arXiv subjects

H. A. Mook

Publications and source records attributed to H. A. Mook.

At least 19 recordsLinked to original sources

arXiv2013

Doping dependence of the spin excitations in Fe-based superconductors Fe1+yTe1-xSex

The Fe1+yTe1-xSex series of materials is one of the prototype families of Fe-based superconductors. To provide further insight into these materials we present systematic inelastic neutron scattering measurements of the low energy spin excitations for x=0.27, 0.36, 0.40, 0.49. These measurements show an evolution of incommensurate spin excitations towards the (1/2 1/2 0) wave vector with doping. Concentrations (x=0.40 and 0.49) which exhibit the most robust superconducting properties have spin excitations closest to (1/2 1/2 0) and also exhibit a strong spin resonance in the spin excitation spectrum below Tc. The resonance signal appears to be closer to (1/2 1/2 0) than the underlying spin excitations. We discuss the possible relationship between superconductivity and spin excitations at the (1/2 1/2 0) wave vector and the role that interstitial Fe may play.

cond-mat.supr-con↗

arXiv2012

The complex multiferroic phase diagram of Mn$_{1-x}$Co$_x$WO$_4$

The complete magnetic and multiferroic phase diagram of Mn$_{1-x}$Co$_{x}$WO$_4$ single crystals is investigated by means of magnetic, heat capacity, and polarization experiments. We show that the ferroelectric polarization $\overrightarrow{P}$ in the multiferroic state abruptly changes its direction twice upon increasing Co content, x. At x$_{c1}$=0.075, $\overrightarrow{P}$ rotates from the $b-$axis into the $a-c$ plane and at x$_{c2}$=0.15 it flips back to the $b-$axis. The origin of the multiple polarization flops is identified as an effect of the Co anisotropy on the orientation and shape of the spin helix leading to thermodynamic instabilities caused by the decrease of the magnitude of the polarization in the corresponding phases. A qualitative description of the ferroelectric polarization is derived by taking into account the intrachain ($c-$axis) as well as the interchain ($a-$axis) exchange pathways connecting the magnetic ions. In a narrow Co concentration range (0.1$\leq$x$\leq$0.15), an intermediate phase, sandwiched between the collinear high-temperature and the helical low-temperature phases, is discovered. The new phase exhibits a collinear and commensurate spin modulation similar to the low-temperature magnetic structure of MnWO$_4$.

cond-mat.str-el↗

arXiv2011

A pseudogap term in the magnetic response of the cuprate superconductors

We combine neutron scattering (INS) and NMR/NQR nuclear spin lattice relaxation rate data to deduce the existence of a new contribution to the magnetic response (dynamic susceptibility) in cuprate superconductors. This contribution, which has yet to be observed with INS, is shown to embody the magnetic pseudogap effects. As such, it explains the long-standing puzzle of pseudogap effects missing from cuprate INS data, dominated by stripe fluctuations, for the dynamic susceptibility at low energies. For La_(1.86)Sr_(0.14)CuO_(4) and YBa_(2)Ba_(3)O_(6.5), the new term is the chief contributor to nuclear spin lattice relaxation at T >> T_(c).

cond-mat.supr-con↗

arXiv2011

Robust Ferroelectric State in Multiferroic Mn$_{1-x}$Zn$_x$WO$_4$

We report the remarkably robust ferroelectric state in the multiferroic compound Mn$_{1-x}$Zn$_x$WO$_4$. The substitution of the magnetic Mn$^{2+}$ with nonmagnetic Zn$^{2+}$ reduces the magnetic exchange and provides control of the various magnetic and multiferroic states of MnWO$_4$. Only 5 % of Zn substitution results in a complete suppression of the frustrated collinear (paraelectric) low temperature phase. The helical magnetic and ferroelectric phase develops as the ground state. The multiferroic state is stable up to a high level of substitution of more than 50 %. The magnetic, thermodynamic, and dielectric properties as well as the ferroelectric polarization of single crystals of Mn$_{1-x}$Zn$_x$WO$_4$ are studied for different substitutions up to x=0.5. The magnetic phases have been identified in single crystal neutron scattering experiments. The ferroelectric polarization scales with the neutron intensity of the incommensurate peak of the helical phase.

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arXiv2010

Magnetic and multiferroic phases of single-crystalline Mn$_{0.85}$Co$_{0.15}$WO$_4$

The magnetic and multiferroic phase diagram of Mn$_{0.85}$Co$_{0.15}$WO$_4$ single crystals is investigated by means of magnetic, heat capacity, dielectric, polarization, and neutron scattering experiments. Three magnetic phase transitions are detected through distinct anomalies in all physical quantities. The ferroelectric polarization is observed only along the b-axis below 10 K but not along the a-axis as recently suggested. The magnetic phases studied by neutron scattering are very complex. Up to four different magnetic structures, partially coexisting at certain temperature ranges, have been identified. Upon decreasing temperature two commensurate phases (AF4, AF1) are followed by an incommensurate phase (AF5) and a second incommensurate phase (AF2) is detected as a minor phase. The ferroelectric polarization is possibly associated with both (AF2 and AF5) phases.

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arXiv2010

Direct observation of paramagnons in palladium

We report an inelastic neutron scattering study of the spin fluctuations in the nearly-ferromagnetic element palladium. Dispersive over-damped collective magnetic excitations or ``paramagnons'' are observed up to 128 meV. We analyze our results in terms of a Moriya-Lonzarich-type spin fluctuation model and estimate the contribution of the spin fluctuations to the low temperature heat capacity. In spite of the paramagnon excitations being relatively strong, their relaxation rates are large. This leads to a small contribution to the low-temperature electronic specific heat.

cond-mat.str-el↗

arXiv2010

Detection of the Unusual Magnetic Orders in the Pseudogap Region of a High-Temperature Superconducting YBa2Cu3O6.6 Crystal by Muon-Spin Relaxation

We present muon spin relaxation (muSR) measurements on a large YBa2Cu3O6.6 single crystal in which two kinds of unusual magnetic order have been detected in the pseudogap region by neutron scattering. A comparison is made to measurements on smaller, higher quality YBa2Cu3Oy single crystals. One type of magnetic order is observed in all samples, but does not evolve significantly with hole doping. A second type of unusual magnetic order is observed only in the YBa2Cu3O6.6 single crystal. This magnetism has an ordered magnetic moment that is quantitatively consistent with the neutron experiments, but is confined to just a small volume of the sample (~ 3%). Our findings do not support theories that ascribe the pseudogap to a state characterized by loop-current order, but instead indicate that dilute impurity phases are the source of the unusual magnetic orders in YBa2Cu3Oy.

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arXiv2009

Unusual Relationship between Magnetism and Superconductivity in FeTe$_{0.5}$Se$_{0.5}$

We use neutron scattering, to study magnetic excitations in crystals near the ideal superconducting composition of FeTe$_{0.5}$Se$_{0.5}$. Two types of excitations are found, a resonance at (0.5, 0.5, 0) and incommensurate fluctuations on either side of this position. We show that the two sets of magnetic excitations behave differently with doping, with the resonance being fixed in position while the incommensurate excitations move as the doping is changed. These unusual results show that a common behavior of the low energy magnetic excitations is not necessary for pairing in these materials.

cond-mat.supr-con↗

arXiv2009

Lattice distortion and magnetic quantum phase transition in CeFeAs(1-x)P(x)O

We use neutron scattering to show that replacing the larger arsenic with smaller phosphorus in CeFeAs(1-x)P(x)O simultaneously suppresses the AF order and orthorhombic distortion near x = 0.4, providing evidence for a magnetic quantum critical point. Furthermore, we find that the pnictogen height in iron arsenide is an important controlling parameter for their electronic and magnetic properties, and may play an important role in electron pairing and superconductivity.

cond-mat.supr-con↗

arXiv2009

Evolution of spin excitations into the superconducting state in FeTe1-xSex

The nature of the superconducting state in the recently discovered Fe-based superconductors1-3 is the subject of intense scrutiny. Neutron scattering investigations have already elucidated a strong correlation between magnetism and superconductivity in the form of a spin resonance in the magnetic excitation spectrum4-7. A central unanswered question concerns the nature of the normal state spin fluctuations which may be responsible for the pairing mechanism. Here we show inelastic neutron scattering measurements of Fe1.04Te0.73Se0.27, not superconducting in bulk, and FeTe0.51Se0.49, a bulk superconductor. These measurements demonstrate that the spin fluctuation spectrum is dominated by two-dimensional incommensurate excitations near the (1/2,1/2) (square lattice (pi,0)) wavevector, the wavevector of interest in other Fe-based superconductors, that extend to energies at least as high as 300 meV. Most importantly, the spin excitations in Fe1+yTe1-xSex exhibit four-fold symmetry about the (1,0) (square lattice (pi,pi)) wavevector and are described by the identical wavevector as the normal state spin excitations in the high-TC cuprates8-12 demonstrating a commonality between the magnetism in these classes of materials which perhaps extends to a common origin for superconductivity.

cond-mat.supr-con↗

arXiv2009

Spin Excitations in BaFe1.84Co0.16As2 Superconductor Observed by Inelastic Neutron Scattering

Superconductivity appears to compete against the spin-density-wave in Fe pnictides. However, optimally cobalt doped samples show a quasi-two-dimensional spin excitation centered at the (0.5, 0.5, L) wavevector, "the spin resonance peak", that is strongly tied to the onset of superconductivity. By inelastic neutron scattering on single crystals we show the similarities and differences of the spin excitations in BaFe1.84Co0.16As2, with respect to the spin excitations in the high-temperature superconducting cuprates. As in the cuprates the resonance occurs as an enhancement to a part of the spin excitation spectrum which extends to higher energy transfer and higher temperature. However, unlike in the cuprates, the resonance peak in this compound is asymmetric in energy.

cond-mat.supr-con↗

arXiv2009

Neutron scattering patterns show Superconductivity in FeTe0.5Se0.5 likely results from itinerant electron fluctuations

The discovery of the Fe pnictide superconductors generated great interest as the structure consists of planes of a magnetic material quite similar to the cuprate superconductors. Fe(Te0.5Se0.5) is a particularly simple system whose planes are isostructural to the FeAs layers found in the originally discovered superconductors of this type. We report here neutron scattering measurements on this material that provide an understanding of the superconductivity. Since the information about the Fermi surface is available both from photoemission and band structure calculations for FeTe, FeSe and other related materials the neutron spectra can be used to see if the itinerant electron picture is valid. The results are consistent with a picture where there are both electron and hole Fermi surfaces that make exact (pi,pi) transitions possible. This would normally favor either a spin or charge density wave state. However, our measurements show the extent of the region where (pi,pi) transitions take place and demonstrate that there are a much larger number of transitions near pi,pi). The near (pi,pi) transitions are observed both above and below Tc and are expected to be strongly pairing. The superconductivity can be attributed to these excitations while the exactly (pi,pi) transitions produce the narrow resonance excitation that appears below Tc.

cond-mat.supr-con↗

arXiv2009

Two-dimensional resonant magnetic excitation in BaFe1.84Co0.16As2

Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wavevectors (1/2 1/2 L) in tetragonal notation. On cooling below TC, a clear resonance peak is observed at this wavevector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) kBTC. This is in good agreement with the canonical value of 5 kBTC observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c-axis, indicating that the magnetic fluctuations are two-dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.

cond-mat.supr-con↗

arXiv2008

Magnetic switching and phase competition in the multiferroic antiferromagnet $\rm Mn_{1-x}Fe_xWO_4$

Elastic neutron scattering is used to study the spin correlations in the multiferroic $\rm Mn_{1-x}Fe_{x}WO_4$ with $x=0.035, 0.05$ and 0.10. The noncollinear, incommensurate (ICM) magnetic structure associated with the ferroelectric (FE) phase in pure $\rm MnWO_4$ is suppressed at $x=0.035$ and completely absent at $x=0.10$. The ICM spin order and FE phase can be restored by applying a magnetic field along the spin easy-axis. The low-$T$ commensurate magnetic structure extends in both H/T with increasing Fe concentration. The systematic evolution of the magnetic and electric properties indicates that the noncollinear ICM spin order results from competing magnetic interactions and its stabilization can be tuned by the internal ($x$) or external (magnetic field) perturbations.

cond-mat.str-el↗

arXiv2008

Magnetic Order of the Iron Spins in NdOFeAs

Polarized and unpolarized neutron diffraction measurements have been carried out to investigate the iron magnetic order in undoped NdOFeAs. Antiferromagnetic order is observed below 141(6) K, which is in close proximity to the structural distortion observed in this material. The magnetic structure consists of chains of parallel spins that are arranged antiparallel between chains, which is the same in-plane spin arrangement as observed in all the other iron oxypnictide materials. Nearest-neighbor spins along the c-axis are antiparallel like LaOFeAs. The ordered moment is 0.25(7) muB, which is the smallest moment found so far in these systems.

cond-mat.supr-con↗

arXiv2008

Low energy spin waves and magnetic interactions in SrFe$_2$As$_2$

We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe$_{2}$As$_{2}$ ($T_{N}=220$ K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7 K), the spectrum of magnetic excitations $S(Q,\hbar ω)$ consists of a Bragg peak at the elastic position ($\hbar ω=0$ meV), a spin gap ($ Δ\leq 6.5$ meV), and sharp spin wave excitations at higher energies. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. On warming across $T_{N} $, the low temperature spin gap rapidly closes, with weak critical scattering and spin-spin correlations in the paramagnetic state. The antiferromagnetic order in SrFe$_{2}$As$_{2}$ is therefore consistent with a first order phase transition, similar to the structural lattice distortion.

cond-mat.supr-con↗

arXiv2008

Magnetic Order versus superconductivity in the Iron-based layered La(O1-xFx)FeAs systems

In high-transition temperature (high-Tc) copper oxides, it is generally believed that antiferromagnetism plays a fundamental role in the superconducting mechanism because superconductivity occurs when mobile electrons or holes are doped into the antiferromagnetic parent compounds. The recent discovery of superconductivity in the rare-earth (R) iron-based oxide systems [RO1-xFxFeAs] has generated enormous interest because these materials are the first noncopper oxide superconductors with Tc exceeding 50 K. The parent (nonsuperconducting) LaOFeAs material is metallic but shows anomalies near 150 K in both resistivity and dc magnetic susceptibility. While optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed with doping electrons to form superconductivity, there has been no direct evidence of the SDW order. Here we use neutron scattering to demonstrate that LaOFeAs undergoes an abrupt structural distortion below ~150 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then followed with the development of long range SDW-type antiferromagnetic order at ~134 K with a small moment but simple magnetic structure. Doping the system with flourine suppresses both the magnetic order and structural distortion in favor of superconductivity. Therefore, much like high-Tc copper oxides, the superconducting regime in these Fe-based materials occurs in close proximity to a long-range ordered antiferromagnetic ground state. Since the discovery of long

cond-mat.supr-con↗

arXiv2008

Observation of Magnetic Order in a ${\rm YBa_2Cu_3O_{6.6}}$ Superconductor

Polarized beam neutron scattering measurements on a highly perfect crystal of ${\rm YBa_2Cu_3O_{6.6}}$ show a distinct magnetic transition with an onset at about 235K, the temperature expected for the pseudogap transition. The moment is found to be about 0.1 $μ_B$ for each sublattice and have a correlation length of at least 75 Å. We found the critical exponent for the magnetic neutron intensity to be 2$β$ =0.37$\pm$ 0.12. This is the proper range for the class of transition that has no specific heat divergence possibly explaining why none is found at the pseudogap transition.

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