The J eff = 1 2 state is manifested in systems with large cubic crystal field splitting and spin-... more The J eff = 1 2 state is manifested in systems with large cubic crystal field splitting and spin-orbit coupling that are comparable to the on-site Coulomb interaction, U. 5d transition metal oxides host parameters in this regime and strong evidence for this state in Sr2IrO4, and additional iridates, has been presented. All the candidates, however, deviate from the cubic crystal field required to provide an unmixed canonical J eff = 1 2 state, impacting the development of a robust model of this novel insulating and magnetic state. We present results that not only show Ca4IrO6 hosts the state, but furthermore uniquely resides close to the ideal case required for an unmixed J eff = 1 2 state.
ORNL-Iridates can hosts a spin-orbit entangled ground state with J eff =1/2 magnetic moments. Map... more ORNL-Iridates can hosts a spin-orbit entangled ground state with J eff =1/2 magnetic moments. Mapping of these pseudo-spins onto several distinct crystal structures has led to the uncovering of a variety of novel properties, such as Kitaev physics and Weyl semimetals. Sr 2 IrO 4 is a canonical example of a J eff =1/2 material and much interest has focused on the predictions of unconventional superconductivity driven by analogies to cuprates. In particular, proximity to a superconducting phase was strongly suggested by the magnetic excitation spectra which revealed striking similarities to La 2 CuO 4 and was able to be described within a pure Heisenberg model consisting of spin-1/2 on a square lattice. We have investigated the series Sr 2 Ir 1−x Ru x O 4 with neutron and resonant x-ray scattering and found an evolution of magnetism from ab-plane to c-axis aligned spins [1]. Despite this change the J eff =1/2 moments remain robust. By probing the excitations we observed a large spin-gap (40 meV) that indicates an alteration of the magnetic anisotropy associated with the parent. [1] S.
We study proximity-induced spin-orbit coupling (SOC) in bilayer graphene/few-layer WSe 2 heterost... more We study proximity-induced spin-orbit coupling (SOC) in bilayer graphene/few-layer WSe 2 heterostructure devices. Contact mode atomic force microscopy (AFM) cleaning yields ultra-clean interfaces and high-mobility devices. In a perpendicular magnetic field, we measure the quantum Hall effect to determine the Landau level structure in the presence of out-of-plane Ising and inplane Rashba SOC. A distinct Landau level crossing pattern emerges when tuning the charge density and displacement field independently with dual gates, originating from a layer-selective SOC proximity effect. Analyzing the Landau level crossings and measured inter-Landau level energy gaps yields the proximity induced SOC energy scale. The Ising SOC is » 2.2 meV, 100 times higher than the intrinsic SOC in graphene, while its sign is consistent with theories predicting a dependence of SOC on interlayer twist angle. The Rashba SOC is ~15 meV. Finally, we infer the magnetic field dependence of the inter-Landau level Coulomb interactions. These ultraclean bilayer graphene/WSe 2 heterostructures provide a high mobility system with the potential to realize novel topological electronic states and manipulate spins in nanostructures.
We report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition ... more We report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition without the assistance of any catalysts. The assembly of single-crystal FeGe2 nanowires with tetragonal C16 crystal structure shows anisotropic magnetic behavior along the radial direction or the growth axial direction, with both antiferromagnetic and ferromagnetic orders. Single FeGe2 nanowire devices were fabricated using e-beam lithography. Electronic transport measurement in these devices show two resistivity anomalies near 250 K and 200 K which are likely signatures of the two spin density wave states in FeGe2.
The dispersive magnetic excitations in Sr2IrO4 have previously been well described within an isos... more The dispersive magnetic excitations in Sr2IrO4 have previously been well described within an isospin-1/2 Heisenberg model on a square lattice that revealed parallels with La2CuO4. Here we investigate the inelastic spectra of Sr2Ir0.8Ru0.2O4 with resonant inelastic x-ray scattering (RIXS) at the Ir L3-edge. The results are well described using linear spin-wave theory within a similar Heisenberg model applicable to Sr2IrO4, however the disorder induced by the substitution of 20% Ir 4+ ions for Ru 4+ removes longer range exchange interactions. A large spin-gap (40 meV) is measured indicating strong anisotropy from spin-orbit coupling that is manifest due to the altered magnetic structure in Sr2Ir0.8Ru0.2O4 with c-axis aligned moments compared to the basal plane moments in the parent. Collectively the results indicate the robustness of a Heisenberg model description even when the magnetic structure is altered and the J e↵ =1/2 moments diluted.
Spin Correlated Scattering and Metal-Semiconductor Transition in CMR Manganites
AIP Conference Proceedings, 2006
The CMR manganites are one of the most interesting and widely noticed objects in the physical and... more The CMR manganites are one of the most interesting and widely noticed objects in the physical and materials sciences based on their possible application on CMR properties and sensor technology at low-temperature, strong correlation characteristic, ordering of spin/charge/orbital including lattice degree of freedom. In this paper, a Kondo-like transport was observed in the term of a metal-semiconductor transition at low
Pr-substituted YBCO system has been studied by positron lifetime experiment in the range from 0.0... more Pr-substituted YBCO system has been studied by positron lifetime experiment in the range from 0.0 to 1.0. The present results show that the value of short lifetime s 1 in PrBa 2 Cu 3 O 7Àd is nearly the same as in the isostructural YBa 2 Cu 3 O 7Àd and the defect-related positron lifetime component s 2 decreases as a function of Pr-substitution x. The increase of local electron density n e reflects the electronic intense localization. These experimental results are interpreted in terms of the localization of hole carriers in the Cu-O chains on Pr substitution due to disorder in the Ba and Pr planes by the partly occupation of Ba 2+ ion site with Pr 3+ ion. These will destroy the intrinsic conductivity of the Cu-O chains and so impresses the superconductivity above x=0.6 Pr-substitution concentration.
Magnetic/NIR-responsive drug carrier, multicolor cell imaging, and enhanced photothermal therapy of gold capped magnetite-fluorescent carbon hybrid nanoparticles
Nanoscale, Jan 24, 2015
This paper reports a type of multifunctional hybrid nanoparticle (NP) composed of gold nanocrysta... more This paper reports a type of multifunctional hybrid nanoparticle (NP) composed of gold nanocrystals coated on and/or embedded in a magnetite-fluorescent porous carbon core-shell NP template (Fe3O4@PC-CDs-Au) for biomedical applications, including magnetic/NIR-responsive drug release, multicolor cell imaging, and enhanced photothermal therapy. The synthesis of the Fe3O4@PC-CDs-Au NPs firstly involves the preparation of core-shell template NPs with magnetite nanocrystals clustered in the cores and fluorescent carbon dots (CDs) embedded in a porous carbon shell, followed by an in situ reduction of silver ions (Ag(+)) loaded in the porous carbon shell and a subsequent replacement of Ag NPs with Au NPs through a galvanic replacement reaction using HAuCl4 as a precursor. The Fe3O4@PC-CDs-Au NPs can enter the intracellular region and light up mouse melanoma B16F10 cells in multicolor mode. The porous carbon shell, anchored with hydrophilic hydroxyl/carboxyl groups, endows the Fe3O4@PC-CDs-...
A metastable phase α-FeSi 2 was epitaxially stabilized on silicon substrate using pulsed laser de... more A metastable phase α-FeSi 2 was epitaxially stabilized on silicon substrate using pulsed laser deposition. Nonmetallic and ferromagnetic behaviors were tailored on α-FeSi 2 (111) thin films, while the bulk material of α-FeSi 2 is metallic and nonmagnetic. The transport property of the films renders two different conducting states with a strong crossover at 50 K which is accompanied by an onset of ferromagnetic transition as well as a substantial magnetoresistance. These experimental results are discussed in terms of the unusual electronic structure of α-FeSi 2 obtained within density functional calculations and Boltzmann transport calculations with and without strain. Our finding sheds light on achieving ferromagnetic semiconductors through both the structure and doping tailoring, and provides an example of a tailored material with rich functionalities for both basic research and practical applications.
High-resolution anisotropic magneto-resistance measurement (AMR) was used to detailed study the t... more High-resolution anisotropic magneto-resistance measurement (AMR) was used to detailed study the training effect in exchange biased CoO/Co bi-layer. The sample was cooled to 10 K from room temperature in the magnetic cooling field of 4000 Oe. Then we used 1500 Oe declined perturbation field to pin the magnetization orientation of the FM layer. The perturbation field forms certain angle Θ with the cooling field direction in-plane to re-induce the untrained state. The dependence of the untrained state on the angle between the direction of perturbation field and cooling field has been investigated. The AMR results reveal that the re-induced degree of untrained state is strongly correlated to the angle Θ. The exchange bias field H E for different Θ has been determined from the AMR results, which is in apparent agreement with the Meiklejohn-Bean model. The recover degree of untrained state is the largest when the angle is 75˚, which is different from the traditional view point that untrained state should be the maximum when it is perpendicular. The training effect is related to the FM spin orientation, which can induce the change of the interfacial AFM spin reorientation with different angles.
While the assembled 1D magnetic nanoparticle (NP) chains have demonstrated synergistic magnetic e... more While the assembled 1D magnetic nanoparticle (NP) chains have demonstrated synergistic magnetic effects from the individual NPs, it is essential to prepare new 1D NP chains that can combine the magnetism with other important material properties for multifunctional applications. This paper reports the fabrication and multifunctional investigation of a new type of 1D NP chains that combine the magnetic properties with fluorescent properties, photothermal conversion ability, and drug carrier function. The building block NPs are composed of magnetic Fe 3 O 4 nanocrystals clustered in the core and fluorescent carbon dots embedded in the mesoporous carbon shell with hydroxyl/carboxyl groups anchored on their surface. These NPs can assemble under the induction of external magnetic field and form stable 1D NP chains of diameter ∼90 nm and length ∼3 μm via the hydrogen bonding and π−π stacking linkage of the carbon shell. The resulted 1D hybrid NP chains not only demonstrate much higher magnetic resonance imaging (MRI) contrasting ability than the dispersed building block NPs, but also enter into intracellular region and light up the B16F10 cells under a laser excitation with strong and stable fluorescence. While the mesoporous carbon shell provides high drug loading capacity, the embedded fluorescent carbon dots convert near-infrared (NIR) light to heat, and hence kill the tumor cells efficiently and enhance the drug release rate to further improve the therapeutic efficacy under NIR irradiation. Such designed 1D magnetic-fluorescent hybrid NP chains with enhanced MRI contrast, fluorescent imaging ability, and combined chemo-/ photothermal therapeutic ability have great potential for various biomedical applications.
Small (Weinheim an der Bergstrasse, Germany), Jan 26, 2015
Anisotropic Fe3 O4 octahedrons are obtained via a simple solvothermal synthesis with appropriate ... more Anisotropic Fe3 O4 octahedrons are obtained via a simple solvothermal synthesis with appropriate sizes for various technological applications. A complete suite of materials characterization methods confirms the magnetite phase for these structures, which exhibit substantial saturation magnetization and intriguing morphologies for a wide range of applications.
Magnetic field induced step-like transport jump is found at low temperature for Pr doped phasesep... more Magnetic field induced step-like transport jump is found at low temperature for Pr doped phaseseparated manganites. Magnetization measurements indicate that spin-stiffness constant D exhibits large magnetic field dependence. Moreover, there exists a remarkable change of spin-stiffness constant corresponding to the variety of the step-like transport jump. This suggests a possible existence of quantum orbital transfer resulting from the change of orbital ordering (OO) modes and accompanying with spin reorientation and carrier delocalization. The carrier delocalization is closely related to the coupling of charge-orbital and can be proved from the low temperature resistivity minimum behavior. The phase diagram was determined. These all show the importance of the strong interaction of charge-orbital and/or spin-orbital in the adjacent long-range charge-ordered antiferromagnetic phase, which depends sensitively on the OO modes.
The J eff = 1 2 state is manifested in systems with large cubic crystal field splitting and spin-... more The J eff = 1 2 state is manifested in systems with large cubic crystal field splitting and spin-orbit coupling that are comparable to the on-site Coulomb interaction, U. 5d transition metal oxides host parameters in this regime and strong evidence for this state in Sr2IrO4, and additional iridates, has been presented. All the candidates, however, deviate from the cubic crystal field required to provide an unmixed canonical J eff = 1 2 state, impacting the development of a robust model of this novel insulating and magnetic state. We present results that not only show Ca4IrO6 hosts the state, but furthermore uniquely resides close to the ideal case required for an unmixed J eff = 1 2 state.
ORNL-Iridates can hosts a spin-orbit entangled ground state with J eff =1/2 magnetic moments. Map... more ORNL-Iridates can hosts a spin-orbit entangled ground state with J eff =1/2 magnetic moments. Mapping of these pseudo-spins onto several distinct crystal structures has led to the uncovering of a variety of novel properties, such as Kitaev physics and Weyl semimetals. Sr 2 IrO 4 is a canonical example of a J eff =1/2 material and much interest has focused on the predictions of unconventional superconductivity driven by analogies to cuprates. In particular, proximity to a superconducting phase was strongly suggested by the magnetic excitation spectra which revealed striking similarities to La 2 CuO 4 and was able to be described within a pure Heisenberg model consisting of spin-1/2 on a square lattice. We have investigated the series Sr 2 Ir 1−x Ru x O 4 with neutron and resonant x-ray scattering and found an evolution of magnetism from ab-plane to c-axis aligned spins [1]. Despite this change the J eff =1/2 moments remain robust. By probing the excitations we observed a large spin-gap (40 meV) that indicates an alteration of the magnetic anisotropy associated with the parent. [1] S.
We study proximity-induced spin-orbit coupling (SOC) in bilayer graphene/few-layer WSe 2 heterost... more We study proximity-induced spin-orbit coupling (SOC) in bilayer graphene/few-layer WSe 2 heterostructure devices. Contact mode atomic force microscopy (AFM) cleaning yields ultra-clean interfaces and high-mobility devices. In a perpendicular magnetic field, we measure the quantum Hall effect to determine the Landau level structure in the presence of out-of-plane Ising and inplane Rashba SOC. A distinct Landau level crossing pattern emerges when tuning the charge density and displacement field independently with dual gates, originating from a layer-selective SOC proximity effect. Analyzing the Landau level crossings and measured inter-Landau level energy gaps yields the proximity induced SOC energy scale. The Ising SOC is » 2.2 meV, 100 times higher than the intrinsic SOC in graphene, while its sign is consistent with theories predicting a dependence of SOC on interlayer twist angle. The Rashba SOC is ~15 meV. Finally, we infer the magnetic field dependence of the inter-Landau level Coulomb interactions. These ultraclean bilayer graphene/WSe 2 heterostructures provide a high mobility system with the potential to realize novel topological electronic states and manipulate spins in nanostructures.
We report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition ... more We report the synthesis of single-crystal iron germanium nanowires via chemical vapor deposition without the assistance of any catalysts. The assembly of single-crystal FeGe2 nanowires with tetragonal C16 crystal structure shows anisotropic magnetic behavior along the radial direction or the growth axial direction, with both antiferromagnetic and ferromagnetic orders. Single FeGe2 nanowire devices were fabricated using e-beam lithography. Electronic transport measurement in these devices show two resistivity anomalies near 250 K and 200 K which are likely signatures of the two spin density wave states in FeGe2.
The dispersive magnetic excitations in Sr2IrO4 have previously been well described within an isos... more The dispersive magnetic excitations in Sr2IrO4 have previously been well described within an isospin-1/2 Heisenberg model on a square lattice that revealed parallels with La2CuO4. Here we investigate the inelastic spectra of Sr2Ir0.8Ru0.2O4 with resonant inelastic x-ray scattering (RIXS) at the Ir L3-edge. The results are well described using linear spin-wave theory within a similar Heisenberg model applicable to Sr2IrO4, however the disorder induced by the substitution of 20% Ir 4+ ions for Ru 4+ removes longer range exchange interactions. A large spin-gap (40 meV) is measured indicating strong anisotropy from spin-orbit coupling that is manifest due to the altered magnetic structure in Sr2Ir0.8Ru0.2O4 with c-axis aligned moments compared to the basal plane moments in the parent. Collectively the results indicate the robustness of a Heisenberg model description even when the magnetic structure is altered and the J e↵ =1/2 moments diluted.
Spin Correlated Scattering and Metal-Semiconductor Transition in CMR Manganites
AIP Conference Proceedings, 2006
The CMR manganites are one of the most interesting and widely noticed objects in the physical and... more The CMR manganites are one of the most interesting and widely noticed objects in the physical and materials sciences based on their possible application on CMR properties and sensor technology at low-temperature, strong correlation characteristic, ordering of spin/charge/orbital including lattice degree of freedom. In this paper, a Kondo-like transport was observed in the term of a metal-semiconductor transition at low
Pr-substituted YBCO system has been studied by positron lifetime experiment in the range from 0.0... more Pr-substituted YBCO system has been studied by positron lifetime experiment in the range from 0.0 to 1.0. The present results show that the value of short lifetime s 1 in PrBa 2 Cu 3 O 7Àd is nearly the same as in the isostructural YBa 2 Cu 3 O 7Àd and the defect-related positron lifetime component s 2 decreases as a function of Pr-substitution x. The increase of local electron density n e reflects the electronic intense localization. These experimental results are interpreted in terms of the localization of hole carriers in the Cu-O chains on Pr substitution due to disorder in the Ba and Pr planes by the partly occupation of Ba 2+ ion site with Pr 3+ ion. These will destroy the intrinsic conductivity of the Cu-O chains and so impresses the superconductivity above x=0.6 Pr-substitution concentration.
Magnetic/NIR-responsive drug carrier, multicolor cell imaging, and enhanced photothermal therapy of gold capped magnetite-fluorescent carbon hybrid nanoparticles
Nanoscale, Jan 24, 2015
This paper reports a type of multifunctional hybrid nanoparticle (NP) composed of gold nanocrysta... more This paper reports a type of multifunctional hybrid nanoparticle (NP) composed of gold nanocrystals coated on and/or embedded in a magnetite-fluorescent porous carbon core-shell NP template (Fe3O4@PC-CDs-Au) for biomedical applications, including magnetic/NIR-responsive drug release, multicolor cell imaging, and enhanced photothermal therapy. The synthesis of the Fe3O4@PC-CDs-Au NPs firstly involves the preparation of core-shell template NPs with magnetite nanocrystals clustered in the cores and fluorescent carbon dots (CDs) embedded in a porous carbon shell, followed by an in situ reduction of silver ions (Ag(+)) loaded in the porous carbon shell and a subsequent replacement of Ag NPs with Au NPs through a galvanic replacement reaction using HAuCl4 as a precursor. The Fe3O4@PC-CDs-Au NPs can enter the intracellular region and light up mouse melanoma B16F10 cells in multicolor mode. The porous carbon shell, anchored with hydrophilic hydroxyl/carboxyl groups, endows the Fe3O4@PC-CDs-...
A metastable phase α-FeSi 2 was epitaxially stabilized on silicon substrate using pulsed laser de... more A metastable phase α-FeSi 2 was epitaxially stabilized on silicon substrate using pulsed laser deposition. Nonmetallic and ferromagnetic behaviors were tailored on α-FeSi 2 (111) thin films, while the bulk material of α-FeSi 2 is metallic and nonmagnetic. The transport property of the films renders two different conducting states with a strong crossover at 50 K which is accompanied by an onset of ferromagnetic transition as well as a substantial magnetoresistance. These experimental results are discussed in terms of the unusual electronic structure of α-FeSi 2 obtained within density functional calculations and Boltzmann transport calculations with and without strain. Our finding sheds light on achieving ferromagnetic semiconductors through both the structure and doping tailoring, and provides an example of a tailored material with rich functionalities for both basic research and practical applications.
High-resolution anisotropic magneto-resistance measurement (AMR) was used to detailed study the t... more High-resolution anisotropic magneto-resistance measurement (AMR) was used to detailed study the training effect in exchange biased CoO/Co bi-layer. The sample was cooled to 10 K from room temperature in the magnetic cooling field of 4000 Oe. Then we used 1500 Oe declined perturbation field to pin the magnetization orientation of the FM layer. The perturbation field forms certain angle Θ with the cooling field direction in-plane to re-induce the untrained state. The dependence of the untrained state on the angle between the direction of perturbation field and cooling field has been investigated. The AMR results reveal that the re-induced degree of untrained state is strongly correlated to the angle Θ. The exchange bias field H E for different Θ has been determined from the AMR results, which is in apparent agreement with the Meiklejohn-Bean model. The recover degree of untrained state is the largest when the angle is 75˚, which is different from the traditional view point that untrained state should be the maximum when it is perpendicular. The training effect is related to the FM spin orientation, which can induce the change of the interfacial AFM spin reorientation with different angles.
While the assembled 1D magnetic nanoparticle (NP) chains have demonstrated synergistic magnetic e... more While the assembled 1D magnetic nanoparticle (NP) chains have demonstrated synergistic magnetic effects from the individual NPs, it is essential to prepare new 1D NP chains that can combine the magnetism with other important material properties for multifunctional applications. This paper reports the fabrication and multifunctional investigation of a new type of 1D NP chains that combine the magnetic properties with fluorescent properties, photothermal conversion ability, and drug carrier function. The building block NPs are composed of magnetic Fe 3 O 4 nanocrystals clustered in the core and fluorescent carbon dots embedded in the mesoporous carbon shell with hydroxyl/carboxyl groups anchored on their surface. These NPs can assemble under the induction of external magnetic field and form stable 1D NP chains of diameter ∼90 nm and length ∼3 μm via the hydrogen bonding and π−π stacking linkage of the carbon shell. The resulted 1D hybrid NP chains not only demonstrate much higher magnetic resonance imaging (MRI) contrasting ability than the dispersed building block NPs, but also enter into intracellular region and light up the B16F10 cells under a laser excitation with strong and stable fluorescence. While the mesoporous carbon shell provides high drug loading capacity, the embedded fluorescent carbon dots convert near-infrared (NIR) light to heat, and hence kill the tumor cells efficiently and enhance the drug release rate to further improve the therapeutic efficacy under NIR irradiation. Such designed 1D magnetic-fluorescent hybrid NP chains with enhanced MRI contrast, fluorescent imaging ability, and combined chemo-/ photothermal therapeutic ability have great potential for various biomedical applications.
Small (Weinheim an der Bergstrasse, Germany), Jan 26, 2015
Anisotropic Fe3 O4 octahedrons are obtained via a simple solvothermal synthesis with appropriate ... more Anisotropic Fe3 O4 octahedrons are obtained via a simple solvothermal synthesis with appropriate sizes for various technological applications. A complete suite of materials characterization methods confirms the magnetite phase for these structures, which exhibit substantial saturation magnetization and intriguing morphologies for a wide range of applications.
Magnetic field induced step-like transport jump is found at low temperature for Pr doped phasesep... more Magnetic field induced step-like transport jump is found at low temperature for Pr doped phaseseparated manganites. Magnetization measurements indicate that spin-stiffness constant D exhibits large magnetic field dependence. Moreover, there exists a remarkable change of spin-stiffness constant corresponding to the variety of the step-like transport jump. This suggests a possible existence of quantum orbital transfer resulting from the change of orbital ordering (OO) modes and accompanying with spin reorientation and carrier delocalization. The carrier delocalization is closely related to the coupling of charge-orbital and can be proved from the low temperature resistivity minimum behavior. The phase diagram was determined. These all show the importance of the strong interaction of charge-orbital and/or spin-orbital in the adjacent long-range charge-ordered antiferromagnetic phase, which depends sensitively on the OO modes.
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Papers by Guixin Cao