breakdown and terminating at 0.06
T for , remains unchanged under pressure, whereas the antiferromagnetic
critical field increases from 0.06 T () to 0.29 T ( GPa),
resulting in a crossing of and . Our results are very
similar to those on Yb(RhCo)Si, proving that the Co-induced
disorder can not be the reason for the detachment of both scales under chemical
pressure
Quasiparticles of strongly correlated Fermi liquids at high temperatures and in high magnetic fields
Strongly correlated Fermi systems are among the most intriguing, best
experimentally studied and fundamental systems in physics. There is, however,
lack of theoretical understanding in this field of physics. The ideas based on
the concepts like Kondo lattice and involving quantum and thermal fluctuations
at a quantum critical point have been used to explain the unusual physics.
Alas, being suggested to describe one property, these approaches fail to
explain the others. This means a real crisis in theory suggesting that there is
a hidden fundamental law of nature. It turns out that the hidden fundamental
law is well forgotten old one directly related to the Landau---Migdal
quasiparticles, while the basic properties and the scaling behavior of the
strongly correlated systems can be described within the framework of the
fermion condensation quantum phase transition (FCQPT). The phase transition
comprises the extended quasiparticle paradigm that allows us to explain the
non-Fermi liquid (NFL) behavior observed in these systems. In contrast to the
Landau paradigm stating that the quasiparticle effective mass is a constant,
the effective mass of new quasiparticles strongly depends on temperature,
magnetic field, pressure, and other parameters. Our observations are in good
agreement with experimental facts and show that FCQPT is responsible for the
observed NFL behavior and quasiparticles survive both high temperatures and
high magnetic fields.Comment: 17 pages, 17 figures. Dedicated to 100th anniversary of A.B.Migdal
birthda
Orbital-selective Mott transitions: Heavy fermions and beyond
Quantum phase transitions in metals are often accompanied by violations of
Fermi liquid behavior in the quantum critical regime. Particularly fascinating
are transitions beyond the Landau-Ginzburg-Wilson concept of a local order
parameter. The breakdown of the Kondo effect in heavy-fermion metals
constitutes a prime example of such a transition. Here, the strongly correlated
f electrons become localized and disappear from the Fermi surface, implying
that the transition is equivalent to an orbital-selective Mott transition, as
has been discussed for multi-band transition-metal oxides. In this article,
available theoretical descriptions for orbital-selective Mott transitions will
be reviewed, with an emphasis on conceptual aspects like the distinction
between different low-temperature phases and the structure of the global phase
diagram. Selected results for quantum critical properties will be listed as
well. Finally, a brief overview is given on experiments which have been
interpreted in terms of orbital-selective Mott physics.Comment: 29 pages, 4 figs, mini-review prepared for a special issue of JLT
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