a r X i v :c o n d -m a t /0606514v 1 [c o n d -m a t .m e s -h a l l ] 20 J u n 2006
Magnetic-?eld-induced binding of few-electron systems in shallow quantum dots
B.Szafran,S.Bednarek
Faculty of Physics and Applied Computer Science,
AGH University of Science and Technology,PL-30059Krak′o w,Poland
F.M.Peeters
Departement Fysica,Universiteit Antwerpen,Groenenborgerlaan 171,B-2020Antwerpen,Belgium
(Dated:February 6,2008)Binding of few-electron systems in two-dimensional potential cavities in the presence of an external magnetic ?eld is studied with the exact diagonalization approach.We demonstrate that for shallow cavities the few-electron system becomes bound only under the application of a strong magnetic ?eld.The critical value of the depth of the cavity allowing the formation of a bound state decreases with magnetic ?eld in a non-smooth fashion,due to the increasing angular momentum of the ?rst bound state.In the high magnetic ?eld limit the binding energies and the critical values for the depth of the potential cavity allowing the formation of a bound system tend to the classical values.
PACS numbers:https://www.doczj.com/doc/b81267730.html,,73.43.-f,85.35.Be
Quantum dots [1](QDs)formed by weak con?nement potentials [2]are susceptible to perturbations by external charge defects and interface roughness [3].The spectra of disordered QDs [2,4]exhibit complex behavior in an external magnetic ?eld including pairing and bunching of the charging lines,indicating the formation of multiple charged puddles in local potential minima inside the large QD area.In the quantum Hall e?ect disorder is held re-sponsible for the formation of localized electron reservoirs that allows for the pinning of the Fermi level between the Landau levels [5].Coulomb blockade of charge con?ned within these tiny reservoirs in quantum Hall systems have recently been demonstrated [6]through scanning probe measurements.In this paper we consider electrons local-ized in a nanometer-size reservoir created by a potential ?uctuation.The purpose of the present work is to de-termine the role of the magnetic ?eld in the formation of bound few-electron states.
The present study is related to the problem of the ex-istence of bound excited states of the negative hydrogen ion H ?or charged donor center D ?in a semiconduc-tor.In the absence of a magnetic ?eld the H ?has only one bound state [7],but an in?nite number of weakly bound and arbitrarily extended excited states appears in weak magnetic ?eld (B →0+)due to the interplay of the magnetic ?eld induced localization and the attractive force exerted on the extra electron by the neutral atom H ,or donor D 0[8].This force results from the elec-trostatic potential of D 0which is attractive everywhere [8].In two-dimensional quantum wells the D 0potential for the extra electron is repulsive at large distances [9]which prevents the formation of extended excited bound states in the weak magnetic ?eld limit.For o?-center D ?in quantum wells magnetic ?eld induces angular mo-mentum transitions are found [10],well-known from the theory of QDs [11].States with angular momenta L >4cease to be bound which results in magnetic ?eld induced
D ?evaporation [10].
In the symmetric gauge the Hamiltonian of the elec-tron in the magnetic ?eld (0,0,B )contains a diamag-netic term V d (x,y )=mω2c (x 2
+y 2)/8similar to the two-dimensional harmonic oscillator potential with the cy-clotron frequency ωc =eB/m .Due to the parabolic form of V d one often assumes [12]that a strong magnetic ?eld overcomes the Coulomb repulsion between the electrons and makes them orbit around the center of mass even in the absence of an external con?nement potential.Below we demonstrate,that although indeed the magnetic ?eld facilitates the binding of few-electron states,even in the B →∞limit a ?nite external potential is necessary to stabilize the system.Moreover,we show that the po-tential parameters stabilizing the system in the B →∞limit can be determined using classical physics.
For the purpose of the present study we need a con-?nement model realistic enough to account for the un-bound states.We choose the QD model of a shallow two-dimensional Gaussian well.The Gaussian potential is not only the simplest choice but is also realized in verti-cal QDs [13]with small [14]gates.The discussed physics is not limited to a Gaussian potential and is independent of the speci?c choice of the con?nement provided that it has a ?nite size and depth.A weak magnetic ?eld in-duces an in?nite number of single-electron states weakly bound in the cavity.On the other hand the binding of extended two-electron states at weak magnetic ?elds is excluded since the second electron outside the cavity per-ceives only the Coulomb repulsion of the electron bound in it.In this paper we look for the e?ect of the magnetic ?eld that overcomes the Coulomb blockade and allows binding of additional electrons in the cavity.This e?ect can,for example,be used to entangle spins [15]of the electrons localized in the cavity and the unbound elec-trons.
Hamiltonian of a single electron with the magnetic
2?eld(0,0,B)perpendicular to the plane of con?nement
reads(symmetric gauge),H=?ˉh2?2/2m+V d(x,y)?
1
3
tion to a more weakly localized state of lower interaction energy is more favorable.At
high magnetic ?elds the angular momentum transitions [11]keep the size of the system close to the size of its classical counterpart [19].The angular momentum transitions are associated with the ground-state spin oscillations.States with even L correspond to spin singlets (S =0),and those with odd L to spin triplets (S =1).
For V 0=5.4meV bound states appear only in the presence of an external magnetic ?eld (see Fig.2).Notice that,for shallower QDs the ?rst bound state has non-zero L .We found that states of speci?c L are bound only within a ?nite interval of the magnetic ?eld (see the binding energy for L =3at V 0=5.4meV plotted by the dashed line).
0.0
0.1
0.2
0.3
0.4
0.5
1 / B [ 1/ T ]
0.00.2
0.4
0.6
E B - E B
c l a s s
[m e V ]
6.5V 0
= 7 meV
65.4
4.8
4.4
4.18
N=2
FIG.3:(color online)Binding energy of the two-electron sys-tem minus the classical binding energy vs the inverse of the magnetic ?eld for depth of the potential from V 0=7meV (highest curve)to 4.18meV (lowest curve).The thin dashed straight lines are given by 0.39/B ,0.78/B and 1.28/B .
The horizonal dashed lines in Fig.2show the val-ues of the binding energies obtained for classical point charges [19,20],via minimization of the two-electron po-tential energy E class
B =e 2
w ?2V (w/2,0)with respect to the interelectron distance w .The quantum binding energies tend to the classical values at high B .This is shown clearly when we plot the di?erence between the quantum and classical values as function of the inverse of the magnetic ?eld (see Fig.3).We conclude that the binding energy tend as 1/B to the classical value in the limit of high magnetic ?elds [21].The classical charac-ter of the binding is due to the point localization of the single-electron LLL wave functions in the high magnetic ?eld.
The cusps of the three-electron binding energy are due to both the N =3and N =2angular momentum tran-sitions (cf.Fig.4).The 1/B convergence of the binding energy to the classical limit is also found for three elec-trons (see Fig.5).
Fig.6shows the ’phase diagram’for the maximum number of bound electrons as function of the well depth
B [T]
E B
[m e V ]
FIG.4:(color online)Binding energy of the three-electron
system vs the magnetic ?eld for three values of the poten-tial depth.Horizontal dashed lines show the binding energies of the classical system.The energy cusps corresponding to the angular momentum transitions in the three-(two-)elec-tron ground state are linked by thin solid (dotted)lines.The numbers below the lowest curve list L for N =3.
0.0
0.10.20.30.40.5
1 / B [ 1/ T ]
0.00.4
0.8
1.2
E B
- E B
c l a s s [m e V ]
V 0
= 14 meV
12
m e V
10 meV
N=3
FIG.5:(color online)Same as Fig.3but for N =3.The thin dashed straight lines are given by 6.6/B ,5.4/B and 4.1/B .
and the magnetic ?eld.The thin lines joining the cusps of the phase diagram boundaries corresponding to N =2angular momentum transitions.The cusps related to the N =3angular-momentum transitions are marked with arrows.Dashed lines show the depth of the potential well which allows binding of the second and third classi-cal electron [20].In Fig.7we plot the phase diagram as function of 1/B to demonstrate the convergence of the phase boundaries to the classical result at in?nite mag-netic ?eld.
In an external magnetic ?eld the diamagnetic e?ect in-creases faster the free-electron LLL than the energies of the states localized in the cavity.This leads to the energy shift of the ionization threshold observed in a magnetic ?eld.According to our results the magnetic ?eld induces the formation of bound electron states in QDs.Such a formation was actually observed in single-electron charg-
4
B [T]
V 0 [m e V ]
FIG.6:(color online)The maximum number of bound elec-trons on the V 0/B plane.Below the lowest curve only one electron can be bound,and above the highest the third elec-tron becomes bound,between the curves the maximum num-ber of bound electrons is two.The horizontal dashed lines show the classical depths of the well allowing binding of the second and third electron.
0.0
0.4
0.8
1.2
1.6
2.0
1/B [1/T]
468
1012
14V 0 [m e V ]
N=3
N=2
N=1
FIG.7:(color online)Same as Fig.6but in 1/B scale.The
thin dashed straight lines are given by V class
N +A N /B ,with V class 2=4.073meV,V class 3=8.245meV,A 2=0.7,A 3=5.
ing experiments on self-assembled QDs [22].The QDs [22]at low magnetic ?elds bind six electrons.Only for B >8T the capacitance signal related to the binding of sixth and seventh electron emerge from the background of the LLL wetting layer charging (see Fig.3of Ref.[22]).Self-assembled QDs have a ?xed depth of the con?nement potential,but the phase diagram (Figs.6and 7)can be experimentally determined in the capacitance measure-ment of the vertical quantum dot device [13].The shape of the con?nement potential is exactly Gaussian for a proper choice of the size of the gate (see Fig.6of [14])forming a small electrostatic QD.The depth of the poten-tial is simply changed by the gate voltage.The number of bound electrons can be determined for any magnetic ?eld by counting the capacitance peaks appearing as function of the gate voltage below the two-dimensional electron gas charging (like the one of Fig.3Ref.[22]).
The discussed e?ect can be used to entangle electron spins [15].One needs a cavity which at B =0binds only a single electron.The second electron is bound by the
?eld and one can arrange for the bound state to have zero spin (even L ).After the magnetic ?eld is switched o?the second electron will become unbound and is repelled by the dot and thus goes with its spin state entangled with the localized electron.
In summary,we have shown that a strong magnetic ?eld induces the formation of bound few-electron states in potential cavities that are deep enough to bind the classical point electrons.
Acknowledgment This paper was supported by the Foundation for Polish Science,the Flemish Science Foun-dation and the EU-NoE:SANDiE.
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茶成分与健康 ----王岳飞 茶成分与健康，可以从三个方面来诠释：第一，就是茶叶中的化学成分和特征成分是什么；第二，中国茶为什么分为六大茶类；第三，环境因子对茶的品质有什么影响。 好，我们首先来看一下，整个茶叶的化学成分的总表。第一层意思，现在已经经过分离鉴定的化合物有七百多种在茶叶里面。也就是说在茶叶里的化学成分多如牛毛。 第二，我们看到有人在采茶叶或者是炒茶叶，你就可能会问他，几斤鲜叶可以做一斤茶。其实都是，四斤做一斤。为什么呢？因为水分占鲜叶的四分之三，干物质则仅占四分之一。也就是说水和干物质的比例是3:1左右。所以四斤鲜叶做一斤干茶的化学原理，就是水分的含量在75%-78%，干物质在22%--25%。这是很重要的一个成分。做到干茶以后，像龙井茶的水含量，是3%--5%.这个要求是比较高的，一般6%-7%就可以了。5%--6%，就可以达到名优绿茶的规格了。 第三，茶叶里这七百多种成分里面，含量最多的有十几类，就是课件上这些，大家看一下蛋白质的含量是多少？20%-30%。糖类中主要是纤维素，有20%-25%.第三类含量较多的是多酚类，即是茶多酚，18%-36%.脂类含量是8%。生物碱是3%-5%。氨基酸是1%-4%.在此比较重要的是，茶多酚类，生物碱和氨基酸的含量。 在此，我们可以吧这七百多种成分，十几类，分为四个层面去理解。 第一个层面叫产量成分，产量成分就是每亩地有多少茶叶产量，产量里面到底是什么成分。就是含量最高的这四种，即蛋白质，糖类，茶多酚类和脂类。这四类加起来达90%以上。1980s以前，我们出口茶叶，对一亩地的考核就是考核的茶叶产量。那时候没有名优绿茶，最重视的就是产量。1990s至今，我们的名优绿茶发展的很好，名优绿茶不讲产量，12年西湖龙井的市场价最贵三万元一斤，出口的珠茶五六块钱一斤。那么三万块钱的茶叶利润有多少？我觉得应该有25000.但是珠茶如果也卖到三万块钱一斤，它的利润可能只有250.这就是品质比产量要重要的一个实证。我们讲的这四个成分要是提高了，茶叶的产量也就提高了。可是品质主要依赖于哪个成分呢？ 品质啊，取决于，各个成分的比例关系，就好像人的五官，你像成龙的鼻子过大了，就不帅了。所以品质，一定要讲究比例协调。我们学到评茶里颜色香气味道，它的内质，是由不同的化学成分决定的。 第三个层面，是营养成分。很多人爱问，喝茶是不是营养成分很多啊？我们也都告诉别人确有很多营养成分。但是这个回答应该是对错参半的。为什么呢？什么叫营养成分呢？营养成分就是用来维持生命的，你想一下，我们不吃饭，只喝茶水能不能活？绝对不能。我们不吃饭，只喝白开水能活七天。如果喝茶水，也和白开水差不了多少。从这个层面可以感觉到，茶叶里好像营养成分确实不多啊。但是如果你把茶渣吃了，那你可能可以活下来。不过也不会活得太好。所以他的绝对量和营养成分也并不是太多。但是我为什么说对错参半呢？因为茶叶里的营养成分种类繁多。大家看一下，六大食品营养素，五类四十四种人体所必须的营养素，可见喝茶虽然维持生命是不行的，但是拿它来补充营养是可以的。 我们喝茶是为了什么呢？这就是第四个层面，功效成分。我们喝茶最主要的
How to manage time Time treats everyone fairly that we all have 24 hours per day. Some of us are capable to make good use of time while some find it hard to do so. Knowing how to manage them is essential in our life. Take myself as an example. When I was still a senior high student, I was fully occupied with my studies. Therefore, I hardly had spare time to have fun or develop my hobbies. But things were changed after I entered university. I got more free time than ever before. But ironically, I found it difficult to adjust this kind of brand-new school life and there was no such thing called time management on my mind. It was not until the second year that I realized I had wasted my whole year doing nothing. I could have taken up a Spanish course. I could have read ten books about the stories of successful people. I could have applied for a part-time job to earn some working experiences. B ut I didn’t spend my time on any of them. I felt guilty whenever I looked back to the moments that I just sat around doing nothing. It’s said that better late than never. At least I had the consciousness that I should stop wasting my time. Making up my mind is the first step for me to learn to manage my time. Next, I wrote a timetable, setting some targets that I had to finish each day. For instance, on Monday, I must read two pieces of news and review all the lessons that I have learnt on that day. By the way, the daily plan that I made was flexible. If there’s something unexpected that I had to finish first, I would reduce the time for resting or delay my target to the next day. Also, I would try to achieve those targets ahead of time that I planed so that I could reserve some more time to relax or do something out of my plan. At the beginning, it’s kind of difficult to s tick to the plan. But as time went by, having a plan for time in advance became a part of my life. At the same time, I gradually became a well-organized person. Now I’ve grasped the time management skill and I’m able to use my time efficiently.
英语演讲稿：未来的工作 这篇《英语演讲稿范文：未来的工作》，是特地，希望对大家有所帮助！ 热门演讲推荐：竞聘演讲稿 | 国旗下演讲稿 | 英语演讲稿 | 师德师风演讲稿 | 年会主持词 | 领导致辞 everybody good afternoon:. first of all thank the teacher gave me a story in my own future ideal job. everyone has a dream job. my dream is to bee a boss, own a pany. in order to achieve my dreams, i need to find a good job, to accumulate some experience and wealth, it is the necessary things of course, in the school good achievement and rich knowledge is also very important. good achievement and rich experience can let me work to make the right choice, have more opportunities and achievements. at the same time, munication is very important, because it determines whether my pany has a good future development. so i need to exercise their municative ability. i need to use all of the free time to learn
论高等学校办学类型及其分类(一) 摘要：实行分类办学，促进高校多样化发展，不仅是高校自身发展的需要，更是社会进步的内在要求。当前，随着我国高等教育改革的不断深入，高等教育事业的不断向前发展，我国高等教育大众化进程进一步加快，高等教育呈现出多样化、层次化、个性化发展趋势，面对新的发展趋势，如何科学合理地对高校进行分类，从而既有利于构建中国高等教育的合理的体系结构，又有利于发挥高等学校的办学积极性，直接关系到大学的科学定位和发展方向，关系到政府管理政策和大学评价标准的制定，关系到国家对教育资源公平合理的分配，关系到中国多层次、多类型、多样化高等教育的全面、协调、可持续发展。因此，对我国各级各类高校进行科学合理的分类，这是目前我国高等教育改革和发展所面临的重要课题。 关键词：高等学校；办学；分类；发展 一、高等学校分类办学的意义 长期以来，在高等教育结构与体系研究中，高校分类办学既是一个世界性的难题，又是关系到中国高等教育能否持续发展的关键。由于分类标准不同，对高校划分的结果也就完全不一样。根据专业的总体性质与结构，高校一般可分为综合类院校、理工类院校、师范类院校、单科类(农、医、林、水)院校；根据组织的基本职能与特征，高校一般可分为研究型院校、教学研究型院校、教学型院校和高职高专学校；根据办学质量与社会声誉，高校一般可分为国际一流、国内一流、省内一流、同类一流等院校。高校办学如何能找准自己的位置，科学分类是关键。 自1999年我国高校连续扩招以来，在校学生人数激增，新升格的高校越来越多，盲目追求高层次、追求“综合化”以及新一轮的办学“大而全”的浪潮正成为我国高等教育事业发展的新热点。因此，如何尽快建立起中国高等学校的类型和层次划分标准，引导我国不同类型和层次的高等学校准确和合理定位，切实做到各安其位，各得其所，并在自己的类型和层次上办出特色、争创一流；如何通过政策架构和制度设计，构建起结构优化、层次清晰、分工明确、相互衔接的高等教育系统，形成精英型大学和大众型高校合理分工、共同发展的格局，有效扭转目前我国高校中普遍存在的分类不清、定位不明、特色不显、目标趋同、盲目升格、模式单一的不良局面；如何实现中国高等教育结构整体上的优化，最大限度地提高高等教育资源的利用率，发挥高等教育系统的整体功能，从而既有利于满足广大人民群众接受高等教育的愿望，又有利于创建一批世界一流大学和高水平研究型大学，使我国高等教育能为全面建设社会主义和谐社会和推进社会主义现代化建设提供源源不断、丰富多样的人才资源，等等，所有这些问题，都是新世纪我国高等教育进一步深化改革中迫切需要解决的重大课题，这也是近年来我国高校分类和定位问题越来越受到高等教育理论界与实际工作者密切关注的原因所在。 进入21世纪，随着我国高等教育大众化进程的不断加快，以及高等教育改革的进一步深化，“创建一流大学”、“大学教育国际化”等口号提得越来越响，可谓是耳熟能详，然而何谓一流的大学？在强调大学国际化的同时，如何看待大学的民族化与地方化？我国的大学改革能否照搬欧美国家的大学发展模式？如何正确处理大学与政府、市场三者之间的关系，确立大学科学的治理结构，推动大学制度创新？上述这些问题，分明地凸显出来，成为高教界甚至整个社会普遍关注的重要议题。因此，对高校进行科学分类，确立高校在整个社会系统和高等教育系统中的准确位置，对于推动我国高等教育大众化进程，深化高等教育的改革和发展，具有及其重要的意义。 二、高等学校分类办学的原则 20世纪中期以来，随着世界经济一体化、文化发展多元化、信息技术网络化发展趋势的不断增强，伴随社会主义小康社会建设的全面推进，我国高等教育迅速得到发展。2005年全国高等教育毛入学率达到23%，高等教育大众化不断向前发展。然而，我们应该清楚地看到，
关于坚持的英语演讲稿 Results are not important, but they can persist for many years as a commemoration of. Many years ago, as a result of habits and overeating formed one of obesity, as well as indicators of overall physical disorders, so that affects my work and life. In friends to encourage and supervise, the participated in the team Now considered to have been more than three years, neither the fine rain, regardless of winter heat, a day out with 5:00 time. The beginning, have been discouraged, suffering, and disappointment, but in the end of the urging of friends, to re-get up, stand on the playground. 成绩并不重要，但可以作为坚持多年晨跑的一个纪念。多年前，由于庸懒习惯和暴饮暴食，形成了一身的肥胖，以及体检指标的全盘失常，以致于影响到了我的工作和生活。在好友的鼓励和督促下，参加了晨跑队伍。现在算来，已经三年多了，无论天晴下雨，不管寒冬酷暑，每天五点准时起来出门晨跑。开始时，也曾气馁过、痛苦过、失望过，但最后都在好友们的催促下，重新爬起来，站到了操场上。 In fact, I did not build big, nor strong muscles, not a sport-born people. Over the past few years to adhere to it, because I have a team behind, the strength of a strongteam here, very grateful to our team, for a long time, we encourage each other, and with sweat, enjoying common health happy. For example, Friends of the several run in order to maintain order and unable to attend the 10，000 meters race, and they are always concerned about the brothers and promptly inform the place and time, gives us confidence and courage. At the same time, also came on their own inner desire and pursuit for a good health, who wrote many of their own log in order to refuel for their own, and inspiring. 其实我没有高大身材，也没健壮肌肉，天生不属于运动型的人。几年来能够坚持下来，因为我的背后有一个团队，有着强大团队的力量，在这里，非常感谢我们的晨跑队，长期以来，我们相互鼓励着，一起流汗，共同享受着健康带来的快
现行各种高校学历教育种类及区别为了提高国民教育，使更多的人获得学习的机会，国家提供了多种继续教育的途径。尤其在大学学历教育上，除了普通高考外，还提供了成人高考、自考、广播电视大学、网络教育四种渠道，为那些没有机会通过高考，进入大学校园学习的人提供了更多的选择和可能。但是很多需要机会的人并不知道自己还有多少种选择，更不清楚这几种教育方式之间的差别。 【入学门槛对比】 成人高等教育入学考试是全国统一招生考试，由学校根据考试结果择优录取，统考每年举行一次。 自考考生自行参加单科考试，合格一门，发一门的合格证书，所有科目合格后，方可颁发学历证书。每年两次考试。 广播电视大学，需具备要求的相关学历方可注册入学。 成人网络教育由招生院校自主命题考试试题及科目，具备该类型资质院校，国家承认学历的仅有68所。 总的来说，自学考试和广播电视大学实行的是“宽进严出”，基本没有什么入学门槛；而成人高考参加的是全国统考，网络考试则由各招生院校自主考试。从考试的难易程度来看，部分网络教育大学因知名度高，所招专业又是学校强势专业，故入学要求也相对较高，试题也有一定难度。
【学习方式对比】 成人高考主要有脱产学习和业余学习、函授学习等学习方式。 自考则以自学为主。 网络教育有两种授课方式，一是通过网络进行在线授课，学员可以通过网络浏览课间，完成作业，通过BBS进行提问和沟通，二是集中面授。不少希望进入自己心仪的名校进行学习的外地学生，可以通过报考网络学院，接受远程授课以完成学业。 广播电视大学有网上课堂和面授两种方式。 四种途径都可以利用业余时间进行学习。 【招生对象对比】 成人高考、网络大学、广播电视大学的招生范围包括在职、从业人员和社会其他人员，一般要求高中起点报考专、本科的，必须是获得普通高中、中等专业学校或职业技术学校毕业证书或同等学历者。专科起点报考本科的，应具有国民教育系列的专科或专科以上毕业证书。 成人高考主要以招收本地户籍生为主，外地学生也可以进行函授学习，网络大学和广播电视大学由于自身的授课方式的优势，招生范围没有地域上的限制。 【学习年限对比】
茶叶的主要成分 谈到饮茶的作用，要从茶叶中的主要成分说起，各种茶的主要成分是一样的，因为不管什么茶，从植物学角度来看，它们首先都是茶叶，之所以会有那么多的茶叶种类，例如楼主提到的“红茶、绿茶、茉莉花茶、铁观音、普洱”等等，一来是原料茶树所属的种略有差别，但主要还是加工方法和加工工艺的不同，一般来说，它们只会改变茶叶的口感和香气，不会改变茶叶中的成分。 茶叶中的主要成分及其作用： 1、有机酸 茶叶中有机酸种类较多，含量约为干物质的3%左右。 茶叶中的有机酸多为游离有机酸，如苹果酸、柠檬酸、琥珀酸、草酸等。在制茶过程中形成的有机酸，有棕榈酸、亚油酸、乙烯酸等。 茶叶中的有机酸是香气的主要成分之一，现已发现茶叶香气成分中有机酸的种类达25种，有些有机酸本身虽无香气，但经氧化后转化为香气成分，如亚油酸等；有些有机酸是香气成分的良好吸附剂，如棕榈酸等。 虽然茶叶中柠檬酸对人体无直接危害，但可以促进体内钙的排泄和沉积，如长期大量摄入，有可能导致低钙血症。 2、茶多酚： 茶多酚是茶叶中酚类物质及其衍生物的总称，并不是一种物质，因此常称为多酚类，约占干物质总量的20%～35%。过去茶多酚又称作茶鞣质、茶单宁。 茶多酚的功效主要有：能消除有害自由基、抗衰老、抗辐射、抑制癌细胞、抗菌杀菌、抑制艾滋病病毒。 可以与食物中的铁元素发生反应，生成难以溶解的新物质，时间一长引起人体缺铁，甚至诱发贫血症。 3、氨基酸： 分析表明，茶叶中至少含有25种氨基酸，人体必需的氨基酸是8种，茶叶中就含有6种。氨基酸是构建生物机体的众多生物活性大分子之一，是构建细胞、修复组织的基础材料。 4、蛋白质： 茶叶中的蛋白质含量占干物质量的20-30%，能溶于水直接被利用的蛋白质含量仅占1-2%，所以茶叶中蛋白质含量不是很高，这部分水溶性蛋白质是形成茶汤滋味的成分之一。 蛋白质是维持机体的生长、组成、更新和修补人体组织的重要材料,通过氧化作用为人体提供能量。 需要说明的是，蛋白质遇到茶多酚，会产生很难溶解的鞣酸蛋白，对人体不利。 5、生物碱： 茶叶中的生物碱包括咖啡碱、可可碱和条碱。其中以咖啡碱的含量最多，约占2%～5%；其他含量甚微，所以茶叶中的生物碱含量常以测定咖啡碱的含量为代表。咖啡碱易溶于水，是形成茶叶滋味的重要物质。 咖啡碱，具有兴奋大脑神经和促进心脏机能亢进的作用。过量饮（食）用，能扰乱胃液的正
1.How to build a business that lasts100years 0:11Imagine that you are a product designer.And you've designed a product,a new type of product,called the human immune system.You're pitching this product to a skeptical,strictly no-nonsense manager.Let's call him Bob.I think we all know at least one Bob,right?How would that go? 0:34Bob,I've got this incredible idea for a completely new type of personal health product.It's called the human immune system.I can see from your face that you're having some problems with this.Don't worry.I know it's very complicated.I don't want to take you through the gory details,I just want to tell you about some of the amazing features of this product.First of all,it cleverly uses redundancy by having millions of copies of each component--leukocytes,white blood cells--before they're actually needed,to create a massive buffer against the unexpected.And it cleverly leverages diversity by having not just leukocytes but B cells,T cells,natural killer cells,antibodies.The components don't really matter.The point is that together,this diversity of different approaches can cope with more or less anything that evolution has been able to throw up.And the design is completely modular.You have the surface barrier of the human skin,you have the very rapidly reacting innate immune system and then you have the highly targeted adaptive immune system.The point is,that if one system fails,another can take over,creating a virtually foolproof system. 1:54I can see I'm losing you,Bob,but stay with me,because here is the really killer feature.The product is completely adaptive.It's able to actually develop targeted antibodies to threats that it's never even met before.It actually also does this with incredible prudence,detecting and reacting to every tiny threat,and furthermore, remembering every previous threat,in case they are ever encountered again.What I'm pitching you today is actually not a stand-alone product.The product is embedded in the larger system of the human body,and it works in complete harmony with that system,to create this unprecedented level of biological protection.So Bob,just tell me honestly,what do you think of my product? 2:47And Bob may say something like,I sincerely appreciate the effort and passion that have gone into your presentation,blah blah blah-- 2:56(Laughter) 2:58But honestly,it's total nonsense.You seem to be saying that the key selling points of your product are that it is inefficient and complex.Didn't they teach you 80-20?And furthermore,you're saying that this product is siloed.It overreacts, makes things up as it goes along and is actually designed for somebody else's benefit. I'm sorry to break it to you,but I don't think this one is a winner.
关于工作的优秀英语演讲稿 Different people have various ambitions. Some want to be engineers or doctors in the future. Some want to be scientists or businessmen. Still some wish to be teachers or lawers when they grow up in the days to come. Unlike other people, I prefer to be a farmer. However, it is not easy to be a farmer for Iwill be looked upon by others. Anyway,what I am trying to do is to make great contributions to agriculture. It is well known that farming is the basic of the country. Above all, farming is not only a challenge but also a good opportunity for the young. We can also make a big profit by growing vegetables and food in a scientific way. Besides we can apply what we have learned in school to farming. Thus our countryside will become more and more properous. I believe that any man with knowledge can do whatever they can so long as this job can meet his or her interest. All the working position can provide him with a good chance to become a talent. 1 ————来源网络整理，仅供供参考
大学生如何选择不同类型形式企业 大学生如何选择不同类型形式企业现如今社会，大学生就业困难已成一种趋势，那么就业困难，自然便会有更多的毕业生走向了自主创业道路。通过自己或者与他人合作的方式来自主创业，给自己一个就业的机会同时也给社会创造了就业岗位。但创业并不是那么简单的是，大学生无论是在资金还是经验方面都不足的，这就要求大学生要充分的掌握相关的创业知识，包括公司制度、企业形式等，这些方面都要考虑到。 那么接下来就来谈谈大学生创业时，如何选择不同类型形式企业。当前社会企业形式有很多诸如:有限责任公司、合伙公司、一人有限责任公司、个人独资企业、股份有限公司、私营企业、股份合作制企业等等。但根据大学生的实际情况而言，合伙公司、一人有限责任公司、个人独资企业、有限责任公司这几种比较适合初创业的大学生。下面就来对这几种企业形式进行分析。 合伙公司。合伙公司是私人企业直接结合而成的形态。各所有者共同对结合而成的公司财产负责。同时，出资者要承担无限连带责任。合伙人的无限责任是指合伙人的责任范围不是以其出资额或其在合伙企业财产中的份额为限，而是以其全部的个人财产为限。合伙企业不具有法人资格，属于自然人企业;合伙企业的设立和内部管理是以合伙协议为基础的;合伙人对合伙企业的债务承担无限连带清偿责任;合伙人共同出资、合伙经营、共享收益、
共担风险，具有很强的人合性。 合伙人之间是风雨同舟、荣辱与共的关系，因此，合伙企业的风险和责任相对于独资企业要分散一些。合伙企业的资金来源和企业信用能力有限，不能发行股票和债券，这使得合伙企业的规模不可能太大。合伙人的责任比公司股东的责任大得多，合伙人之间的连带责任使合伙人需要对其合伙人的经营行为负责，更加重了合伙人的风险。由于合伙企业具有浓重的人合性，任何一个合伙人破产、死亡或退伙都有可能导致合伙企业解散，因而其存续期限不可能很长。 一人有限责任公司。一人有限责任公司是指由一名股东持有公司的全部出资的有限责任公司。一人有限责任公司的注册资本最低限额为人民币十万元并且股东应当一次足额缴纳公司章程规定的出资额。此外一人有限责任公司的股东不能证明公司财产独立于股东自己的财产的，应当对公司债务承担连带责任。一人有限责任公司的成立对于一个大学生，尤其是刚刚毕业的大学生来说是很困难甚至是不可能的。首先就在于它最低的注册限额为10万元，并且需要一次性足额缴纳。 如果再加上公司成立时的其他开销，对于刚刚毕业的大学生来说这是一笔巨额支出，根本无法实现，即使实现了，可能接下来的资金周转等其他问题也会接踵而至，这必然会给公司的接下来发展带来很大的困难。当然，再则，一人责任公司要求唯一的股东承担其债务连带责任的，如果一旦公司还没有起步就陷入了危机的时候，对于刚刚创业有没有资金资本的大学生来说无疑是最惨的打击。所以不建议大学生一开始就通过一人有限责任公司
茶的主要成分检测 一，水分 占比：茶鲜叶含水量75%~78%，干茶4%~6% 性质：含水量越高，茶叶越易变质，含量超过12%时，茶叶内部各种化学反应还在继续进行，且茶还能被氧气氧化变质。 检测方法：GBT8304-2002 二，灰分 概念：茶叶经过高温灼烧后残留下的无机物质的统称。 占比：4%~7%。灰分含量过高，茶品质越差，灰度中可溶性部分含量越高，茶的品质越好。 检测方法：
三，茶多酚 概念：茶叶中30多种酚类物质的总称。 占比：占鲜叶干物质的33% 组成： 1，儿茶素：含量最高，占多酚类物质的70%~80%，具有收敛性，苦涩味较重。 2，黄酮类物质：又叫花黄素 3，花青素：又叫花色素，具有明显的苦味，在干旱高温季节容易形成并积累，因此夏茶具有明显的苦涩味。 4，酚酸：含量较小，味苦涩。 检测方法： GB/T 8313-2002 茶叶中多酚类物质能与亚铁离子形成紫褐色络合物，用分光光度法测定其含量。四，蛋白质和氨基酸
蛋白质： 1，组成：谷蛋白（含量最大），白蛋白（溶于水，对茶汤滋味起到积极作用），球蛋白，精蛋白组成。 2，性质：在鲜叶加工中，部分蛋白质可以分解为氨基酸，具有花香和鲜味。 氨基酸 1，性质：溶于水，具有鲜爽味，决定着茶汤的品质的鲜爽度。 2，组成：茶氨酸（占比50%以上，嫩芽嫩茎中占比最大）天门冬氨酸，谷氨酸，精氨酸，丝氨酸 检测方法： 凯氏定氨法测定茶叶中蛋白质 试剂及主要仪器 1、浓硫酸(AR) 2、硫酸钾(AR) 3、硫酸铜(AR) 4、3.0%NaOH溶液 5、2%硼酸溶液 6、0.05mol/L的标准HCL溶液 7、0.1%混合指示剂：0.1%甲基红酒精溶液和0.1%甲基蓝酒精溶液，按1:1混合。上海创赛提供甲基红指示剂盒，商品编号：C015217-5ml*2，价格20元。 8、微量凯氏定氨仪 处理样品 准确称取样品2g，小心移入烧瓶中，再CuSO4 1g,K2SO4 10g和浓H2S04 50mL.用电炉加热消化至液体变成蓝绿透明后，冷却，定容至100mL。上海创赛提供2-溴
尊敬的领导，老师，亲爱的同学们， 大家好！我是5班的梁浩东。今天早上我坐车来学校的路上，我仔细观察了路上形形色色的人，有开着小车衣着精致的叔叔阿姨，有市场带着倦容的卖各种早点的阿姨，还有偶尔穿梭于人群中衣衫褴褛的乞丐。于是我问自己，十几年后我会成为怎样的自己，想成为社会成功人士还是碌碌无为的人呢，答案肯定是前者。那么十几年后我怎样才能如愿以偿呢，成为一个受人尊重，有价值的人呢？正如我今天演讲的题目是：自主管理。 大家都知道爱玩是我们孩子的天性，学习也是我们的责任和义务。要怎样处理好这些矛盾，提高自主管理呢？ 首先，我们要有小主人翁思想，自己做自己的主人，要认识到我们学习，生活这一切都是我们自己走自己的人生路，并不是为了报答父母，更不是为了敷衍老师。 我认为自主管理又可以理解为自我管理，在学习和生活中无处不在，比如通过老师，小组长来管理约束行为和同学们对自身行为的管理都属于自我管理。比如我们到一个旅游景点，看到一块大石头，有的同学特别兴奋，会想在上面刻上：某某某到此一游话。这时你就需要自我管理，你需要提醒自己，这样做会破坏景点，而且是一种素质低下的表现。你设想一下，如果别人家小孩去你家墙上乱涂乱画，你是何种感受。同样我们把自主管理放到学习上，在我们想偷懒，想逃避，想放弃的时候，我们可以通过自主管理来避免这些，通过他人或者自己的力量来完成。例如我会制定作息时间计划表，里面包括学习，运动，玩耍等内容的完成时间。那些学校学习尖子，他们学习好是智商高于我们吗，其实不然，在我所了解的哪些优秀的学霸传授经验里，就提到要能够自我管理，规范好学习时间的分分秒秒，只有辛勤的付出，才能取得优异成绩。 在现实生活中，无数成功人士告诉我们自主管理的重要性。十几年后我想成为一位优秀的，为国家多做贡献的人。亲爱的同学们，你们们？让我们从现在开始重视和执行自主管理，十几年后成为那个你想成为的人。 谢谢大家！
关于工作的英语演讲稿 【篇一：关于工作的英语演讲稿】 关于工作的英语演讲稿 different people have various ambitions. some want to be engineers or doctors in the future. some want to be scientists or businessmen. still some wish to be teachers or lawers when they grow up in the days to come. unlike other people, i prefer to be a farmer. however, it is not easy to be a farmer for iwill be looked upon by others. anyway,what i am trying to do is to make great contributions to agriculture. it is well known that farming is the basic of the country. above all, farming is not only a challenge but also a good opportunity for the young. we can also make a big profit by growing vegetables and food in a scientific way. besides we can apply what we have learned in school to farming. thus our countryside will become more and more properous. i believe that any man with knowledge can do whatever they can so long as this job can meet his or her interest. all the working position can provide him with a good chance to become a talent. 【篇二：关于责任感的英语演讲稿】 im grateful that ive been given this opportunity to stand here as a spokesman. facing all of you on the stage, i have the exciting feeling of participating in this speech competition. the topic today is what we cannot afford to lose. if you ask me this question, i must tell you that i think the answer is a word---- responsibility. in my elementary years, there was a little girl in the class who worked very hard, however she could never do satisfactorily in her lessons. the teacher asked me to help her, and it was obvious that she expected a lot from me. but as a young boy, i was so restless and thoughtless, i always tried to get more time to play and enjoy myself. so she was always slighted over by me. one day before the final exam, she came up to me and said, could you please explain this to me? i can not understand it. i
全国重点大学专业实力排行榜 我国，各地大学数不胜数，每个学校也都有自己的优势专业。不少人认为好的大学，它所有的专业就都是好的，其实不然。那么今天在职读研网的老师就为大家整理一下中国各大学的优势，以便于大家去更好的选择适合自己的专业和院校。 综合实力排名 在由12个学科门类组成的科研综合实力排行榜中，共有15所大学获得A++级别。浙江大学、北京大学(分数线,专业设置)、清华大学获前三名。第4名至第15名依次是：上海交通大学、复旦大学、南京大学、武汉大学、四川大学、中山大学、华中科技大学、山东大学、吉林大
学、哈尔滨工业大学、中国科学技术大学、南开大学 社会科学类学科排名 在由哲学、经济学、法学、教育学、文学、历史学、管理学、艺术学8个学科门类组成社会科学实力排行榜中，共有15所大学获得A++级别。北京大学、中国人民大学、南京大学获前三名。第4名至第15名依次是：清华大学、武汉大学、复旦大学、北京师范大学、南开大学、浙江大学、中山大学、吉林大学、厦门大学、四川大学、山东大学、华东师范大学 自然科学类学科排名 在由理学、工学、农学、医学4个学科门类组成自然科学实力排行榜中，共有14所大学获得A++级别。浙江大学、清华大学、北京大学获前三名。第4名至
第14名依次是：上海交通大学、复旦大学、南京大学、哈尔滨工业大学、四川大学、华中科技大学、中山大学、中国科学技术大学、山东大学、吉林大学、武汉大学 工学类学科排名 在工学实力排行榜中，共有14所大学获得A++级别。清华大学、浙江大学、哈尔滨工业大学获前三名。第4名至第14名依次是：上海交通大学、天津大学、北京航空航天大学、华南理工大学、华中科技大学、东南大学、西安交通大学、大连理工大学、西北工业大学、同济大学、重庆大学 理学类学科排名 在理学实力排行榜中，共有13所大学获得A++级别。北京大学、南京大学、浙江大学获前三名。第4名至第13名依次