55 research outputs found
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 (SR). Short-range static magnetism occurs below the
freezing temperature Tg ~ 0.7 K, which prevents the system from accessing the
putative ferromagnetic quantum critical point. In the SR, 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.Comment: 5 pages, 4 figures, accepted for publication in Physical Review
Letters, Link:
http://prl.aps.org/accepted/6207bYdaGef1483c419928305372ce2d4419eb96
Cascade of magnetic field induced Lifshitz transitions in the ferromagnetic Kondo lattice material YbNi4P2
A ferromagnetic quantum critical point is thought not to exist in two and
three-dimensional metallic systems yet is realized in the Kondo lattice
compound YbNi4(P,As)2, possibly due to its one-dimensionality. It is crucial to
investigate the dimensionality of the Fermi surface of YbNi4P2 experimentally
but common probes such as ARPES and quantum oscillation measurements are
lacking. Here, we studied the magnetic field dependence of transport and
thermodynamic properties of YbNi4P2. The Kondo effect is continuously
suppressed and additionally we identify nine Lifshitz transitions between 0.4
and 18 T. We analyze the transport coefficients in detail and identify the type
of Lifshitz transitions as neck or void type to gain information on the Fermi
surface of YbNi4P2. The large number of Lifshitz transitions observed within
this small energy window is unprecedented and results from the particular flat
renormalized band structure with strong 4f-electron character shaped by the
Kondo lattice effect.Comment: 6 pages, 4 figure
Relationship between transport anisotropy and nematicity in FeSe
We thank the Max Planck Society for financial support. C. W. H., A. P. M., and C. T. acknowledge support by the DFG (DE) through the Collaborative Research Centre SFB 1143 (Projects C09 and A04). C. T. acknowledges support by the DFG (DE) through the Cluster of Excellence on Complexity and Topology in Quantum Matter ct.qmat (EXC 2147). Work in Japan was supported by Grants-in-Aid for Scientific Research (KAKENHI) (No. JP19H00649 and No. JP18H05227), and Grant-in-Aid for Scientific Research on innovative areas “Quantum Liquid Crystals” (No. JP19H05824 and No. JP20H05162) from Japan Society for the Promotion of Science (JSPS).The mechanism behind the nematicity of FeSe is not known. Through elastoresitivity measurements it has been shown to be an electronic instability. However, thus far measurements have extended only to small strains, where the response is linear. Here, we apply large elastic strains to FeSe and perform two types of measurement. (1) Using applied strain to control twinning, the nematic resistive anisotropy at temperatures below the nematic transition temperature Ts is determined. (2) Resistive anisotropy is measured as nematicity is induced through applied strain at fixed temperature above Ts. In both cases, as nematicity strengthens, the resistive anisotropy peaks at about 7%, then decreases. Below ≈40 K, the nematic resistive anisotropy changes sign. We discuss possible implications of this behavior for theories of nematicity. In addition, we report the following. (1) Under experimentally accessible conditions with bulk crystals, stress, rather than strain, is the conjugate field to the nematicity of FeSe. (2) At low temperatures the twin boundary resistance is ∼10% of the sample resistance, and must be properly subtracted to extract intrinsic resistivities. (3) Biaxial in-plane compression increases both in-plane resistivity and the superconducting critical temperature Tc, consistent with a strong role of the yz orbital in the electronic correlations.Publisher PDFPeer reviewe
Low temperature thermodynamic properties near the field-induced quantum critical point in DTN
We present a comprehensive experimental and theoretical investigation of the
thermodynamic properties: specific heat, magnetization and thermal expansion in
the vicinity of the field-induced quantum critical point (QCP) around the lower
critical field \,T in DTN . A behavior in the
specific heat and magnetization is observed at very low temperatures at
that is consistent with the universality class of Bose-Einstein
condensation of magnons. The temperature dependence of the thermal expansion
coefficient at shows minor deviations from the expected
behavior. Our experimental study is complemented by analytical calculations and
Quantum Monte Carlo simulations, which reproduce nicely the measured
quantities. We analyze the thermal and the magnetic Gr\"{u}neisen parameters
that are ideal quantities to identify QCPs. Both parameters diverge at
with the expected power law. By using the Ehrenfest relations at the
second order phase transition, we are able to estimate the pressure
dependencies of the characteristic temperature and field scales.Comment: 11 paged, 10 figures, submitted to PR
Intermediate magnetization state and competing orders in Dy2Ti2O7 and Ho2Ti2O7
We thank R. Moessner, C. Castelnovo and M. Gingras for helpful discussions, and the financial support of ANPCYT through PICT 2013-2004 and PICT 2014-2618 and CONICET (Argentina), the EPSRC and the Royal Society (UK).Among the frustrated magnetic materials, spin-ice stands out as a particularly interesting system. Residual entropy, freezing and glassiness, Kasteleyn transitions and fractionalization of excitations in three dimensions all stem from a simple classical Hamiltonian. But is the usual spin-ice Hamiltonian a correct description of the experimental systems? Here we address this issue by measuring magnetic susceptibility in the two most studied spin-ice compounds, Dy2Ti2O7 and Ho2Ti2O7, using a vector magnet. Using these results, and guided by a theoretical analysis of possible distortions to the pyrochlore lattice, we construct an effective Hamiltonian and explore it using Monte Carlo simulations. We show how this Hamiltonian reproduces the experimental results, including the formation of a phase of intermediate polarization, and gives important information about the possible ground state of real spin-ice systems. Our work suggests an unusual situation in which distortions might contribute to the preservation rather than relief of the effects of frustration.Publisher PDFPeer reviewe
Topological metamagnetism : thermodynamics and dynamics of the transition in spin ice under uniaxial compression
This work was carried out within the framework of a Max-Planck independent research group on strongly correlated systems. We acknowledge financial support from the Deutsche Forschungsgemeinschaft through SFB 1143 (Project No. 247310070) and Cluster of Excellence ct.qmat (EXC 2147, Project No. 390858490), EPSRC (EP/T028637/1), ShanghaiTech University, Agencia Nacional de Promoción Científica y Tecnológica through PICT 2017-2347, and Consejo Nacional de Investigaciones Científicas y Técnicas through PIP 0446.Metamagnetic transitions are analogs of a pressure-driven gas-liquid transition in water. In insulators, they are marked by a superlinear increase in the magnetization that occurs at a field strength set by the spin exchange interactions. Here we study topological metamagnets, in which the magnetization is itself a topological quantity and for which we find a single transition line for two materials with substantially different magnetic interactions: the spin ices Dy2Ti2O7 and Ho2Ti2O7. We study single crystals under magnetic field and stress applied along the [001] direction and show that this transition, of the Kasteleyn type, has a magnetization versus field curve with upward convexity and a distinctive asymmetric peak in the susceptibility. We also show that the dynamical response of Ho2Ti2O7 is sensitive to changes in the Ho3+ environment induced by compression along [001]. Uniaxial compression may open up experimental access to equilibrium properties of spin ice at lower temperatures.Publisher PDFPeer reviewe
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
Single-ion Kondo Scaling of the Coherent Fermi Liquid Regime in Ce1-xLaxNi2Ge2
Thermodynamic and transport properties of the La-diluted Kondo lattice
CeNi2Ge2 were studied in a wide temperature range. The Ce-rich alloys
Ce1-xLaxNi2Ge2 were found to exhibit distinct features of the coherent heavy
Fermi liquid. At intermediate compositions (0.7 <= x <= 0.9) non-Fermi liquid
properties have been observed, followed by the local Fermi liquid behavior in
the dilute limit. The 4f-electron contribution to the specific heat was found
to follow the predictions of the Kondo impurity model both in the local as well
as coherent regimes, with the characteristic Kondo temperature decreasing
rapidly from about 30 K for the parent compound CeNi2Ge2 to about 1K in the
most dilute samples. The specific heat does not show any evidence for the
emergence of a new characteristic energy scale related to the formation of the
coherent Kondo lattice.Comment: to appear in Physical Review Letter
Pair-breaking quantum phase transition in superconducting nanowires
A quantum phase transition (QPT) between distinct ground states of matter is
a wide-spread phenomenon in nature, yet there are only a few experimentally
accessible systems where the microscopic mechanism of the transition can be
tested and understood. These cases are unique and form the experimentally
established foundation for our understanding of quantum critical phenomena.
Here we report the discovery that a magnetic-field-driven QPT in
superconducting nanowires - a prototypical 1d-system - can be fully explained
by the critical theory of pair-breaking transitions characterized by a
correlation length exponent and dynamic critical exponent . We find that in the quantum critical regime, the electrical
conductivity is in agreement with a theoretically predicted scaling function
and, moreover, that the theory quantitatively describes the dependence of
conductivity on the critical temperature, field magnitude and orientation,
nanowire cross sectional area, and microscopic parameters of the nanowire
material. At the critical field, the conductivity follows a
dependence predicted by phenomenological scaling theories and more recently
obtained within a holographic framework. Our work uncovers the microscopic
processes governing the transition: The pair-breaking effect of the magnetic
field on interacting Cooper pairs overdamped by their coupling to electronic
degrees of freedom. It also reveals the universal character of continuous
quantum phase transitions.Comment: 22 pages, 5 figure
Ferromagnetic Quantum Criticality in the Quasi-One-Dimensional Heavy Fermion Metal YbNi4P2
We present a new Kondo-lattice system, YbNi4P2, which is a clean
heavy-fermion metal with a severely reduced ferromagnetic ordering temperature
at T_C=0.17K, evidenced by distinct anomalies in susceptibility, specific-heat,
and resistivity measurements. The ferromagnetic nature of the transition, with
only a small ordered moment of ~0.05mu_B, is established by a diverging
susceptibility at T_C with huge absolute values in the ferromagnetically
ordered state, severely reduced by small magnetic fields. Furthermore, YbNi4P2
is a stoichiometric system with a quasi-one-dimensional crystal and electronic
structure and strong correlation effects which dominate the low temperature
properties. This is reflected by a stronger-than-logarithmically diverging
Sommerfeld coefficient and a linear-in-T resistivity above T_C which cannot be
explained by any current theoretical predictions. These exciting
characteristics are unique among all correlated electron systems and make this
an interesting material for further in-depth investigations.Comment: 14 pages, 6 figure
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