摘要 :
We report electronic Raman scattering measurements of Sr(Fe_(1-x)Co_x)_2As_2 single crystals in their magnetic-spin density wave (SDW) phase. The spectra display multiple, polarization-resolved SDW gaps as expected in a band-foldi...
展开
We report electronic Raman scattering measurements of Sr(Fe_(1-x)Co_x)_2As_2 single crystals in their magnetic-spin density wave (SDW) phase. The spectra display multiple, polarization-resolved SDW gaps as expected in a band-folding itinerant picture for a multiband system. The temperature dependence of the SDW gaps reveals an unusual evolution of the reconstructed electronic structure with at least one gap being activated only well below the magnetic SDW transition T_N. A comparison with temperature dependent Hall measurements allows us to assign this activated behavior to a change in the Fermi surface topology deep in the SDW phase, which we attribute to the disappearance of a holelike Fermi pocket. Our results highlight the strong sensitivity of the low energy electronic structure to temperature in iron-arsenide superconductors.
收起
摘要 :
Higher-harmonic spin density waves (SDW's) in Mn_3Si have been investigated by neutron diffraction, and the third-harmonic satellites have been found to exist at low temperatures at the 3Q positions expected from the propagation w...
展开
Higher-harmonic spin density waves (SDW's) in Mn_3Si have been investigated by neutron diffraction, and the third-harmonic satellites have been found to exist at low temperatures at the 3Q positions expected from the propagation wave vector Q of the fundamental satellite. The magnitude of the magnetic moments of the third harmonics is about 1/40 that of the fundamental. The third-harmonic satellites did not follow the usual temperature variation of a magnetization curve, but disappeared abruptly above 9 K. This is far below the Neel temperature of 23 K, and the temperature dependence suggests a first-order-type phase transition. Associated with this transition the intensity of the fundamental satellites changed slightly and the presence of the third harmonics suppressed the amplitude of the fundamental SDW. This transition was confirmed by specific heat measurements showing a small hump at 8.5 K, which persists to high magnetic fields of more than 6 T. The possibility of the presence of second-harmonic satellites is suggested, but it is not as clear as that of third-harmonic satellites in the present experiment. The existence of the third-harmonic satellites suggests that a transverse sinusoidal structure with spins modulating in the (110) or (210) plane is the most probable spin structure of Mn_3Si.
收起
摘要 :
We systematically investigate temperature- and spectrally dependent optical reflectivity dynamics in AAs_2Fe_2, (A = Ba, Sr, and Eu), iron-based superconductors parent spin-density wave (SDW) compounds. Two different relaxation pr...
展开
We systematically investigate temperature- and spectrally dependent optical reflectivity dynamics in AAs_2Fe_2, (A = Ba, Sr, and Eu), iron-based superconductors parent spin-density wave (SDW) compounds. Two different relaxation processes are identified. The behavior of the slower process, which is strongly sensitive to the magnetostructural transition, is analyzed in the framework of the relaxation-bottleneck model involving magnons. The results are compared to recent time-resolved angular photoemissiori results (TR-ARPES) and possible alternative assignment of the slower relaxation to the magnetostructural order parameter relaxation is discussed.
收起
摘要 :
Recent Fe x-ray emission spectroscopy experiments [H. Gretarsson et al., Phys. Rev. B 84, 100509 (2011)] unveiled sizable local moments in iron pnictides in the room-temperature paramagnetic state. In an effort to further clarify ...
展开
Recent Fe x-ray emission spectroscopy experiments [H. Gretarsson et al., Phys. Rev. B 84, 100509 (2011)] unveiled sizable local moments in iron pnictides in the room-temperature paramagnetic state. In an effort to further clarify the notion of coexisting magnetic moments and itinerant carriers in iron pnictides we focus on the interactions between the two subsystems. At a moderate on-site Coulomb repulsion the intra-atomic Hund's interaction leads to the formation of nonzero ("bare") local moments on the Fe sites. We show that the Kondo-like exchange with the itinerant electrons may significantly renormalize the "bare" value of the moments manifested in different experiments. In turn, the itinerant carriers scatter on the renormalized moments that remain disordered in the paramagnetic phase. On the one hand, the scattering mechanism is responsible for high values of resistivity of the stoichiometric pnictides at the temperature of their transition into the antiferromagnetic phase, and on the other hand, it washes out the fine details of the Fermi surfaces. The results are rigorous and were obtained without use of any Born-type approximation. It also turned out that the value of the local moment and the inverse free time for scattering of carriers on the moments tend in the limit of the strong Kondo exchange to the finite universal values. The independence of the results on the on-site Coulomb repulsion is then illustrated in the frameworks of a simplistic model. It is shown that the spin density wave transition is driven by the Ruderman-Kittel-Kasuya-Yosida interactions between the renormalized moments via exchange by the electron-hole pairs. The applicability of the Boltzmann approach to transport in multiband pnictides is discussed briefly.
收起
摘要 :
The antiferromagnetic (AFM) order and structural distortion in the LaFe(As_(1_x)Sb_x)O system have been investigated by neutron powder diffraction and physical properties measurements. Polycrystalline samples of LaFe(As_(1_x)Sb_x)...
展开
The antiferromagnetic (AFM) order and structural distortion in the LaFe(As_(1_x)Sb_x)O system have been investigated by neutron powder diffraction and physical properties measurements. Polycrystalline samples of LaFe(As_(1_x)Sb_x)O (x < 0.5) were prepared using solid state synthesis at ambient and high pressure. We find that the isoelectronic substitution of Sb for As decreases the structural and magnetic transition temperatures, but, contrary to the effects of phosphorus substitution, superconductivity is not induced. Instead a slight increase in the Fe-magnetic moment is observed.
收起
摘要 :
We study the Cn_(1-x) Re_x phase diagram finding that its phase transition temperature towards an antiferromagnetic order T_N follows a quantum [(x_c - x)/x_c]~Ψ law, with Ψ = 1/2, from the quantum critical point (QCP) at x_c = ...
展开
We study the Cn_(1-x) Re_x phase diagram finding that its phase transition temperature towards an antiferromagnetic order T_N follows a quantum [(x_c - x)/x_c]~Ψ law, with Ψ = 1/2, from the quantum critical point (QCP) at x_c = 0.25 up to T_N ≈ 600 K. We compare this system to others in order to understand why this elemental material is affected by the QCP up to such unusually high temperatures. We determine a general criterion for the crossover, as a function of an external parameter such as concentration, from the region controlled solely by thermal fluctuations to that where quantum effects become observable. The properties of materials with low coherence lengths will thus be altered far away from the QCP.
收起
摘要 :
We have performed a detailed study of the evolution of the charge-density wave (CDW) and spin-density wave (SDW) on the surface of nanoscale Cr islands grown on W( 110). Using low-temperature scanning tunneling microscopy (STM). w...
展开
We have performed a detailed study of the evolution of the charge-density wave (CDW) and spin-density wave (SDW) on the surface of nanoscale Cr islands grown on W( 110). Using low-temperature scanning tunneling microscopy (STM). we find a striking nonmonotonic thickness dependence of the CDW wavelength at 50 K. As the local island thickness (-) decreases from about 50 to 5.2 nm, the CDW wavelength gradually increases from the bulk value by approximately 30%. We find a gap without any CDW at coverages (-)_(gap) between 5.2 and 3.7 nm. Spin-resolved STM data reveal that within this CDW gap the SDW modulation also disappears on the Cr island surface. At (-) ＜ 3.7 nm the CDW reappears. This unusual behavior of CDW and SDW in Cr nanoislands can be understood by a reorientation of the SDW wave vector Q which is potentially driven by the pinning of the SDW node at the island surface at (-)gap.
收起
摘要 :
Time-resolved photoinduced reflectivity is measured in the spin-density-wave phase of the itinerant antiferromagnet UPtGa_5. Two relaxation components were seen: (a) a slow component whose amplitude appears below T_N, and relaxati...
展开
Time-resolved photoinduced reflectivity is measured in the spin-density-wave phase of the itinerant antiferromagnet UPtGa_5. Two relaxation components were seen: (a) a slow component whose amplitude appears below T_N, and relaxation time τ_(slow) exhibits an upturn near T_N, and (b) the fast component persists at all temperatures, with the relaxation time τ_(fast), also exhibiting an upturn near T_N. Comparing with pump-probe data on UNiGa_5, the differences are explained in the context of UPtGa_5 having A-type (rather than G-type) antiferromagnetism, resulting in partial Fermi surface nesting, partial gapping, and consequently finite density of states at the Fermi surface.
收起
摘要 :
We report neutron-scattering studies on static magnetic orders and spin excitations in the Fe-based chalco-genide system Fe_(1+δ)Se_xTe_(1-x) with different Fe and Se compositions. Short-range static magnetic order with an in-pla...
展开
We report neutron-scattering studies on static magnetic orders and spin excitations in the Fe-based chalco-genide system Fe_(1+δ)Se_xTe_(1-x) with different Fe and Se compositions. Short-range static magnetic order with an in-plane wave vector near the (0.5,0) (using the two-Fe unit cell), together with strong low-energy magnetic excitations is found in all nonsuperconducting samples for Se doping up to 45%. When the static order disappears and bulk superconductivity emerges, the spectral weight of the magnetic excitations shifts to the region of reciprocal space near the in-plane wave vector (0.5, 0.5), corresponding to "collinear" spin correlations. Our results suggest that there is a strong correlation between superconductivity and the character of the magnetic order/fluctuations in this system. Excess Fe appears to be important for stabilizing the magnetic order that competes with superconductivity.
收起
摘要 :
We have performed systematic resistivity and inelastic neutron scattering measurements on Fe_(0.98-z)Ni_zTe_(0.5)Se_(0.5) samples to study the impact of Ni substitution on the transport properties and the low-energy (≤12 meV) mag...
展开
We have performed systematic resistivity and inelastic neutron scattering measurements on Fe_(0.98-z)Ni_zTe_(0.5)Se_(0.5) samples to study the impact of Ni substitution on the transport properties and the low-energy (≤12 meV) magnetic excitations. It is found that, with increasing Ni doping, both the conductivity and superconductivity are gradually suppressed; in contrast, the low-energy magnetic spectral weight changes little. Comparing with the impact of Co and Cu substitution, we find that the effects on conductivity and superconductivity for the same degree of substitution grow systematically as the atomic number of the substituent deviates from that of Fe. The impact of the substituents as scattering centers appears to be greater than any contribution to carrier concentration. The fact that low-energy magnetic spectral weight is not reduced by increased electron scattering indicates that the existence of antiferromagnetic correlations does not depend on electronic states close to the Fermi energy.
收起
