Pengcheng Dai's group works at Rice University in the Physics Department on condensed matter physics.

Experimental condensed matter physics uses neutrons as probes to study strongly correlated electron systems. The microscopic origin of high-transition temperature (Tc) superconductivity, magnetism and electron-lattice coupling in high-Tc superconductors. Magnetism and lattice effects in colossal magnetoresistance (CMR) manganese oxides. Quantum criticality in Ru-based oxide materials and other transitional metal oxides. Our work is supported by the National Science Foundation and the Department of Energy.

Updates:

[Feb 12th, 2016]   This week, the PI won 2016 Neutron Scattering Society of America (NSSA) Sustained Research Prize and become a fellow of NSSA (see http://neutronscattering.org/ for details). This would not have been possible for the hard work and collaborative environment of the group over the past decade. I wish to use this opportunity to thanks the students and all our group's collaborators for the success of the group.

[Aug 20th, 2015]   In today's Rev. Mod. Phys, the PI published a paper summarizing the progress in neutron scattering studies of iron based superconductors in the field over the past 7 years. Most of the work in the PI's group was carried out by students and postdoscs, without their contribution, this would not have been possible. See Rev. Mod. Phys. 87, 855 (2015) for details.

[Jul 31st, 2014]   In today's Science Express, our group with the IOP student Xingye Lu as a leading author, published a paper detailing our discovery of pressure effect in sin excitations of iron pnictides. Previous transport measurements have reveal strong resistivity anisotropy and the microscopic origin of this effect is an issue of debate. In the present experiment, we showed that spin excitations in the system also display 90 degree rotational anisotropy at a temperature similar to resistivity anisotropy. Our data suggest that resistivity anisotropy is correlated with spin nematic phase. See DOI: 10.1126/science.1251853. for the paper. Rice also carried a story on this at: See Rice News Story.

[May 28th, 2014]   In Today's PRL, our group published a paper described inelastic neutron scattering determination of the entire spin wave spectra in NaFeAs, the parent compounds of the 111 family of iron-based pnictide superconductors. We find that the spin wave spectra in this family of materials has a narrower magnetic bandwidth. Compared with DMFT calculation, we conclude the pnictogen height is the controlling parameter in determining the electron correlations and the bandwidth of the system (Phys. Rev. Lett. 112, 217202 (2014).).

[Dec 30th, 2013]   In today's PRX, our collaboration with Prof. Yuan Li of Beijing University yielded a nice paper detecting the longitudinal spin excitations in BaFe2As2. Since the discovery of iron pnictide superconductors, it has been debated whether magnetism in these materials arises from local moment or itinerant electrons. There is no dirctly proof, until now, that itinerant electrons indeed have a contribution to the spin waves in the parent compounds of iron pnictides. This is represented as longitudinal spin excitations, which is forbidden in a local moment system. For details, see PRX 3, 041036 (2013).

[Dec 4th, 2013]   In today's Nature Communication, our group published in paper detailing the relationship between the correlations between magnetism and superconductivity in iron pnictides superconductors. In BCS superconductors, high-Tc superconductivity required two important ingredients, a strong electron-lattice coupling and a high Debye temperature for the underlying lattice. By carrying out systematic measurements at ISIS on iron pnictides with different Tc's, we have shown that high-Tc superconductivity in iron pnictides requires both strong electron-spin excitations coupling and large magnetic exchange coupling, similar to BCS superconductors except that magnetism is important for electron pairing. This work took a long time and three dedicated group members to do, and was lead by Meng Wang, Chenglin Zhang, and Xingye Lu. We are also very fortunate to collaborate with ISIS instrument scientists E. A. Goremychkin and Toby Perring as well as Thomas Maier, Zhiping Yin, Kristjan Haule, and Gabriel Kotliar for theoretical calculations. See Nature Communications 4, 2874 (2013) for details.

[Nov 12th, 2013]   The neutron spin resonance, a collective magnetic excitation occurring below the superconducting transition temperature at the antiferromagnetic ordering wave vector, has been studied for more than 20 years. It was initially found in YBa2Cu3O6+x family of copper oxide superconductors, and has since been found in other cuprate superconductors, heavy Fermion superconductors, and more recently in iron-based superconductors. In spite of intensive efforts, its microscopic origin is still an issue under debate. One of the unique features of iron pnictides is that these materials are multiband superconductors. If the neutron spin resonance is indeed related to the superconducting electron pairing, it should be sensitive to the superconducting gap energy anisotropy, which may be different at different bands. In today's PRL (PRL 111, 207002 (2013)), our group published a paper lead by Chenglin Zhang on the discovery of double neutron spin resonance in underdoped NaFeCoAs. This work is in close collaboration with our Rice colleague Qimiao Si's group. The discovery of such a mode suggest inelastic neutron scattering can be used to probe superconducting gap anisotropy.

[Oct 25th, 2013]   In today's PRB paper, we present comprehensive inelastic neutron scattering studies of evolution of spin excitations in electron-doped iron pnictides. We confirmed the earlier results and showed that low-energy spin excitation-itinerant electron coupling is important for superconductivity in these materials. See PRB 88, 144516 (2013).

[Oct 17th, 2013]   In today's PRB paper, our group report the discovery of an in-plane spin excitation anisotropy in the orthorhombic phase of NaFeAs. These results suggest the presence of spin anisotropy already in the paramagnetic state. See PRB 88, 134512 (2013).

[Sep 5th, 2013]   In this week's PRL, we have two papers published in the same issue. The first paper deals with NMR meausrements on pressure dependence of the magnetic susceptibility. The results are consistent with magnetism in this material has two component, itinerant and localized component. Our contribution to the paper is to provide high quality samples and revise the paper. In the second paper, lead by the IOP's Huiqian Luo, is our group's work using polarized neutron scattering to study dynamic susceptibility anisotropy. We discovered an isotropic to anisotropic spin excitation transition at a temperature in the tetragonal phase of Ni-doped Ba122. The outcome suggests that neutron polarization analysis is a good probe of spin-orbit coupling in iron pnictides.

[Aug 9th, 2013]   In early August, our group has officially moved to Rice University from the University of Tennessee. Our first paper using Rice address was published August 9th in PRB with Chenglin Zhang as the leading author. In the paper, we showed that the neutron spin resonance is a sharp excitation in Co-doped NaFeAs. The results are in strong support of s+- symmetry and cannot be explained by s++ electron pairing symmetry.

[Jun 18th, 2013]   In today's PRL, our IOP/UTK graduate student Xingye Lu and collaborators published a paper detailing the electronic and magnetic phase diagram in BaFe2-xNixAs2. These results reveal a avoided magnetic bicritical point near optimal superconductivity and magnetoelastic coupling. They suggest that AF to superconductivity transition is first order like, different from earlier work on Co-doped Ba122 family of materials. See Phys. Rev. Lett. 110, 257001 (2013) .

[Feb 12th, 2013]   In today's Nature Communication, our collaborators at PSI, using resonant inelastic X-ray scattering to study single crystals grown in our lab, published a paper detailing high-energy spin excitations in optimally hole-doped iron pnictide superconductor (Ba,K)Fe2As2. These results reveal that high-energy spin excitations in hole-doped iron pnictide soften compared with the parent compound BaFe2As2. These resulst are amazing as they suggest that X-ray scattering can now compete with neutron scattering in probing high-energy spin excitations. We have also done neutron scattering by mapping out the entire spin excitations spectra in (Ba,K)Fe2As2. A comparison of our results with that of electron-doped, and hole over doped samples will hopefully be published soon (the paper has been submitted sometime ago). See Nature Communications 4, 1470 (2013) .

[Feb 8th, 2013]   In today'sPRB online publication, our graduate student Miaoyin Wang published a paper on studying temperature dependence of spin waves in insulating Rb0.8Fe1.6Se2. This material was thought to be very close to the Mott insulator. Our temperature dependence of the spin wave spectra instead showed that paramagnetic spin excitations in this material is much different from those of BaFe2As2 and FeTe. Therefore, the system is more likely to be a semiconductor. This suggests that superconductivity in alkaline iron selenide may not be near a Mott insulating phase as initially thought. See PRB 87, 064409 (2013) .

[Feb 7th, 2013]   In today'sPRB online publication, our group's Chenglin Zhang published a paper on studying spin excitations in optimally (Ba,K)Fe2As2 using polarized neutron scattering. His key findings are that the low-energy spin excitations in (Ba,K)Fe2As2 are mostly anisotropic and c-axis polarized. Therefore, spin-orbit coupling in optimally hole-doped (Ba,K)Fe2As2 may be important in determining its magnetic properties. See PRB 87, 081101(R) (2013) .

[Dec 1st, 2012]   It has been almost five years since the discovery of iron-based superconductors. The field has made a lot of progress over the period. On Nov. 17th, Profs. Nanlin Wang, Hideo Hosono, and the PI edited a Book on the topic of iron-based superconductors with a title "Iron-based Superconductors - Materials, Properties and Mechanisms". The book is now published by Pan Stanford publishing company . It is also available for puchasing on Amazon.com for $179. The price is rather expensive, but the editors and authors of the book chapters make NO profit whatsoever from the book.

[Nov 29th, 2012]   Today, PI is elected as AAAS fellow, thanks to the hard work of our group's former/current graduate students and postdocs, and also to all collaborators. Without them, none of our group's work would have been possible. See the nice article written by Catherine, who really gives the PI too much credit. Thanks, Catherine.

[Oct 17th, 2012]   In today's PRB, our group (with leading author fromer graduate student Leland Harriger, now at NIST) reports the temperature dependence of paramagnetic spin excitations in BaFe2As2. Our key finding is that paramagnetic scattering in this sytem has very little temperature dependence up to room temperature or 2.1TN. See Phys. Rev. B 86, 140403 (2012).

[Oct 11th, 2012]   In today's PRB paper, our group's IOP student Meng Wang and coworkers published an article describing spin dynamics, transport, and angle resolved photoemission study of superconducting and nonsuperconducting LiFeAs. our key conclusion is that LiFeAs is an intrinsically electron overdoped superconductors, thus LiFeAs is no different from other iron-based superconductors. See Phys. Rev. B 86, 144511 (2012).

[Oct 4th, 2012]   In today's Nature Physics on-line publication, the PI, together with Jiangping Hu and Elbio Dagotto, has published a review article discussing recent progress in magnetism of iron-based superconductors. Since the discovery of antiferromagnetic order in the parent compounds of these materials first by our group, the microscopic origin of the magnetism has been in intensive debates. In the present review aticle, we summarize recent progress and discuss the emerging consensus regarding the magnetism. It seems that magnetism in these materials is due to a combined local and itinerant electrons interacting in these interesting materials. For a detailed discussion of the paper see: See Nature Physics 8, 709-718 (2012).

[Jun 18th, 2012]   In today's Phys. Rev. B, Mengshu Liu, a graduate student in the group, published a paper describing polarized neutron scattering experiment studying electron-overdoped Ba(Fe,Ni)2As2. She found that the magnetic excitations in this materials are isotropic in both the normal and superconducting state. This suggests that the neutron spin resonance is consistent with a singlet-to-triplet excitation scenario. For more details, See Phys. Rev. B 85, 214516 (2012) .

[Jun 12th, 2012]   In today's Phys. Rev. Lett., our group member at IOP, Dr. Huiqian Luo has carried out a systematic measurements on the doping evolution of the AF order in Ni-doped BaFe2As2. Surprisingly, he found that near optimal superconductivity, short-range incommensurate AF order, instead of long -range commensurate AF order, competes with superconductivity. This means there is NO conventional magnetic quantum critical point near optimal superconductivity, as many have speculated. The work also suggest AF order to superconductivity transition is frist-order like, instead of conventional second order phase transition where Neel temperature is suppressed to zero with zero moment. For more details, See Phys. Rev. Lett. 108, 247002 (2012) .

[May 31st, 2012]   In today's Phys. Rev. Lett. Shan Lei of IOP in collaboration with us have carried out a STM experiment on optimally doped (Ba,K)Fe2As2 and discovered an electronic mode in the tunneling spectra very much similar to the neutron spin resonance seen in the same material. This suggests that spin excitations are coupled to the itinerant electrons responsible for superconductivity. This is very much similar to electron-doped cuprate work our group did with Boston college group. So may be pnictides and cuprates have the same magnetic origin for superconductivity after all. For more details, See Phys. Rev. Lett. 108, 227002 (2012) .

[Apr 13th, 2012]   Contragulations to Graduate student Miaoyin Wang and undergraduate student Tucker Netherton for winning Extraordinary Professional Promise Award in Chancellor's Honors program. They have done well. See http://www.phys.utk.edu/news/2012/news_chancellors_04112012.html for details.

[Mar 26th, 2012]   In today's Nature Physics on-line publication, our group (with the leading author graduate student Mengshu Liu) has published a paper completely determined the absolute intensity of magnetic excitations in optimally electron-doped iron-based superconductor BaFe1.9Ni0.1As2. An outstanding question about iron-based superconductors has been whether or not their magnetic characteristics are dominated by itinerant or localized magnetic moments. Absolute measurements and calculations of the magnetic response of undoped and Ni-doped BaFe2As2 indicate the latter. For a detailed discussion of the paper see: See Nature Physics (2012) doi:10.1038/nphys2268. For a general discuss of the meaning of this work, see a nice description by Catherine Longmire of the physics department. .

[Dec 6th, 2011]   In today's Nature Communication on-line only publication, our group (with the leading author graduate student Miaoyin Wang) has published a paper completely determine the effective magnetic exchange couplings in the insulating RbFe1.6Se2 antiferromagnet. This material is particularly interesting because it may have a superconducting microscopic origin different from all other known Fe-based superconductors. For a detailed discussion of the paper see: See Nature Communications 2, 580 (2011) . For a general discuss of the meaning of this work, see a nice description by Catherine Longmire of the physics department. .

[Oct 19th, 2011]   In today's Scientific Reports online publication, we (with Chenglin Zhang and Meng Wang being the leading authors) have finally published our paper on optimally doped (Ba,K)Fe2As2. We have shown experimentally that the spin excitations in hole-doped iron-based superconductors are longitudinally elongated, in contrast with the transverse elongated electron-doped materials. In addition, we have shown that spin excitations for energies below the resonance change its lingshape at Tc. This paper was finished last year and submitted Nature Materials (arXiv.org > cond-mat > arXiv:1012.4065). But after three rounds of referees, it was rejected and transferred to Scientific Reports. Since this is the first single crystal spin excitations measurements on hole-doped iron superconductors, we still believe that the results are important. See Scientific Reports 1, 115 (2011) .

[Aug 24th, 2011]   In today'sPRB online publication, we (with Leland Harriger being the leading author) have finally published our paper concerning the overall spin waves in BaFe2As2, the prototypical FeAs-based parent compound. We have for the first time completely determined the overall spin wave spectra, both below and above T_N. This paper was originally submitted to PRL, but after two rounds of reviewing and an appeal to the associate editor, it is finally deemed not important enough for PRL. However, we firmly believe that the paper is much more important than many of the PRL's we ourself publish. Only time can tell whether the referees or we are correct concerning the importance of this paper. See PRB 84, 054544 (2011) .

[Jun 24th, 2011]   In today'sPRB online publication, our group's IOP student Meng Wang published his first paper on studying spin excitations in LiFeAs. His key findings are that spin excitations in LiFeAs are antiferromagnetic and similar to all other Fe-based superconductors. Therefore, the microscopic origin for superconductivity in LiFeAs may be no different from all other Fe-based superconductors. See PRB 83, 220515(R) (2011) .

[May 23rd, 2011]   In today's Nature Physics online publication, our group in collaboration of Vydia's group Boston College published a paper detailing electron-spin excitation coupling in an electron-doped copper oxide superconductor. Essentially, we had discovered that the evolution of spin excitations as a function of oxygen annealing is in direct association with the evolution of the electronic bosonic modes detected by electron tunneling spectroscopy. This observation essentially indicates that bosons coupled to electrons are spin excitations, thus electron-spin interactions are the mostly likely reason for high-Tc superconductivity in electron-doped cuprates. See doi:10.1038/nphys2006 .

[Mar 14th, 2011]   In today's PRB, Miaoyin Wang, a student of the group, published a PRB on magnetic field effect on static AF order and spin excitations in underdoped BaFeNiAs superconductor. Miaoyin's key findings include the discovery of the magnetic field-induced AF order as a competing order parameter for superconductivity. This paper was chosen as editor's choice for PRB. For more information, See Phys. Rev. B. 83, 094516 (2011).

[Feb 4th, 2011]   In today's PRL, Olly Lipscombe, a postdoc of the group, published a PRL on spin waves of FeTe, the parent compound of FeSe superconductors. Olly's key findings include the discovery that the second nearest neighbor exchange coupling is important for superconductivity in Fe(Te,Se) and FeAs-based superconductors. For more information, See Phys. Rev. Lett. 106, 057004 (2011).

[Oct 4th, 2010]   In today's PRL, Shiliang Li, former postdoc of the group, published his first leading author PRL on FeSeTe family of superconductors. Shiliang's key findings include the discovery of an hourglasslike dispersion in the normal state of FeSeTe and demonstrate that such excitations are not directly associated with superconductivity, and appear to be connected with Fermi surface nesting of the FeSeTe system. For more information, See Phys. Rev. Lett. 105, 157002 (2010).

[Aug 18th, 2010]   In Today's Phys. Rev. B, our group published a paper describing the first polarized neutron scattering experiment on the neutron spin resonance in BaFeNiAs superconductor. Surprisingly, we found that the resonance in FeAs-based superconductors are not isotropic in space, and rather it only appears in moment paralletl to the FeAs plane. These results suggest that mode is NOT the long-sought singlet-to-triplet excitation. This paper was choosen as editor's choice at PRB. For more information, see See Phys. Rev. B 82, 064515 (2010).

[May 12th, 2010]   In Today's Phys. Rev. B, our group published a paper describing c-axis magnetic field effect on the neutron spin resonance in BaFeNiAs superconductor. Our experiment was the first that showed a c-axis aligned magnetic field can shift the energy of the resonance downward, consistent with the idea that the mode is probing superconducting electronic gap. This paper was choosen as editor's choice at PRB. For more information, see See Phys. Rev. B 8, 180505(R) (2010).

[Jan 8th, 2010]   In today's Phys. Rev. Lett. on line publication, our group (with postdoc Clarina de la Cruz the leading author) reports the electronic phase phase diagram of CeFeAsPO as a function of P-doping. The our key finding in the paper is the presence of a magnetic quantum critical point in the phase diagram of CeFeAs1-xPxO series of materials. These results suggest quantum critical point may have something to do with the exotic electronic properties of the materials. See Phys. Rev. Lett. 104, 017204 (2010). for details.

[Dec 31st, 2009]   Contragulations to Jun Zhao, the current graduate student of the group, for just winning the Miller Fellowship at UC-Berkeley. Miller Fellowship is probably the most prestigeous named postdoc fellowship in the country. Jun's work speaks for itself, and he is certainly deserving of the Miller Fellowship. Happy new year, all group members, from the PI in Beijing. For more information on Miller institute and Miller Fellowships, please See Miller Institute for details.

[Oct 14th, 2009]   In today's Phys. Rev. Lett. on line publication, our group in collaboration with Dr. Feng Ye of ORNL report our elastic neutron scattering work on layered PrCaMnO4 manganites. The key point of the paper is that electron-doping the material will phase separate from rest of the charge/orbital ordered CE structure. Electronic phase separation is also seen in cuprates near 1/8 doping. See Phys. Rev. Lett. 103, 167202 (2009). for details.

[Oct 6th, 2009]   Congratulations to senior graduate student Jun Zhao, who just won GMAG (Topical Group on Magnetism and its applications) Student Dissertation Award from APS. This is the second award to a graduate student from the group. Fromer graduate student Stephen Wilson won this award in 2006-2007, and will become junior faculty in Boston College from January 2010. Jun will give an invited talk at APS March meeting in Portland 2010, receive a $500 prize and up to an additional $250 in travel expenses to attend the meeting.

[Oct 5th, 2009]   The nature paper our group published last year on the discovery of static antiferromagnetic order in LaFeAsO (Nature 453, 899 (2008)) has been cited 353 time over the course of past year and is now designated as a "Current Classics" Mulitdisciplinary paper for June 2009 by Thomson Reuters ScienceWach.com. See for MP3 podcast on the paper in sciencewatch.com.

[Sep 18th, 2009]   Recently, the former postdoc in the group Shiliang Li and PI wrote a short review article summarizing recent progress in FeAs-based superconductors. These results are published in the popular Chinese Physics magazine in Chinese, Wu li 38, 644-650 (2009).

[Aug 20th, 2009]   In today's Phys. Rev. Lett. on line publication, our group (with leading author graduate student Leland Harriger) report our inelastic neutron scattering work on lightly Ni-doped BaFe2As2. Essentially, what we found was that the c-axis magnetic exchange coupling decreases very rapidly with increasing electron-doping. This may be the reason why structural and magnetic phase transitions are separated as soon as electrons are doped into their parent compounds. See Phys. Rev. Lett. 103, 087005 (2009). for details.

[Jul 13th, 2009]   In today's Nature Physics Advanced Online Publication, our group (with leading author graduate student Jun Zhao) published a paper completely determine the magnetic exchange coupling in one of the parent compounds of FeAs based superconductors. The material we focused on is CaFe2As2. The central debate in understanding the magnetism in these materials has been whether it is more appropriate to use localized or itinerant point of view to describe it. In our paper, we showed that answer may be both are needed in order to understand the spin wave excitations in these materials. This work establishes the basis from which future superconductivity and electron/hole doping induced changes can be studies. See doi:10.1038/nphys1336 .

[Jul 9th, 2009]   This week, our group reports our elastic neutron scattering experiment on NaFeAs (111) family of FeAs-based superconductors. We found that collinear antiferromagnetic structure is a common feature for the parent compounds of all FeAs-based superconductors. For more details, see See Phys. Rev. B 80, 020504(R) (2009).

[May 20th, 2009]   A new review article by PI's long term collaborator Jeff Lynn and PI had just appeared in Physica C 469, 469 (2009). This work summarizes the magnetic neutron scattering work on iron arsenide superconductors over the past 12 months. See group's publication for more details.

[Mar 13th, 2009]   In a new paper entitled "Inelastic Neutron-Scattering Measurements of a Three-Dimensional Spin Resonance in the FeAs-Based BaFe1:9Ni0:1As2 Superconductor" by Chi et al. from the group, we report our discovery of three-dimensional neutron spin resonance. This is anologous to the famous 41 meV neutron resonance seen in cuprates. For details, see: Phys. Rev. Lett. 102, 107006 (2009) .

[Dec 7th, 2008]   Recently, the PI has been elected as Fellow of American Physical Society with the citation “For his contribution to understand fundamental properties of magnetic excitations in high-transition temperature superconductors, f-electron heavy Fermions, and colossal magneto-resistance manganites.” Again, this would not have been possible without the dedicated hardwork of my current/former students/postdoc, and collaborators over the years. See http://www.aps.org/programs/honors/fellowships/archive-all.cfm?initial=&year=2008. for details and other elected fellows.

[Nov 21st, 2008]   In a new paper published in the Nov. 21 issue of PRL, we report our determination of the Ce crystal electric field level and its relationship with superconductivity in CeFeAsOF. The leading author of this paper is graduate student Songxue Chi, who has defended his Ph. D thesis and will graduate in December, 2008. He will be working as an instrument scientist at NIST center for neutron research from January, 2009. See Phys. Rev. Lett. 101, 217002 (2008).

[Oct 26th, 2008]   In a new paper published online Oct. 26, 2008, in Nature Materials, we report our determination of the structural and magnetic phase diagram of CeFeAsO_{1-x}F_x as a function of F-doping x. The leading author of this paper is graduate student Jun Zhao. The phase diagram of this material shows considerable similarities with the high-Tc cuprate superconductors. These results are an important addition to the effort to find out where superconductivity in these iron–arsenic alloys arises. See Nature Materials 7, 953-959 (2008). for the paper, and Nature Materials 7, 927-928 (2008). for news and view by Steve Kivelson and Yong Yao.

[Oct 15th, 2008]   A new paper from the group lead by graduate student Jun Zhao just appeared in PRL. In this paper, we describe first inelastic neutron scattering experiment designed to study the spin wave excitations in the parent compound of FeAs-based superconductor SrFe2As2. Our group has been active in studying these materials since their initial discovery some 6 months ago. See Phys. Rev. Lett. 101, 167203 (2008)). for more details.

[Oct 14th, 2008]   Congratulations to Leland Harriger who passed the department Ph. D qualifying exam and can now focus on his Ph. D thesis work. We also want to welcome two new graduate students joining the group. They are Ms. Mengshu Liu from Zhengzhou University (where PI got his BS degree some 24 years ago) and Mr. Miaoyin Wang from Shanghai Jiao Tong University. Welcome is also due to Dr. Chenglin Zhang, who just joined us after obtaining his Ph. D degree from Rutgers University working with Sang Cheong.

[Oct 7th, 2008]   From September 1, 2008, the PI is promoted to the Joint Institute for Advanced Materials (JIAM) Chair of Excellence at the University of Tennessee and Oak Ridge National Laboratory. This would not have been possible wihout the hard work of my former and current students/postdocs from them the PI has learned and continuous to learn a great deal. It is their work that made this possible.

[Aug 7th, 2008]   Our group has recently published three papers in Physical Review B summarizing progress made on FeAs-based superconductors. In the most recent paper with graduate student Jun Zhao being the leading author, we use elastic neutron scattering to determine the magnetic structure of SrFe2As2. This work [PRB 78, 140504(R) (2008)] was chosen as editor's choice for PRB.

[May 28th, 2008]   Understanding superconductivity has been a long term goal of the condensed matter physicists. The recent discovery of superconductivity in Fe-based oxypnictides has reinvigorated the field of high-Tc superconductivity. On May 28 issue of advance on line publication, our group reports the discovery of static antiferromagnetic order in the parent compound of the nonsuperconducting LOFeAs, and showed that the static order disappears when the system is doped to induce superconductivity. This phenomenon is very reminiscent of that the copper oxide superconductors, a class of high-Tc superconducting materials that were discovered in 1986, but its superconductivity mechanism is still elusive in spite of more than 20 years of work. With another class of material that exhibits high-Tc superconductivity, condensed matter physicists can finally compare and contrast these two classes of materials and ask what are the key ingredients for high-Tc superconductivity. In the present paper published by Nature advance online publication, doi: 10.1038/nature07057 , we find the magnetism aspect of the new superconductors to have remarkably similar to those of cuprates. Of course, more work has to be done to sort out the detailed comparison of these two classes of superconductors. For more information, see UT press release ; NIST press release ; and Physics department story .

[Apr 9th, 2008]   On Chancellor's honor night, April 9th, both the PI and graduate student Songxue Chi have been recognized for research work on strongly correlated electron materials. The PI won the "Research and Creative Achievement" award while Songxue was recognized as having "Extraordinary Professional Promise". See Research and Creative Achievement and Extraordinary Professional Promise

[Apr 7th, 2008]   In a new paper recently posted on cond-mat, we report the discovery of static antiferromagnetic order in the parent compound of the newly discovered FeAs based superconductor LaOFeAs, and the static AF order disappears when the system becomes superconducting. This work is lead by postdoc Clarina dela Cruz in collaboration with NIST/AMES scientists (Drs. Q. Huang, J. W. Lynn, J. Zarestsky) and Prof. Nanlin Wang's group at the institute of Physics, Chinese Academy of Sciences. See: arXiv: 0804.0795

[Mar 25th, 2008]   A new paper from the group lead by Songxue Chi describing a detailed neutron scattering experiment and theory to determine the crystal electric field levles on PrOsAs has been published in Phys. Rev. B 77, 094428 (2008) .

[Mar 25th, 2008]   A new paper from the group lead by Songxue Chi describing a detailed neutron scattering experiment and theory to determine the crystal electric field levles on PrOsAs has been published in Phys. Rev. B 77, 094428 (2008) .

[Dec 18th, 2007]   A new paper from the group lead by Shiliang Li describing a detailed neutron scattering experiment on spin excitations of an underdoped superconducting YBa2Cu3O6.45 (Tc=48 K) has been published in Phys. Rev. B 77, 014523 (2008) . We find that the low-temperature spin excitations in YBCO family of compounds change dramatically when the system is tuned across the metal-to-insulator crossover. Our results suggest that the spin resonance and spin gap in YBCO might be a consequence of metallic ground state superconductors.

[Dec 13th, 2007]   Understanding the mechanism of high-Tc superconductivity in doped copper oxides remains one of the most daunting problems in condensed matter physics. Perhaps the most debated current issue in the field of high-Tc superconductivity is the microscopic origin of the superconducting 'glue' that binds electrons into superconducting pairs. While the interaction that pairs the electrons to form the superconducting state is mediated by lattice vibrations (phonons) in traditional superconductors, the leading contenders in the case of high-Tc superconductors are lattice vibrations (phonons) and spin excitations, with the additional possibility of pairing without mediators. Using scanning tunneling microscopy, our collaborators in Boston College and us report spatially resolved, reproducible spectroscopy of the electron-doped superconductor PLCCO that reveal a collective mode in the material's electronic excitations at ~10.5 meV [See Nature 450, 1058-1061 (2007).]. This is consistent with magnetic neutron resonance observed earlier on the same sample by our group and inconsistent with oxygen optical phonon modes [see Nature 442, 59 (2006) and update July 2006]. Our results thus suggest an electronic origin of the mode — and possibly the superconducting 'glue' — consistent with the involvement of spin-excitations rather than phonons.

[Oct 11th, 2007]   A new paper discussing the Fermi-surface evolution as a function of annealing in electron-doped cuprates has just appeared in PRL. This is a collaborative work between our group and the group of Hong Ding at Boston College. See See PRL 99, 157002 (2007).

[Sep 21st, 2007]   A new paper from the group on the evolution of quantum spin excitations through field-induced superconducting-normal state phase transition has just been published in PNAS. See PNAS 104, 15259 (2007).

[Jul 5th, 2007]   A new paper with Jun Zhao as the leading author describing the observation of the neutron resonance in electron-doped Nd1.85Ce0.15CuO4 has now been published in Phys. Rev. Lett. This paper follows the group's discovery of the mode last year in electron-doped Pr0.88LaCe0.12CuO4, thus proving the mode is a general feature of optimally electron-doped materials. See Phys. Rev. Lett. 99, 017001 (2007) for more details.

[Jul 4th, 2007]   A new review article lead by group's long time collaborator Prof. Jiandi Zhang of FIU has just been published in J. Phys.: Condens. Matt. This paper summarizes the our collaborative efforts to understand spin excitations in Colossal magnetoresistance manganites. See J. Phys. Condens. Matter 19, 315204 (2007). for more details.

[Jun 20th, 2007]   A new paper lead by graduate student Songxue Chi on spin-lattice coupling in layered CMR manganite Pr0.5Ca1.5MnO4 has just been published by PNAS. This is a part of Songxue's Ph. D thesis, which is to focus on layered manganites and filled skutterudite PrOs4As12. see: PNAS 104, 11796-11801 (2007).

[Apr 23rd, 2007]   Congratulations to Stephen Wilson for winning the Fowler-Marion Outstanding Graduate Student Award at the physics department's annunal honor's day. This award recognizes a senior graduate student for his outstanding Ph. D thesis work and citizenship.

[Apr 7th, 2007]   A paper describing spin wave excitations of CMR manganites La1-xCaxMnO3 (x=0.25, 0.3) throughout Brillouin zone has been published in PRB. This work completes the group's efforts over the past 9 years to study spin excitations in three-dimensional CMR materials. For more details see PRB 75, 144407 (2007).

[Mar 30th, 2007]   Congratulations to graduate student Stephen Wilson, who has successfully defended his Ph. D thesis on March 30th. Stephen will work with Bob Birgeneau at UC-Berkeley and LBL from June 1, 2007.

[Jan 17th, 2007]   A new paper entitled " Microscopic annealing process and its impact on superconductivity in T'-structure electron-doped copper oxides" from the group lead by former student Hye Jung Kang has been published as an article in Nature Materials. See Nature Materials 6, 224-229 (2007) for article and for News and Views by Hidenori Takagi.

[Nov 20th, 2006]   Congratulations to graduate student Stephen Wilson for winning the 2006-2007 GMAG (Topical Group on Magnetism and its applications) Student Dissertation Award. Stephen will give an invited talk at APS March meeting in Denver, receive a $500 prize and up to an additional $250 in travel expenses to attend the meeting.

[Oct 31st, 2006]   A new short review article to celebrate 20 years since the discovery of high-transition temperature superconductors, written in Chinese by PI and postdoc Shiliang Li, was published in Physics, a Chinese popular phyiscs monthly journal. See Wuli 35, 837 (2006) .

[Aug 28th, 2006]   A new paper entitled "Evolution of the low-energy spin dynamics in electron-doped high-transition temperature superconductor Pr0.88LaCe0.12CuO4" by Wilson et al has been published in PRB. For a reprint, see: Phys. Rev. B 74, 144514 (2006) .

[Aug 11th, 2006]   A new paper entitled "Magnetic energy change available to superconducting condensation in optimally doped YBa2Cu3O6.95" by Hyungje Woo, Pengcheng Dai, S. M. Hayden, H. A. Mook, T. Dahm, D. J. Scalapino, T. G. Perring, and F. Dogan has been accepted by journal Nature Physics and will appear shortly. For complete paper, click here.

[Jul 5th, 2006]   A new paper from the group on discovery of the magnetic resonance in electron-doped high-Tc superconductor with graduate student Stephen Wilson as the lead author just appeared in journal Nature; See: Nature 442, 59 (2006) . For press releases from NIST and UT see: NIST press release and UT press release .

[Jul 3rd, 2006]   The PI's, Pengcheng Dai's, promotion to the rank of Professor of Physics and Astronomy at The University of Tennessee has been approved by the Board of Trustees and will become effective on August 1, 2006.

[Jun 22nd, 2006]   A new paper on spontaneous spin-lattice coupling in CuFeO2 by Feng Ye, a close collaborator of the group, had just appeared in PRB as a Rapid Communication. See F. Ye et al., Phys. Rev. B 73, 220404(R) (2006).

[May 1st, 2006]   Our group's collaboration paper with Prof. Brian Maple's group at UCSD on electronic and magnetic properties of PrOs4As12 just appeared in the inaugural article by Maple et al. in Proceedings of the National Academy of Sciences (PNAS). See:PNAS 103, 6783-6789 (2006).

[Apr 28th, 2006]   Graduate student Jun Zhao won the Paul H. Stelson Fellowship in Physics for Professonal promise in Physics hornor's day.

[Apr 24th, 2006]   A new paper on spin excitations of PrLaCeCuO4 from the group with graduate student Stephen Wilson as the lead author just appeared in PRL. See: Phys. Rev. Lett. 96, 157001 (2006).

[Jan 30th, 2006]   A new paper on CMR manganite from the group just appeared in PRL. See: Phys. Rev. Lett. 96, 047204 (2006).

[Jan 27th, 2006]   New graduate student Chad Bircher joined group after passing his qualifying exam.

[Aug 10th, 2005]   New student Jun Zhao joined our group.

[Aug 5th, 2005]   New website is developed by Serena Dai and Alice Gu.

[Jul 20th, 2005]   Pengcheng has given a talk at UCSB: Microscopic Evidence for a Magnetic Quantum Critical Point in Electron Doped Superconductivity .

[Jun 1st, 2005]   Electronically competing phases and their magnetic field dependence in electron-doped nonsuperconducting and superconducting Pr0.88 LaCe0.12 CuO4± delta is published in Phys. Rev. B 71, 214512 (2005).

[May, 2005]   Hyejung Kang was awarded her Ph. D and now is working at NIST.

[Mar 1st, 2005]   Electronic inhomogeneity and competing phases in electron-doped superconducting Pr0.88 La Ce0.12 Cu O4- delta is published in Phys. Rev. B 71, 100502 (2005).

[Feb 11th, 2005]   Quantum Critical Scaling and origin of Non-Fermi-Liquid behavior in Sc1-xUxPd3 is published in Phys. Rev. Lett. 94, 056402 (2005).

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Created by Serena Dai and Alice Gu, 2005. Updated October 2013 by Yu Song.