計算凝聚態(tài)物理研究課件

1、計算凝聚態(tài)物理研究HPCfirst principles calculationsmy work in computational physics1946年2月15日，第一臺電子數(shù)字計算機ENIAC問世，這標志著計算機時代的到來。時鐘頻率100 KHz，能在1秒鐘的時間內(nèi)完成5000次加法運算。Jaguar, which is located at the Department of Energys Oak Ridge Leadership Computing Facility and was upgraded earlier this year, posted a 1.75 petaflop

2、/s performance speed running the Linpack benchmark. Jaguar roared ahead with new processors bringing the theoretical peak capability to 2.3 petaflop/s and nearly a quarter of a million cores. One petaflop/s refers to one quadrillion calculations per second. (Year 2009, being No. 1 till now)US DOE AS

3、CI systems are claiming the first four positions of the TOP500. The new IBM ASCI White system at Lawrence Livermore National Laboratory is the new number one with 4.9 TFlop/s Linpack performance. This system is built with 512 nodes, each of which contains 16 IBM Power3 processors using a shared memo

4、ry. This type of hierarchical architecture is more and more common for systems used in HPC .(Year 2000)*11月15日21日在美國菲尼克斯舉行的SC2003超級計算機會議上，第22屆國際高性能計算機性能TOP500排行榜如期揭曉。從榜單上看，雖然第一名仍被基于矢量技術的日本地球模擬器以35.86Tflop/s的Linpack性能值穩(wěn)占，第二名也依然是Linpack值為13.88Tflop/s、安裝于美國能源部洛斯阿拉莫斯國家實驗室的ASCI Q，但是從第三名開始便引出了本屆排行榜的第一個亮點機

5、群（Cluster）系統(tǒng)的提升，包括最高名次的提升和總數(shù)量的提升。 A Chinese system called Nebulae, build from a Dawning TC3600 Blade system with Intel X5650 processors and NVidia Tesla C2050 GPUs is now the fastest in theoretical peak performance at 2.98 PFlop/s and No. 2 with a Linpack performance of 1.271 PFlop/s. This is the hi

6、ghest rank a Chinese system ever achieved. There are now 2 Chinese systems in the TOP10 and 24 in the TOP500 overall. Top 10 in 2010應用是最欠缺的意味機會也最多My cluster at School of Physics系統(tǒng)總的雙精度浮點峰值理論性能達24萬億次每秒（2.4T）(峰值計算能力公式=主頻*每個時鐘周期執(zhí)行指令數(shù)*CPU核數(shù)*系統(tǒng)CPU總數(shù);即,2.66*4*4*56=2384Gflop) One of the applications in sci

7、entific research-ab initio calculations of condensed matter*simulations in other scales: molecular dynamicsfinite element methodFirst Principles Calculations Based on Density Functional Theory ( DFT )What does “first principles” mean?-Also called ab initio calculations-Means from the beginning-No pa

8、rameters from experiments usedWhat can we calculate?-Electronic structure of condensed matter-magnetic properties-optical properties-structural optimization of defects-molecular dynamics simulationsHistory of Density Functional TheoryWALTER KOHN JOHN A. POPLE 1998 Nobel Laureate in Chemistry for his

9、 development of computational methods in quantum chemistry 1998 Nobel Laureate in Chemistry for his development of the density-functional theory History of Density Functional TheoryBust of Schrdinger, in the courtyard arcade of the main building, University of Vienna, Austria.The Schrodinger Equatio

10、n (in 1926):Or the static one:For hydrogen atom:History of Density Functional TheoryH2 (or He) with two electronsearly approachesHeitler and London (1927)Mullikan (1928)James and Coolidge (1933, the most successful ) depends on M parameters M33N , N is the number of electrons M can be significantly

11、reducedSystem with more electrons (e.g. N=100)M3300 10150-Density functional theory provides just a solution to such large systemsHistory of Density Functional TheoryThe basic lemma of Hohenberg-KohnThe ground state density n(r) of a bound system of interacting electrons in some external potential v

12、(r) determines this potential uniquely.or v(r) = Fn(r) or to say: n(r) completely characterizes the systemthe system is completely specified by n(r)The Hohenberg-Kohn Variational Principle or where History of Density Functional TheoryVariational principle: E = min En(r) We have single electron equat

13、ion (Kohn-Sham): andWhereThe total energy of the system:Approximation for Excn(r) The local density approximation (LDA)Beyond the local density approximationGGA (generalized gradient approximation)meta-GGAHybrid functionals that includes the HF exact exchangeFor a uniform electron gas:Simple but the

14、 results are surprisingly usefulAbout the VASP code1. Written by Georg Kresse, Martijn Marsman, and Jurgen FurthmullerComputational Physics, Faculty of PhysicsUniversitat WienSensengasse 8, A-1130 Wien, Austria2. http:/cms.mpi.univie.ac.at/VASP/3. CharacteristicsThe code is easy to use: just prepare

15、 four files for each calculationThe system that modeled can be as large as having several hundred atomsThe results are reliableIt is ever developing to include more functionsThe code is well parallel programmed so it runs fast enoughIt is used worldwideProjects in my groupDoping of graphenehydrogen

16、fuel cellPt catalystcarbon alloy to replace Ptwhich kind of structure is the keyhow to realize itGrowth mechanism of semiconductorshow to dope themhow to reduce the effect of extended defectsConductivity at complex oxide interfacesurface effectsdoping effectsMy recent works in the material research1

17、. Passivation of the grain boundaries (dislocations) in semiconductors2. Enhancing dopant solubility via epitaxial surfactant growth3. Extra large hydrogen bonding at the solid-solution interfacePassivation of the grain boundaries (dislocations) in semiconductors Why the passivation is important?The

18、 band gap of CdTe is 1.45 eV, ideal for solar energy.Conversion efficiency record: 16.5%.Back contactLarge density ofgrain boundariesDetrimental effect of grain boundaries-recombination center Valence BandConduction BandEgValence BandConduction BandPassivation of the grain boundaries (dislocations)

19、in semiconductors Why passivation is important?The band gap of CdTe is 1.45 eV, ideal for solar energy.Conversion efficiency record: 16.5%.Back contactLarge density ofgrain boundariesCdCl2 heat treatmentCu from the back contact Atomic model of the grain boundariesCdTeCd-core112111S2I2S7S1I1S4S6S3S5e

20、ach core is mirror symmetricThe cores are periodic along 111In Te-core, the Te atms at S1 and Cd at S2 both form dimersIn Cd-core the Cd at S4 and Te at S5 do not form dimersFormed by incorporating two (111) surfacesTe at S3 (Cd at S6) is 5-coordinatedTe at S1 (Cd at S4) is 4(3)-coordinatedCd at S2

21、(Te at S5) is 4(3)-coordinated39Te-coreDOS of the grain boundaries I IIITe-coreCd-core II112111States below CBMStates above VBMDeep statesIIIIIICBMVBMDOSS2I2S7S1I1S4S6S3S5Both Cl and Cu prefer the GBs than bulkCl and Cu both favor in the Te-coreCl favors substitutional (S1) other than interstitialCu

22、 favors substitutional (S2) other than interstitialWhere do the impurities locate?Te-coreCd-corePassivation effect of ClIII IIICBMVBMTe-coreClCd-coreS2S7S1Only half of the Te atoms at S1 are needed to be substitutedThe deep states at S2 and S7 are not affected. The passivation is localized. Passivat

23、ion effect of ClII IIICBMVBMIClCd-coreIIIIIICBMVBMClCd-coreI2S7Cl cant passivate Cd-core.Partial charge density of the deep states in region IIPassivation effect of CuIICBMVBMTe-coreClCuCd-coreCd-coreTe-coreClCuIThe passivation of the deep levels in Te-core is complete.Passivation effect of CuIIIIIC

24、BMVBMICuCd-coreCu cant passivate Cd-core.ResultsThe passivations of Cl and Cu are successful in Te-core but not in Cd-coreFor high efficiency solar cells, Te-core may dominateTo completely passivate the Te-core, Cl and Cu are both neededCo-passivation might be applied to other materials like pc-Si a

25、nd pc-GaAsFor detail, please refer to Phys. Rev. Lett. 101, 155501 (2008). Enhancing dopant solubility via epitaxial surfactant growthWith Sb or Bi at the surface, the density of Zn dopant can increase by an order of magnitude.GaP:Zn (100)Sb(Bi)Howard, Chapman, and Stringfellow, J. Appl. Phys. 100,

26、044904 (2006).Zhu, Liu, and Stringfellow, Phys. Rev. Lett. 101, 196103 (2008) H(a) -P(b) -P,H(d) -Sb,HGaPZnGaHSbSbZnGaHAtomic models of the dual-surfactant effectSb(c) -SbFormation energies of the substitutional Zn in sublayerSurfacesFormation energy (Ef) in eVP-terminated (-P)= 2.24 + mGa - mZnSb-t

27、erminated (-Sb)= 2.59 + mGa - mZnP-terminated with H (-P, H)= 2.63 + mGa - mZnSb-terminated with H (-Sb, H)= 1.78 + mGa - mZnBulk= 2.80 + mGa - mZn Band structures of the surfaces Dashed lines are Fermi levels Energy zero is the VBM of GaP Bands A and B are localized surface states In P-, they are s

28、hallow states and locate around VBM In Sb-, these states are much higher but empty In Sb,H-, they are totally filled The electronic origin of the dual-surfactant effectThe surfactant Sb provides the deep levelsH will pin the Fermi level as high as possible (close to CBM)The level of the Zn substitut

29、ional is always around VBMThe higher the Fermi level locates, the more energy the charge transfer gains. This makes the Zn incorporation more favorable. VBCBZnGaSb A general rule?The key of this concept is to find the appropriate surfactants that generate high (low) levels that can transfer electron

30、s (holes) to dopant acceptor (donor) levels in p-type (n-type) doping, thus significantly lowering the formation energy of dopant defects. Sole surfactant Te induced enhancement of N solubility in ZnSe The Te induced states are filledNo H is neededConsistent with the experiments, see e.g., Gu et al,

31、 J. Electron. Mater. 31, 799 (2002)Lin et al, , Appl. Phys. Lett. 76, 2205 (2000). Surfactants for enhancing p-type doing of ZnO with epitaxial growth p-type ZnO can be realized if the solubility of Ag or Cu can be significantly increasedS, Se, or Te may act as surfactantsH helpsDetail: Phys. Rev. B

32、 80, 073305 (2009) Extra large hydrogen bonding at the solid-solution interfaceMultiple Exciton Generation (MEG) effect for nano crystalsShape control is an important issue in nano crystals growth.Facts of PbS:Eg 0.4 eV with Rocksalt structure (NaCl)(100) is the natural cleavage surface Lee et al, J

33、ACS, 2002(100)(111)Why PbS ?Energies of PbS surfaces in vacuum(100)(110)(111)*SPb0.058(100)(111)(110)(eV/2)*For (111) data, it is for a pair of Pb- and S-terminated surfacesDependence of surface energy on the growth conditionmS=0, S in bulk elemental phasemS=-1.13, Pb in bulk elemental phaseAdsorption of CH3NH2S-2S-1Pb(100)(111)1.85NCHsurfaceCoverageAdsorption energy(eV/molecule)(111) ML0.80 ML0.55(100) ML0.45 ML0.34Adsorption of methylamine is not enough to convert the energy order of PbS surfacesDependence of surface energy on the growth conditionmS=0, S in bulk elemental phasemS