arXiv subjects

F. Ronning

Publications and source records attributed to F. Ronning.

At least 163 records · Page 9Linked to original sources

arXiv2006

Magnetic Excitations of the 2-D Sm Spin Layers in Sm(La,Sr)CuO4

We present specific heat and susceptibility data on Sm(La,Sr)CuO4 in magnetic fields up to 9 T and temperatures down to 100 mK. We find a broad peak in specific heat which is insensitive to magnetic field at a temperature of 1.5 K with a value of 2.65 J/mol K. The magnetic susceptibility at 5 T continues to increase down to 2 K, the lowest temperature measured. The data suggest that the Sm spin system may be an ideal realization of the frustrated Heisenberg antiferromagnet on the square lattice.

cond-mat.str-el↗

arXiv2006

Hidden Magnetism and Quantum Criticality in the Heavy Fermion Superconductor CeRhIn5

With understood exceptions, conventional superconductivity does not coexist with long-range magnetic order[1]. In contrast, unconventional superconductivity develops near a boundary separating magnetically ordered and magnetically disordered phases[2,3]. A maximum in the superconducting transition temperature Tc develops where this boundary extrapolates to T=0 K, suggesting that fluctuations associated with this magnetic quantum-critical point are essential for unconventional superconductivity[4,5]. Invariably though, unconventional superconductivity hides the magnetic boundary when T < Tc, preventing proof of a magnetic quantum-critical point[5]. Here we report specific heat measurements of the pressure-tuned unconventional superconductor CeRhIn5 in which we find a line of quantum-phase transitions induced inside the superconducting state by an applied magnetic field. This quantum-critical line separates a phase of coexisting antiferromagnetism and superconductivity from a purely unconventional superconducting phase and terminates at a quantum tetracritical point where the magnetic field completely suppresses superconductivity. The T->0 K magnetic field-pressure phase diagram of CeRhIn5 is well described with a theoretical model[6,7] developed to explain field-induced magnetism in the high-Tc cuprates but in which a clear delineation of quantum-phase boundaries has not been possible. These experiments establish a common relationship among hidden magnetism, quantum criticality and unconventional superconductivity in cuprate and heavy-electron systems, such as CeRhIn5.

cond-mat.supr-con↗

arXiv2006

Anomalous high energy dispersion in photoemission spectra from insulating cuprates

Angle resolved photoelectron spectroscopic measurements have been performed on an insulating cuprate Ca_2CuO_2Cl_2. High resolution data taken along the Γto (pi,pi) cut show an additional dispersive feature that merges with the known dispersion of the lowest binding energy feature, which follows the usual strongly renormalized dispersion of ~0.35 eV. This higher energy part reveals a dispersion that is very close to the unrenormalized band predicted by band theory. A transfer of spectral weight from the low energy feature to the high energy feature is observed as the Γpoint is approached. By comparing with theoretical calculations the high energy feature observed here demonstrates that the incoherent portion of the spectral function has significant structure in momentum space due to the presence of various energy scales.

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arXiv2006

Pressure Study of Quantum Criticality in CeCoIn5

We report resistivity measurements in the normal state of CeCoIn5 down to 40 mK and simultaneously in magnetic fields up to 9 T in the [001] crystallographic direction and under pressures up to 1.3 GPa. At ambient pressure the data are consistent with a field tuned quantum critical point coincident with the superconducting upper critical field H_{c2}, as observed previously. We find that with increasing pressure the quantum critical point moves inside the superconducting dome to lower fields. Thus, we can rule out that superconductivity is directly responsible for the non-Fermi liquid behavior in CeCoIn5. Instead, the data point toward an antiferromagnetic quantum critical point scenario.

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arXiv2005

Unpaired Electrons in the Heavy-Fermion Superconductor CeCoIn_{5}

Thermal conductivity and specific heat were measured in the superconducting state of the heavy fermion material Ce_{1-x}La_{x}CoIn_{5}. With increasing impurity concentration x, the suppression of T_{c} is accompanied by the increase in the residual electronic specific heat expected of a d-wave superconductor, but it occurs in parallel with a decrease in residual electronic thermal conductivity. This contrasting behavior reveals the presence of uncondensed electrons coexisting with nodal quasiparticles. An extreme multiband scenario is proposed, with a d-wave superconducting gap on the heavy-electron sheets of the Fermi surface and a negligible gap on the light, three-dimensional pockets.

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arXiv2005

Heat Transport as a Probe of Electron Scattering by Spin Fluctuations: the Case of Antiferromagnetic CeRhIn5

Heat and charge conduction were measured in the heavy-fermion metal CeRhIn5, an antiferromagnet with T_N=3.8 K. The thermal resistivity is found to be proportional to the magnetic entropy, revealing that spin fluctuations are as effective in scattering electrons as they are in disordering local moments. The electrical resistivity, governed by a q^2 weighting of fluctuations, increases monotonically with temperature. In contrast, the difference between thermal and electrical resistivities, characterized by an omega^2 weighting, peaks sharply at T_N and eventually goes to zero at a temperature T^* ~ 8 K. T^* thus emerges as a measure of the characteristic energy of magnetic fluctuations.

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arXiv2005

Delocalized fermions in underdoped cuprates

Low temperature heat transport was used to investigate the ground state of high-purity single crystals of the lightly-doped cuprate YBa$_{2}$Cu$_{3}$O$_{6.33}$. Samples were measured on either side of the superconducting phase boundary, in both zero and applied magnetic field. We report the observation of delocalized fermionic excitations at zero energy in the non-superconducting state, which shows that the ground state of underdoped cuprates is metallic. Its low-energy spectrum appears to be similar to that of the d-wave superconductor, i.e. nodal. The insulating ground state observed in underdoped La$_{2-x}$Sr$_{x}$CuO$_4$ is attributed to the competing spin-density-wave order present in that system.

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arXiv2005

c-axis magnetotransport in CeCoIn$_{5}$

We present the results of out-of-plane electrical transport measurements on the heavy fermion superconductor CeCoIn$_{5}$ at temperatures from 40 mK to 400 K and in magnetic field up to 9 T. For $T <$ 10 K transport measurements show that the zero-field resistivity $ρ_{c}$ changes linearly with temperature and extrapolates nearly to zero at 0 K, indicative of non-Fermi-liquid (nFL) behavior associated with a quantum critical point (QCP). The longitudinal magnetoresistance (LMR) of CeCoIn$_{5}$ for fields applied parallel to the c-axis is negative and scales as $B/(T+T^{*})$ between 50 and 100 K, revealing the presence of a single-impurity Kondo energy scale $T^{*} \sim 2$ K. Beginning at 16 K a small positive LMR feature is evident for fields less than 3 tesla that grows in magnitude with decreasing temperature. For higher fields the LMR is negative and increases in magnitude with decreasing temperature. This sizable negative magnetoresistance scales as $B{^2}/T$ from 2.6 K to roughly 8 K, and it arises from an extrapolated residual resistivity that becomes negative and grows quadratically with field in the nFL temperature regime. Applying a magnetic field along the c-axis with B $>$ B$_{c2}$ restores Fermi-liquid behavior in $ρ_{c}(T)$ at $T$ less than 130 mK. Analysis of the $T{^2}$ resistivity coefficient's field-dependence suggests that the QCP in CeCoIn$_{5}$ is located \emph{below} the upper critical field, inside the superconducting phase. These data indicate that while high-$T$ c-axis transport of CeCoIn$_{5}$ exhibits features typical for a heavy fermion system, low-$T$ transport is governed both by spin fluctuations associated with the QCP and Kondo interactions that are influenced by the underlying complex electronic structure intrinsic to the anisotropic CeCoIn$_{5}$ crystal structure.

cond-mat.str-el↗

arXiv2004

Field-Tuned Quantum Critical Point in CeCoIn$_5$ Near the Superconducting Upper Critical Field

We report a systematic study of high magnetic field specific heat and resistivity in single crystals of CeCoIn_5 for the field oriented in the basal plane (H//ab) of this tetragonal heavy fermion superconductor. We observe a divergent electronic specific heat as well as an enhanced A coefficient of the T^{2} law in resistivity at the lowest temperatures, as the field approaches the upper critical field of the superconducting transition. Together with the results for field along the tetragonal axis (H//c), the emergent picture is that of a magnetic field tuned quantum critical point which exists in the vicinity of the superconducting H_{c2} despite a variation of a factor of 2.4 in H_{c2} for different field orientations. This suggests an underlying physical reason exists for the superconducting H_{c2} to coincide with the quantum critical field. Moreover, we show that the recovery of a Fermi Liquid ground state with increasing magnetic field is more gradual, meaning that the fluctuations responsible for the observed quantum critical phenomena are more robust with respect to magnetic field, when the magnetic field is applied in-plane. Together with the close proximity of the quantum critical point and H_{c2} in CeCoIn_5 for both field orientation, the anisotropy in the recovery of the Fermi liquid state might constitute an important piece of information in identifying the nature of the fluctuations that become critical.

cond-mat.str-el↗

arXiv2004

Superconductivity in CeCoIn5-xSnx: Veil Over an Ordered State or Novel Quantum Critical Point?

Measurements of specific heat and electrical resistivity in magnetic fields up to 9 T along [001] and temperatures down to 50 mK of Sn-substituted CeCoIn5 are reported. The maximal -ln(T) divergence of the specific heat at the upper critical field H_{c2} down to the lowest temperature characteristic of non-Fermi liquid systems at the quantum critical point (QCP), the universal scaling of the Sommerfeld coefficient, and agreement of the data with spin-fluctuation theory, provide strong evidence for quantum criticality at H_{c2} for all x < 0.12 in CeCoIn5-xSnx. These results indicate the "accidental" coincidence of the QCP located near H_{c2} in pure CeCoIn5, in actuality, constitute a novel quantum critical point associated with unconventional superconductivity.

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arXiv2004

Missing Quasiparticles and the Chemical Potential Puzzle in the Doping Evolution of the Cuprate Superconductors

The evolution of Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ from Mott insulator to superconductor was studied using angle-resolved photoemission spectroscopy. By measuring both the excitations near the Fermi energy as well as non-bonding states, we tracked the doping dependence of the electronic structure and the chemical potential with unprecedented precision. Our work reveals failures in the conventional quasiparticle theory, including the broad lineshapes of the insulator and the apparently paradoxical shift of the chemical potential within the Mott gap. To resolve this, we develop a model where the quasiparticle is vanishingly small at half filling and grows upon doping, allowing us to unify properties such as the dispersion and Fermi wavevector with the behavior of the chemical potential.

cond-mat.supr-con↗

arXiv2004

Non-Fermi Liquid behavior in CeIrIn$_5$ near a metamagnetic transition

We present specific heat and resistivity study of CeIrIn5 in magnetic fields up to 17 T and temperature down to 50 mK. Both quantities were measured with the magnetic field parallel to the c-axis (H || [001]) and within the a-b plane (H \perp [001]). Non-Fermi-liquid (NFL) behavior develops above 12 T for H || [001]. The Fermi liquid state is much more robust for H \perp [001] and is suppressed only moderately at the highest applied field. Based on the observed trends and the proximity to a metamagnetic phase transition, which exists at fields above 25 T for H || [001], we suggest that the observed NFL behavior in CeIrIn5 is a consequence of a metamagnetic quantum critical point.

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arXiv2003

Field-Induced Quantum Critical Point in CeCoIn5

The resistivity of the heavy-fermion superconductor CeCoIn5 was measured as a function of temperature, down to 25 mK and in magnetic fields of up to 16 T applied perpendicular to the basal plane. With increasing field, we observe a suppression of the non-Fermi liquid behavior, rho ~ T, and the development of a Fermi liquid state, with its characteristic rho = rho_0 + AT^2 dependence. The field dependence of the T^2 coefficient shows critical behavior with an exponent of 1.37. This is evidence for a field-induced quantum critical point (QCP), occuring at a critical field which coincides, within experimental accuracy, with the superconducting critical field H_c2. We discuss the relation of this field-tuned QCP to a change in the magnetic state, seen as a change in magnetoresistance from positive to negative, at a crossover line that has a common border with the superconducting region below ~ 1 K.

cond-mat.supr-con↗

arXiv2003

Fully Gapped Single-Particle Excitations in the Lightly Doped Cuprates

The low-energy excitations of the lightly doped cuprates were studied by angle-resolved photoemission spectroscopy. A finite gap was measured over the entire Brillouin zone, including along the d_{x^2 - y^2} nodal line. This effect was observed to be generic to the normal states of numerous cuprates, including hole-doped La_{2-x}Sr_{x}CuO_{4} and Ca_{2-x}Na_{x}CuO_{2}Cl_{2} and electron-doped Nd_{2-x}Ce_{x}CuO_{4}. In all compounds, the gap appears to close with increasing carrier doping. We consider various scenarios to explain our results, including the possible effects of chemical disorder, electronic inhomogeneity, and a competing phase.

cond-mat.supr-con↗

arXiv2003

An angle-resolved photoemission spectral function analysis of the electron doped cuprate Nd_1.85Ce_0.15CuO_4

Using methods made possible by recent advances in photoemission technology, we perform an indepth line-shape analysis of the angle-resolved photoemission spectra of the electron doped (n-type) cuprate superconductor Nd_1.85Ce_0.15CuO_4. Unlike for the p-type materials, we only observe weak mass renormalizations near 50-70 meV. This may be indicative of smaller electron-phonon coupling or due to the masking effects of other interactions that make the electron-phonon coupling harder to detect. This latter scenario may suggest limitations of the spectral function analysis in extracting electronic self-energies when some of the interactions are highly momentum dependent.

cond-mat.supr-con↗

arXiv2003

Transport in Ultraclean YBa$_2$Cu$_3$O$_7$: neither Unitary nor Born Impurity Scattering

The thermal conductivity of ultraclean YBa$_2$Cu$_3$O$_7$ was measured at very low temperature in magnetic fields up to 13 T. The temperature and field dependence of the electronic heat conductivity show that two widespread assumptions of transport theory applied to unconventional superconductors fail for clean cuprates: impurity scattering cannot be treated in the usual unitary limit (nor indeed in the Born limit), and scattering of quasiparticles off vortices cannot be neglected. Our study also sheds light on the long-standing puzzle of a sudden onset of a "plateau" in the thermal conductivity of Bi-2212 versus field.

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arXiv2003

Field-Induced Thermal Metal-to-Insulator Transition in Underdoped LSCO

The transport of heat and charge in cuprates was measured in single crystals of La_{2-x}Sr_xCuO_{4+δ} (LSCO) across the doping phase diagram at low temperatures. In underdoped LSCO, the thermal conductivity is found to decrease with increasing magnetic field in the T goes to 0 limit, in striking contrast to the increase observed in all superconductors, including cuprates at higher doping. In heavily underdoped LSCO, where superconductivity can be entirely suppressed with an applied field, we show that a novel thermal metal-to-insulator transition takes place upon going from the superconducting state to the field-induced normal state.

cond-mat.supr-con↗

arXiv2003

Field-induced thermal metal-to-insulator transition in underdoped LSCO

The transport of heat and charge in cuprates was measured in undoped and heavily-underdoped single crystal La_{2-x}Sr_xCuO_{4+delta} (LSCO). In underdoped LSCO, the thermal conductivity is found to decrease with increasing magnetic field in the T --> 0 limit, in striking contrast to the increase observed in all superconductors, including cuprates at higher doping. The suppression of superconductivity with magnetic field shows that a novel thermal metal-to-insulator transition occurs upon going from the superconducting state to the field-induced normal state.

cond-mat.supr-con↗