Pengcheng Dai - Academia.edu (original) (raw)
Papers by Pengcheng Dai
Physical Review B, 2012
We use polarized inelastic neutron scattering to study low-energy spin excitations and their spat... more We use polarized inelastic neutron scattering to study low-energy spin excitations and their spatial anisotropy in electron-overdoped superconducting BaFe1.85Ni0.15As2 (Tc = 14 K). In the normal state, the imaginary part of the dynamic susceptibility, χ ′′ (Q, ω), at the antiferromagnetic (AF) wave vector Q = (0.5, 0.5, 1) increases linearly with energy for E ≤ 13 meV. Upon entering the superconducting state, a spin gap opens below E ≈ 3 meV and a broad neutron spin resonance appears at E ≈ 7 meV. Our careful neutron polarization analysis reveals that χ ′′ (Q, ω) is isotropic for the in-plane and out-of-plane components in both the normal and superconducting states. A comparison of these results with those of undoped BaFe2As2 and optimally electron-doped BaFe1.9Ni0.1As2 (Tc = 20 K) suggests that the spin anisotropy observed in BaFe1.9Ni0.1As2 is likely due to its proximity to the undoped BaFe2As2. Therefore, the neutron spin resonance is mostly isotropic in the optimal and electron overdoped iron pnictides, consistent with a singlet to triplet excitation and isotropic paramagnetic scattering.
Nature, 2000
There is increasing evidence that inhomogeneous distributions of charge and spinÐso-called`stripe... more There is increasing evidence that inhomogeneous distributions of charge and spinÐso-called`striped phases'Ðplay an important role in determining the properties of the high-temperature superconductors. For example, recent neutron-scattering measurements on the YBa 2 Cu 3 O 7-x family of materials show both spin and charge¯uctuations that are consistent with the stripedphase picture. But the¯uctuations associated with a striped phase are expected to be one-dimensional, whereas the magnetic uctuations observed to date appear to display two-dimensional symmetry. We show here that this apparent two-dimensionality results from measurements on twinned crystals, and that similar measurements on substantially detwinned crystals of YBa 2 Cu 3 O 6.6 reveal the one-dimensional character of the magnetic¯uctuations, thus greatly strengthening the striped-phase interpretation. Moreover, our results also suggest that superconductivity originates in charge stripes that extend along the b crystal axis, where the super¯uid density is found to be substantially larger than for the a direction.
We use neutron scattering to systematically investigate the evolution of spin excitations in Ni-d... more We use neutron scattering to systematically investigate the evolution of spin excitations in Ni-doped BaFe2As2. Previous works on underdoped and optimally doped materials have found evidence for spin gap and neutron spin resonance. We use flux method to grow overdoped material approaching to the nonsuperconducting regime. We have carried out inelastic neutron scattering measurements on spin and lattice excitations of
While continual effort in scanning tunneling microscopy (STM) remains focused on the hole-doped c... more While continual effort in scanning tunneling microscopy (STM) remains focused on the hole-doped cuprates less attention has been given to the equally important electron-doped side of the phase diagram. We use a variable temperature UHV STM to investigate the electron-doped cuprate superconductor Pr1-xLaCexCuO4. We explore temperatures 2 K and higher and find a superconducting gap that disappears above Tc. We
It is well known that in the superconducting state, the current carriers are the cooper pairs, wh... more It is well known that in the superconducting state, the current carriers are the cooper pairs, where two electrons (fermions) get paired up by a mediator (glue) and behave as a single boson. In conventional superconductors, lattice vibrations (phonons) act as a glue to pair up the electrons, however, in high Tc superconductors, the mechanism that binds these fermions together
We use high resolution synchrotron X-ray and neutron diffraction to study the geometrically frust... more We use high resolution synchrotron X-ray and neutron diffraction to study the geometrically frustrated triangular lattice antiferromagnet (TLA) CuFeO 2. We show that the occurrence of the two magnetic transitions, at 14 K and 11 K, respectively is accompanied simultaneously by a second-and first-order structural phase transitions from a hexagonal structure to a monoclinic form. This is the first observation of two successive spin-driven structural transitions directly coupled with incommensurate and commensurate magnetic orderings in frustrated TLA systems.
Physical Review Letters, 2007
We use inelastic neutron scattering to probe magnetic excitations of an optimally electron-doped ... more We use inelastic neutron scattering to probe magnetic excitations of an optimally electron-doped superconductor Nd1.85Ce0.15CuO 4−δ above and below its superconducting transition temperature Tc = 25 K. In addition to gradually opening a spin pseudo gap at the antiferromagnetic ordering wavevector Q = (1/2, 1/2, 0), the effect of superconductivity is to form a resonance centered also at Q = (1/2, 1/2, 0) but at energies above the spin pseudo gap. The intensity of the resonance develops like a superconducting order parameter, similar to those for hole-doped superconductors and electron-doped Pr0.88LaCe0.12CuO4. The resonance is therefore a general phenomenon of cuprate superconductors, and must be fundamental to the mechanism of high-Tc superconductivity.
Physical Review Letters, 2010
We measure magnetic quantum oscillations in the underdoped cuprates YBa2Cu3O6+x with x = 0.61, 0.... more We measure magnetic quantum oscillations in the underdoped cuprates YBa2Cu3O6+x with x = 0.61, 0.69, using fields of up to 85 T. The quantum-oscillation frequencies and effective masses obtained suggest that the Fermi energy in the cuprates has a maximum at p ≈ 0.11 − 0.12. On either side, the effective mass may diverge, possibly due to phase transitions associated with the T = 0 limit of the metal-insulator crossover (low-p side), and the postulated topological transition from small to large Fermi surface close to optimal doping (high p side).
Physical Review Letters, 2013
We use inelastic neutron scattering to show that superconductivity in electron-underdoped NaFe 0:... more We use inelastic neutron scattering to show that superconductivity in electron-underdoped NaFe 0:985 Co 0:015 As induces a dispersive sharp resonance near E r1 ¼ 3:25 meV and a broad dispersionless mode at E r2 ¼ 6 meV. However, similar measurements on overdoped superconducting NaFe 0:935 Co 0:045 As find only a single sharp resonance at E r ¼ 7 meV. We connect these results with the observations of angleresolved photoemission spectroscopy that the superconducting gaps in the electron Fermi pockets are anisotropic in the underdoped material but become isotropic in the overdoped case. Our analysis indicates that both the double neutron spin resonances and gap anisotropy originate from the orbital dependence of the superconducting pairing in the iron pnictides. Our discovery also shows the importance of the inelastic neutron scattering in detecting the multiorbital superconducting gap structures of iron pnictides.
Physical Review Letters, 2002
Neutron scattering has been used to measure the charge and spin structure in the highly underdope... more Neutron scattering has been used to measure the charge and spin structure in the highly underdoped superconductor YBa 2 Cu 3 O 6.35. Incommensurate static charge ordering is found that remains at high temperatures. The magnetic pattern is complex with a resonance and incommensurate structure observed at low temperatures. The results clarify the role of striped phases in YBa 2 Cu 3 O 61x superconductors.
Physical Review Letters, 2013
We study the structural and magnetic orders in electron-doped BaFe2−xNixAs2 by high-resolution sy... more We study the structural and magnetic orders in electron-doped BaFe2−xNixAs2 by high-resolution synchrotron X-ray and neutron scatterings. Upon Ni-doping x, the nearly simultaneous tetragonalto-orthorhombic structural (Ts) and antiferromagnetic (TN) phase transitions in BaFe2As2 are gradually suppressed and separated, resulting in Ts > TN with increasing x as was previously observed. However, the temperature separation between Ts and TN decreases with increasing x for x ≥ 0.065, tending towards a quantum bi-critical point near optimal superconductivity at x ≈ 0.1. The zerotemperature transition is preempted by the formation of a secondary incommensurate magnetic phase in the region 0.088 x 0.104, resulting in a finite value of TN ≈ Tc + 10 K above the superconducting dome around x ≈ 0.1. Our results imply an avoided quantum critical point, which is expected to strongly influence the properties of both the normal and superconducting states.
Physical Review B, 2008
Polarized and unpolarized neutron diffraction measurements have been carried out to investigate t... more 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) μ B , which is the smallest moment found so far in these systems.
Physical Review B, 2005
We present comprehensive neutron scattering studies of nonsuperconducting and superconducting ele... more We present comprehensive neutron scattering studies of nonsuperconducting and superconducting electron-doped Pr0.88LaCe0.12CuO 4±δ (PLCCO). At zero field, the transition from antiferromagnetic (AF) as-grown PLCCO to superconductivity without static antiferromagnetism can be achieved by annealing the sample in pure Ar at different temperatures, which also induces an epitaxial (Pr,La,Ce)2O3 phase as an impurity. When the superconductivity first appears in PLCCO, a quasi-two-dimensional (2D) spin-density-wave (SDW) order is also induced, and both coexist with the residual three-dimensional (3D) AF state. A magnetic field applied along the [1, 1, 0] direction parallel to the CuO2 plane induces a "spin-flop" transition, where the noncollinear AF spin structure of PLCCO is transformed into a collinear one. The spin-flop transition is continuous in semiconducting PLCCO, but gradually becomes sharp with increasing doping and the appearance of superconductivity. A c-axis aligned magnetic field that suppresses the superconductivity also enhances the quasi-2D SDW order at (0.5, 0.5, 0) for underdoped PLCCO. However, there is no effect on the 3D AF order in either superconducting or nonsuperconducting samples. Since the same field along the [1, 1, 0] direction in the CuO2 plane has no (or little) effect on the superconductivity, (0.5, 0.5, 0) and (Pr,La,Ce)2O3 impurity positions, we conclude that the c-axis field-induced effect is intrinsic to PLCCO and arises from the suppression of superconductivity.
Physical Review B, 2004
Orbital current order of the d-density-wave type (DDW) has been postulated to explain the pseudog... more Orbital current order of the d-density-wave type (DDW) has been postulated to explain the pseudogap in the high temperature superconductors. We have performed neutron scattering experiments to search for this order and show here the results obtained on an YBa 2 Cu 3 O 6.45 sample using the best neutron spectrometers available. We argue that the data are consistent with a small, largely c-axis-directed moment, found below about 200K.
Physical Review B, 2006
We use inelastic neutron scattering to explore the evolution of the low energy spin dynamics in t... more We use inelastic neutron scattering to explore the evolution of the low energy spin dynamics in the electron-doped cuprate Pr0.88LaCe0.12CuO 4−δ (PLCCO) as the system is tuned from its nonsuperconducting, as-grown antiferromagnetic (AF) state into an optimally-doped superconductor (Tc ≈ 24 K) without static AF order. The low temperature, low energy response of the spin excitations in under-doped samples is coupled to the presence of the AF phase, whereas the low-energy magnetic response for samples near optimal Tc exhibits spin fluctuations surprisingly insensitive to the sample temperature. This evolution of the low energy excitations is consistent with the influence of a quantum critical point in the phase diagram of PLCCO associated with the suppression of the static AF order. We carried out scaling analysis of the data and discuss the influence of quantum critical dynamics in the observed excitation spectrum.
Physical Review B, 2012
We use inelastic neutron scattering to show that superconducting (SC) rubidium iron selenide Rb0.... more We use inelastic neutron scattering to show that superconducting (SC) rubidium iron selenide Rb0.82Fe1.68Se2 exhibits antiferromagnetic (AF) spin excitations near the in-plane wave vector Q = (π, 0) identical to that for iron arsenide superconductors. Moreover, we find that these excitations change from incommensurate to commensurate with increasing energy, and occur at the expense of spin waves associated with the coexisting √ 5 × √ 5 block AF phase. Since angle resolved photoemission experiments reveal no evidence for hole-like Fermi surface at Γ(0, 0), our results suggest that the Q = (π, 0) excitations in SC Rb0.82Fe1.68Se2 come from localized moments and may have a similar origin as the hourglass-like spin excitations in copper oxide superconductors.
Physical Review B, 2010
The discovery of high-transition temperature (high-T c) superconductivity near antiferromagnetism... more The discovery of high-transition temperature (high-T c) superconductivity near antiferromagnetism in iron arsenides raised the possibility of an unconventional superconducting mechansim 1-8. The observation of clear Fermi surfaces and nodeless superconducting gaps by angle resolved photoemission 9-12 suggests that electron pairing in these materials may be mediated by quasiparticle excitations between sign reversed hole and electron Fermi pockets 5-8. Although the presence of a "resonance" in the spin excitation spectrum found by inelastic neutron scattering 13-17 is consistent with this picture 18-20 , there has been no direct evidence connecting the resonance to the superconducting gap energy. Here we show that for the optimally electron doped BaFe 1.9 Ni 0.1 As 2 (T c =20 K, Fig. 1c) iron arsenide
Physical Review B, 2009
By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity ... more By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity in the AFe 2 As 2 ͑A =Ca,Ba,Sr͒ systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of ͑Ba 0.5 K 0.5 ͒Fe 2 As 2 exhibits a nearly linear variation in T at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multigap effects.
Physical Review B, 2012
We use polarized inelastic neutron scattering to study low-energy spin excitations and their spat... more We use polarized inelastic neutron scattering to study low-energy spin excitations and their spatial anisotropy in electron-overdoped superconducting BaFe1.85Ni0.15As2 (Tc = 14 K). In the normal state, the imaginary part of the dynamic susceptibility, χ ′′ (Q, ω), at the antiferromagnetic (AF) wave vector Q = (0.5, 0.5, 1) increases linearly with energy for E ≤ 13 meV. Upon entering the superconducting state, a spin gap opens below E ≈ 3 meV and a broad neutron spin resonance appears at E ≈ 7 meV. Our careful neutron polarization analysis reveals that χ ′′ (Q, ω) is isotropic for the in-plane and out-of-plane components in both the normal and superconducting states. A comparison of these results with those of undoped BaFe2As2 and optimally electron-doped BaFe1.9Ni0.1As2 (Tc = 20 K) suggests that the spin anisotropy observed in BaFe1.9Ni0.1As2 is likely due to its proximity to the undoped BaFe2As2. Therefore, the neutron spin resonance is mostly isotropic in the optimal and electron overdoped iron pnictides, consistent with a singlet to triplet excitation and isotropic paramagnetic scattering.
Nature, 2000
There is increasing evidence that inhomogeneous distributions of charge and spinÐso-called`stripe... more There is increasing evidence that inhomogeneous distributions of charge and spinÐso-called`striped phases'Ðplay an important role in determining the properties of the high-temperature superconductors. For example, recent neutron-scattering measurements on the YBa 2 Cu 3 O 7-x family of materials show both spin and charge¯uctuations that are consistent with the stripedphase picture. But the¯uctuations associated with a striped phase are expected to be one-dimensional, whereas the magnetic uctuations observed to date appear to display two-dimensional symmetry. We show here that this apparent two-dimensionality results from measurements on twinned crystals, and that similar measurements on substantially detwinned crystals of YBa 2 Cu 3 O 6.6 reveal the one-dimensional character of the magnetic¯uctuations, thus greatly strengthening the striped-phase interpretation. Moreover, our results also suggest that superconductivity originates in charge stripes that extend along the b crystal axis, where the super¯uid density is found to be substantially larger than for the a direction.
We use neutron scattering to systematically investigate the evolution of spin excitations in Ni-d... more We use neutron scattering to systematically investigate the evolution of spin excitations in Ni-doped BaFe2As2. Previous works on underdoped and optimally doped materials have found evidence for spin gap and neutron spin resonance. We use flux method to grow overdoped material approaching to the nonsuperconducting regime. We have carried out inelastic neutron scattering measurements on spin and lattice excitations of
While continual effort in scanning tunneling microscopy (STM) remains focused on the hole-doped c... more While continual effort in scanning tunneling microscopy (STM) remains focused on the hole-doped cuprates less attention has been given to the equally important electron-doped side of the phase diagram. We use a variable temperature UHV STM to investigate the electron-doped cuprate superconductor Pr1-xLaCexCuO4. We explore temperatures 2 K and higher and find a superconducting gap that disappears above Tc. We
It is well known that in the superconducting state, the current carriers are the cooper pairs, wh... more It is well known that in the superconducting state, the current carriers are the cooper pairs, where two electrons (fermions) get paired up by a mediator (glue) and behave as a single boson. In conventional superconductors, lattice vibrations (phonons) act as a glue to pair up the electrons, however, in high Tc superconductors, the mechanism that binds these fermions together
We use high resolution synchrotron X-ray and neutron diffraction to study the geometrically frust... more We use high resolution synchrotron X-ray and neutron diffraction to study the geometrically frustrated triangular lattice antiferromagnet (TLA) CuFeO 2. We show that the occurrence of the two magnetic transitions, at 14 K and 11 K, respectively is accompanied simultaneously by a second-and first-order structural phase transitions from a hexagonal structure to a monoclinic form. This is the first observation of two successive spin-driven structural transitions directly coupled with incommensurate and commensurate magnetic orderings in frustrated TLA systems.
Physical Review Letters, 2007
We use inelastic neutron scattering to probe magnetic excitations of an optimally electron-doped ... more We use inelastic neutron scattering to probe magnetic excitations of an optimally electron-doped superconductor Nd1.85Ce0.15CuO 4−δ above and below its superconducting transition temperature Tc = 25 K. In addition to gradually opening a spin pseudo gap at the antiferromagnetic ordering wavevector Q = (1/2, 1/2, 0), the effect of superconductivity is to form a resonance centered also at Q = (1/2, 1/2, 0) but at energies above the spin pseudo gap. The intensity of the resonance develops like a superconducting order parameter, similar to those for hole-doped superconductors and electron-doped Pr0.88LaCe0.12CuO4. The resonance is therefore a general phenomenon of cuprate superconductors, and must be fundamental to the mechanism of high-Tc superconductivity.
Physical Review Letters, 2010
We measure magnetic quantum oscillations in the underdoped cuprates YBa2Cu3O6+x with x = 0.61, 0.... more We measure magnetic quantum oscillations in the underdoped cuprates YBa2Cu3O6+x with x = 0.61, 0.69, using fields of up to 85 T. The quantum-oscillation frequencies and effective masses obtained suggest that the Fermi energy in the cuprates has a maximum at p ≈ 0.11 − 0.12. On either side, the effective mass may diverge, possibly due to phase transitions associated with the T = 0 limit of the metal-insulator crossover (low-p side), and the postulated topological transition from small to large Fermi surface close to optimal doping (high p side).
Physical Review Letters, 2013
We use inelastic neutron scattering to show that superconductivity in electron-underdoped NaFe 0:... more We use inelastic neutron scattering to show that superconductivity in electron-underdoped NaFe 0:985 Co 0:015 As induces a dispersive sharp resonance near E r1 ¼ 3:25 meV and a broad dispersionless mode at E r2 ¼ 6 meV. However, similar measurements on overdoped superconducting NaFe 0:935 Co 0:045 As find only a single sharp resonance at E r ¼ 7 meV. We connect these results with the observations of angleresolved photoemission spectroscopy that the superconducting gaps in the electron Fermi pockets are anisotropic in the underdoped material but become isotropic in the overdoped case. Our analysis indicates that both the double neutron spin resonances and gap anisotropy originate from the orbital dependence of the superconducting pairing in the iron pnictides. Our discovery also shows the importance of the inelastic neutron scattering in detecting the multiorbital superconducting gap structures of iron pnictides.
Physical Review Letters, 2002
Neutron scattering has been used to measure the charge and spin structure in the highly underdope... more Neutron scattering has been used to measure the charge and spin structure in the highly underdoped superconductor YBa 2 Cu 3 O 6.35. Incommensurate static charge ordering is found that remains at high temperatures. The magnetic pattern is complex with a resonance and incommensurate structure observed at low temperatures. The results clarify the role of striped phases in YBa 2 Cu 3 O 61x superconductors.
Physical Review Letters, 2013
We study the structural and magnetic orders in electron-doped BaFe2−xNixAs2 by high-resolution sy... more We study the structural and magnetic orders in electron-doped BaFe2−xNixAs2 by high-resolution synchrotron X-ray and neutron scatterings. Upon Ni-doping x, the nearly simultaneous tetragonalto-orthorhombic structural (Ts) and antiferromagnetic (TN) phase transitions in BaFe2As2 are gradually suppressed and separated, resulting in Ts > TN with increasing x as was previously observed. However, the temperature separation between Ts and TN decreases with increasing x for x ≥ 0.065, tending towards a quantum bi-critical point near optimal superconductivity at x ≈ 0.1. The zerotemperature transition is preempted by the formation of a secondary incommensurate magnetic phase in the region 0.088 x 0.104, resulting in a finite value of TN ≈ Tc + 10 K above the superconducting dome around x ≈ 0.1. Our results imply an avoided quantum critical point, which is expected to strongly influence the properties of both the normal and superconducting states.
Physical Review B, 2008
Polarized and unpolarized neutron diffraction measurements have been carried out to investigate t... more 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) μ B , which is the smallest moment found so far in these systems.
Physical Review B, 2005
We present comprehensive neutron scattering studies of nonsuperconducting and superconducting ele... more We present comprehensive neutron scattering studies of nonsuperconducting and superconducting electron-doped Pr0.88LaCe0.12CuO 4±δ (PLCCO). At zero field, the transition from antiferromagnetic (AF) as-grown PLCCO to superconductivity without static antiferromagnetism can be achieved by annealing the sample in pure Ar at different temperatures, which also induces an epitaxial (Pr,La,Ce)2O3 phase as an impurity. When the superconductivity first appears in PLCCO, a quasi-two-dimensional (2D) spin-density-wave (SDW) order is also induced, and both coexist with the residual three-dimensional (3D) AF state. A magnetic field applied along the [1, 1, 0] direction parallel to the CuO2 plane induces a "spin-flop" transition, where the noncollinear AF spin structure of PLCCO is transformed into a collinear one. The spin-flop transition is continuous in semiconducting PLCCO, but gradually becomes sharp with increasing doping and the appearance of superconductivity. A c-axis aligned magnetic field that suppresses the superconductivity also enhances the quasi-2D SDW order at (0.5, 0.5, 0) for underdoped PLCCO. However, there is no effect on the 3D AF order in either superconducting or nonsuperconducting samples. Since the same field along the [1, 1, 0] direction in the CuO2 plane has no (or little) effect on the superconductivity, (0.5, 0.5, 0) and (Pr,La,Ce)2O3 impurity positions, we conclude that the c-axis field-induced effect is intrinsic to PLCCO and arises from the suppression of superconductivity.
Physical Review B, 2004
Orbital current order of the d-density-wave type (DDW) has been postulated to explain the pseudog... more Orbital current order of the d-density-wave type (DDW) has been postulated to explain the pseudogap in the high temperature superconductors. We have performed neutron scattering experiments to search for this order and show here the results obtained on an YBa 2 Cu 3 O 6.45 sample using the best neutron spectrometers available. We argue that the data are consistent with a small, largely c-axis-directed moment, found below about 200K.
Physical Review B, 2006
We use inelastic neutron scattering to explore the evolution of the low energy spin dynamics in t... more We use inelastic neutron scattering to explore the evolution of the low energy spin dynamics in the electron-doped cuprate Pr0.88LaCe0.12CuO 4−δ (PLCCO) as the system is tuned from its nonsuperconducting, as-grown antiferromagnetic (AF) state into an optimally-doped superconductor (Tc ≈ 24 K) without static AF order. The low temperature, low energy response of the spin excitations in under-doped samples is coupled to the presence of the AF phase, whereas the low-energy magnetic response for samples near optimal Tc exhibits spin fluctuations surprisingly insensitive to the sample temperature. This evolution of the low energy excitations is consistent with the influence of a quantum critical point in the phase diagram of PLCCO associated with the suppression of the static AF order. We carried out scaling analysis of the data and discuss the influence of quantum critical dynamics in the observed excitation spectrum.
Physical Review B, 2012
We use inelastic neutron scattering to show that superconducting (SC) rubidium iron selenide Rb0.... more We use inelastic neutron scattering to show that superconducting (SC) rubidium iron selenide Rb0.82Fe1.68Se2 exhibits antiferromagnetic (AF) spin excitations near the in-plane wave vector Q = (π, 0) identical to that for iron arsenide superconductors. Moreover, we find that these excitations change from incommensurate to commensurate with increasing energy, and occur at the expense of spin waves associated with the coexisting √ 5 × √ 5 block AF phase. Since angle resolved photoemission experiments reveal no evidence for hole-like Fermi surface at Γ(0, 0), our results suggest that the Q = (π, 0) excitations in SC Rb0.82Fe1.68Se2 come from localized moments and may have a similar origin as the hourglass-like spin excitations in copper oxide superconductors.
Physical Review B, 2010
The discovery of high-transition temperature (high-T c) superconductivity near antiferromagnetism... more The discovery of high-transition temperature (high-T c) superconductivity near antiferromagnetism in iron arsenides raised the possibility of an unconventional superconducting mechansim 1-8. The observation of clear Fermi surfaces and nodeless superconducting gaps by angle resolved photoemission 9-12 suggests that electron pairing in these materials may be mediated by quasiparticle excitations between sign reversed hole and electron Fermi pockets 5-8. Although the presence of a "resonance" in the spin excitation spectrum found by inelastic neutron scattering 13-17 is consistent with this picture 18-20 , there has been no direct evidence connecting the resonance to the superconducting gap energy. Here we show that for the optimally electron doped BaFe 1.9 Ni 0.1 As 2 (T c =20 K, Fig. 1c) iron arsenide
Physical Review B, 2009
By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity ... more By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity in the AFe 2 As 2 ͑A =Ca,Ba,Sr͒ systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of ͑Ba 0.5 K 0.5 ͒Fe 2 As 2 exhibits a nearly linear variation in T at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multigap effects.