液化氦的低温，这就是说4K，如果用摄氏温度那就是零下269度的样子。有了液氦就有一个多少算稳定的低温环境，在这个低温环境中能把平常室温下做的实验在液氦中重复一遍，比如，测金属的电阻
http://blog.sciencenet.cn/blog-362400-341209.html
Wednesday, November 27, 2013
qm01 xys01 走过超导之路(7)--无边落木萧萧下
| 主题: zt新语丝：走过超导之路(7)--无边落木萧萧下- 元江（btw跳过看不懂的地方看还是好看） | |
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| 作者 |  zt新语丝：走过超导之路(7)--无边落木萧萧下- 元江（btw跳过看不懂的地方看还是好看） |   | |||||||||
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| 葡萄皮 [个人文集] 加入时间: 2004/02/14 文章: 2398 经验值: 11643 |
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qm01 电子的量子行为与经典行为的区别：一是电子是那种喜欢独居一室的费米子，这是电子能带论的基础；二是电子有个称为‘自旋’的内禀特性，这是自旋电子学和磁电子学的基础
电子的量子行为与经典行为的区别：一是电子是那种喜欢独居一室的费米子，这是电子能带论的基础；二是电子有个称为‘自旋’的内禀特性，这是自旋电子学和磁电子学的基础
http://blog.sciencenet.cn/home.php?mod=space&uid=677221&do=blog&quickforward=1&id=737012
http://blog.sciencenet.cn/home.php?mod=space&uid=677221&do=blog&quickforward=1&id=737012
china01 How China's Stunning $15 Trillion In New Liquidity Blew Bernanke's QE Out Of The Water
Chart Of The Day: How China's Stunning $15 Trillion In New Liquidity Blew Bernanke's QE Out Of The Water
Much has been said about the Fed's attempt to stimulate inflation (instead of
just the stock market) by injecting a record $2.5 trillion in reserves into the
US banking system since the collapse of Lehman (the same goes for the ECB, BOE,
BOJ, etc). Even more has been said about why this money has not been able to
make its way into the broader economy, and instead of forcing inflation - at
least as calculated by the BLS' CPI calculation - to rise above 2% has, by
monetizing a record
amount of US debt issuance, merely succeeded in pushing capital markets to
unseen risk levels as every single dollar of reserves has instead ended up as
assets (and excess deposits as a matched liability) on bank balance sheets.
Much less has been said that of the roughly $2 trillion increase in US bank assets, $2.5 trillion of this has come from the Fed's reserve injections as absent the Fed, US banks have delevered by just under half a trillion dollars in the past 5 years. Because after all, all QE really is, is an attempt to inject money into a deleveraging system and to offset the resulting deflationary effects. Naturally, the Fed would be delighted if instead of banks being addicted to its zero-cost liquidity, they would instead obtain the capital in the old-fashioned way: through private loans. However, since there is essentially no risk when chasing yield and return and allocating reserves to various markets (see JPM CIO and our prior explanation on this topic), whereas there is substantial risk of loss in issuing loans to consumers in an economy that is in a depressionary state when one peels away the propaganda and the curtain of the stock market, banks will always pick the former option when deciding how to allocated the Fed's reserves, even if merely as initial margin on marginable securities.
However, what virtually nothing has been said about, is how China stacks up to the US banking system when one looks at the growth of total Chinese bank assets (on Bloomberg: CNAABTV Index) since the collapse of Lehman.
The answer, shown on the chart below, is nothing short of stunning.
Here is just the change in the past five years:
You read that right: in the past five years the total assets on US bank books have risen by a paltry $2.1 trillion while over the same period, Chinese bank assets have exploded by an unprecedented $15.4 trillion hitting a gargantuan CNY147 trillion or an epic $24 trillion - some two and a half times the GDP of China!
Putting the rate of change in perspective, while the Fed was actively pumping $85 billion per month into US banks for a total of $1 trillion each year, in just the trailing 12 months ended September 30, Chinese bank assets grew by a mind-blowing $3.6 trillion!
Here is how Diapason's Sean Corrigan observed this epic imbalance in liquidity creation:
But what does this mean for the bigger picture? Well, a few things.
For a start, prepare for many more headlines like these: "Chinese buying up California housing", "Hot Money’s Hurried Exit from China", "Following the herd of foreign money into US real estate markets" and many more like these. Because while the world focuses and frets about the Fed's great reflation experiment (which is only set to become bigger not smaller, now that the Fed has thrown all caution about collateral shortage to the wind and will openly pursue NGDP targeting next), China has been quietly injecting nearly three times in liquidity into its own economy (and markets, and foreign economies and markets) as the Fed and the Bank of Japan combined!
To be sure, due to China's still firm control over the exchange of renminbi into USD, the capital flight out of China has not been as dramatic as it would be in a freely CNY-convertible world, although in recent months many stories have emerged showing that enterprising locals from the mainland have found effective ways to circumvent the PBOC's capital controls. And all it would take is for less than 10% of China's new credit creation to "escape" aboard from the Chinese banking system, the bulk of which is quasi nationalized and thus any distinction between prive and public loan creation is immaterial, for the liquidity effect to be as large as one entire year of QE. Needless to say, the more effectively China becomes at depositing all this newly created liquidity, the faster prices of US real estate, the US stock market, and US goods and services in general will rise (something the Fed would be delighted with).
However, while the Fed certainly welcomes this breakneck credit creation in China, the reality is that the bulk of these "assets" are of increasingly lower quality and generate ever lass cash flows, something we covered recently in "Big Trouble In Massive China: "The Nation Might Face Credit Losses Of As Much As $3 Trillion." It is also the reason why China attempted one, promptly aborted, tapering in the summer of 2013, and why the entire third plenum was geared toward economic reform particularly focusing on the country's unsustainable credit (and liquidity) creation machine.
The implications of the above are staggering. If the US stock, and especially bond, market nearly blew a gasket in the summer over tapering fears when just a $10-20 billion reduction in the amount of flow was being thrown about, and the Chinese interbank system almost froze when overnight repo rates exploded to 25% on even more vague speculation of a CNY1 trillion in PBOC tightening, then the world is now fully addicted to about $5 trillion in annual liquidity creation between just the US, Japan and China alone!
Throw in the ECB and BOE as many speculate will happen eventually, and it gets downright surreal.
But more importantly, as with all communicating vessels, global liquidity is now in a constant state of laminar flow - out of central banks: either unadulterated as in the US, Japan, Europe and the UK, or implicit, when Chinese government-backstopped banks create nearly $4 trillion in loans every year. If one issuer of liquidity "tapers", others have to step in. Indeed, as we suggested a few weeks ago, any possibility of a Fed taper would likely involve incremental QE by the Bank of Japan, and vice versa.
However, the biggest workhorse behind the scenes, is neither: it is China. And if something happens to the great Chinese credit-creation dynamo, then we see no way that the rest of the world's central banks will be able to step in with low-powered money creation, to offset the loss of China's liquidity momentum.
Finally, when you lose out on that purchase of a home to a Chinese buyer who bid 50% over asking sight unseen, with no intentions to ever move in, you will finally know why this is happening.
Much less has been said that of the roughly $2 trillion increase in US bank assets, $2.5 trillion of this has come from the Fed's reserve injections as absent the Fed, US banks have delevered by just under half a trillion dollars in the past 5 years. Because after all, all QE really is, is an attempt to inject money into a deleveraging system and to offset the resulting deflationary effects. Naturally, the Fed would be delighted if instead of banks being addicted to its zero-cost liquidity, they would instead obtain the capital in the old-fashioned way: through private loans. However, since there is essentially no risk when chasing yield and return and allocating reserves to various markets (see JPM CIO and our prior explanation on this topic), whereas there is substantial risk of loss in issuing loans to consumers in an economy that is in a depressionary state when one peels away the propaganda and the curtain of the stock market, banks will always pick the former option when deciding how to allocated the Fed's reserves, even if merely as initial margin on marginable securities.
However, what virtually nothing has been said about, is how China stacks up to the US banking system when one looks at the growth of total Chinese bank assets (on Bloomberg: CNAABTV Index) since the collapse of Lehman.
The answer, shown on the chart below, is nothing short of stunning.
Here is just the change in the past five years:
You read that right: in the past five years the total assets on US bank books have risen by a paltry $2.1 trillion while over the same period, Chinese bank assets have exploded by an unprecedented $15.4 trillion hitting a gargantuan CNY147 trillion or an epic $24 trillion - some two and a half times the GDP of China!
Putting the rate of change in perspective, while the Fed was actively pumping $85 billion per month into US banks for a total of $1 trillion each year, in just the trailing 12 months ended September 30, Chinese bank assets grew by a mind-blowing $3.6 trillion!
Here is how Diapason's Sean Corrigan observed this epic imbalance in liquidity creation:
Truly epic flow numbers, and just as unsustainable in the longer-run.Total Chinese banking assets currently stand at some CNY147 trillion, around 2 ½ times GDP. As such, they have doubled in the past four years of increasingly misplaced investment and frantic real estate speculation, adding the equivalent of 140% of average GDP – or, in dollars, $12.5 trillion - to the books. For comparison, over the same period, US banks have added just less than $700 billion, 4.4% of average GDP, 18 times less than their Chinese counterparts – and this in a period when the predominant trend has been for the latter to do whatever it takes to keep commitments off their balance sheets and lurking in the ‘shadows’!
Indeed, the increase in Chinese bank assets during that breakneck quadrennium is equal to no less than seven-eighths of the total outstanding assets of all FDIC-insured institutions! It also compares to 30% of Eurozone bank assets.
But what does this mean for the bigger picture? Well, a few things.
For a start, prepare for many more headlines like these: "Chinese buying up California housing", "Hot Money’s Hurried Exit from China", "Following the herd of foreign money into US real estate markets" and many more like these. Because while the world focuses and frets about the Fed's great reflation experiment (which is only set to become bigger not smaller, now that the Fed has thrown all caution about collateral shortage to the wind and will openly pursue NGDP targeting next), China has been quietly injecting nearly three times in liquidity into its own economy (and markets, and foreign economies and markets) as the Fed and the Bank of Japan combined!
To be sure, due to China's still firm control over the exchange of renminbi into USD, the capital flight out of China has not been as dramatic as it would be in a freely CNY-convertible world, although in recent months many stories have emerged showing that enterprising locals from the mainland have found effective ways to circumvent the PBOC's capital controls. And all it would take is for less than 10% of China's new credit creation to "escape" aboard from the Chinese banking system, the bulk of which is quasi nationalized and thus any distinction between prive and public loan creation is immaterial, for the liquidity effect to be as large as one entire year of QE. Needless to say, the more effectively China becomes at depositing all this newly created liquidity, the faster prices of US real estate, the US stock market, and US goods and services in general will rise (something the Fed would be delighted with).
However, while the Fed certainly welcomes this breakneck credit creation in China, the reality is that the bulk of these "assets" are of increasingly lower quality and generate ever lass cash flows, something we covered recently in "Big Trouble In Massive China: "The Nation Might Face Credit Losses Of As Much As $3 Trillion." It is also the reason why China attempted one, promptly aborted, tapering in the summer of 2013, and why the entire third plenum was geared toward economic reform particularly focusing on the country's unsustainable credit (and liquidity) creation machine.
The implications of the above are staggering. If the US stock, and especially bond, market nearly blew a gasket in the summer over tapering fears when just a $10-20 billion reduction in the amount of flow was being thrown about, and the Chinese interbank system almost froze when overnight repo rates exploded to 25% on even more vague speculation of a CNY1 trillion in PBOC tightening, then the world is now fully addicted to about $5 trillion in annual liquidity creation between just the US, Japan and China alone!
Throw in the ECB and BOE as many speculate will happen eventually, and it gets downright surreal.
But more importantly, as with all communicating vessels, global liquidity is now in a constant state of laminar flow - out of central banks: either unadulterated as in the US, Japan, Europe and the UK, or implicit, when Chinese government-backstopped banks create nearly $4 trillion in loans every year. If one issuer of liquidity "tapers", others have to step in. Indeed, as we suggested a few weeks ago, any possibility of a Fed taper would likely involve incremental QE by the Bank of Japan, and vice versa.
However, the biggest workhorse behind the scenes, is neither: it is China. And if something happens to the great Chinese credit-creation dynamo, then we see no way that the rest of the world's central banks will be able to step in with low-powered money creation, to offset the loss of China's liquidity momentum.
Finally, when you lose out on that purchase of a home to a Chinese buyer who bid 50% over asking sight unseen, with no intentions to ever move in, you will finally know why this is happening.
(59 votes)
leverage01 The Global Leverage Cycle: You Are Here
The Global Leverage Cycle: You Are Here
While one can make an argument that the central banks have now destroyed all
traditional "cycles", including the economic
"virtuous cycle", the business cycle and even the leverage cycle, the
question remains how much longer can the Fed et al defy mean reversion
and all laws of nature associated with it. That said, assuming the fake market
environment we find ourselves in persists for at least another year, this is
what the leverage cycle would look like assuming $10 trillion in global central
bank assets were a pro forma new normal.
Keep a close eye on China: it is on the cusp between the end of the leverage cycle (where as we reported over the past two days, it has been pumping bank assets at the ridiculous pace of $3.5 trillion per year) and on the verge of having its debt bubble bursting. What happens then is unclear.
Some thoughts on the above graphic from SocGen:
For the first time post-crisis, we expect advanced economies in 2014 to see a marked increase in their contribution to global growth. Emerging economies have over the past few years offered a welcome support to global growth, but this relied in part on a build-up of credit that now needs to be paid down. The hope is for advanced economies to take over the baton from the emerging economies as the main driver of global growth. The US is now poised for sustainable recovery and in Japan hopes remain that Abenomics will work. The euro area, however, continues to lag. As such the growth relay from emerging to advanced is likely to prove a bumpy process. Commodity markets will sit at the heart of this dynamic – our strategists look for range-bound markets in 2014.
This new rotation of the global leverage cycle is an integral part of our monetary policy outlook, which we discuss in greater detail in the following sections. Several features are worth noting:
Time for emerging economies to deleverage: Post crisis, emerging economies adopted accommodative economic policies to offset the collapse in demand for their output. Providing a further boost, accommodative monetary policies in advanced economies drove significant financial flows into the region. Combined, these fuelled credit expansion. With the turn in the US interest rate cycle back in the spring, external financing conditions tightened. Moreover, in a number of emerging economies, policymakers have become increasingly concerned by a build-up in leverage; this is not just a story of level, but also one of speed. As seen from our leverage cycle, we believe the emerging economies have now moved to a phase of deleveraging. Our emerging market theme, however, is not just one of a cyclical downturn. As we have highlighted on several occasions, we believe potential growth is structurally slowing and no more so than in China.
China must tame excess capacity: With NFC debt at over 150% of GDP and significant excess capacity, China is ripe for deleveraging. Already in 2013, a notable feature of our forecast has been that the Chinese authorities would resist market pressure to ease monetary policy and further fuel the credit bubble. Nonetheless, shadow bank credit has continued to expand and, with that, problems of excess capacity. China’s challenge now is to deleverage and reform. The two in many ways go hand in hand and we discuss these issues in Boxes 5 and 14. It is worth nothing here that reform in China is tantamount to removing the 100% implicit state guarantee. And looking ahead, even state-backed companies could be allowed to fail. Herein resides also a potential trigger for the risk scenario of a hard landing, should such a company failure be poorly managed and spin out of control.
Japan’s corporate sector to cut savings to invest: Investment and savings are two sides of the same coin and to secure sustainable recovery in Japan, corporations need to reduce savings and invest. The BoJ’s monetary policy is already working through the currency channel and our expectation is to see a pick-up in corporate investment next. This is not just a function of monetary policy, but also the two remaining arrows of Abenomics, namely fiscal stimulus and structural reform. We see significant opportunities medium-term from reform as discussed in Box 13. Short-term, the BoJ is poised to deliver further stimulus and we look for additional asset purchases to be announced early in the new fiscal year (commencing April 1).
US credit cycle is turning: Credit channels have been repaired, household balance sheets deleveraged and excess housing stock unwound. Combined, these lay the foundations for sustainable recovery. In 2013, fiscal tightening exerted a headwind to growth, but this is now easing allowing GDP growth to accelerate to 2.9% in 2014. For the Fed, setting the right monetary policy during this transition will be challenging. A glance at our leverage cycle suggests that the challenge as recovery gains traction over time is to avoid a build-up of excess leverage. This is not an immediate concern to our minds. Although we forecast household credit expansion, our forecast for household income growth is higher, entailing some further reduction of the household debt-to-income ratio.
UK housing credit has been boosted by government measures: Supported by policy initiatives, UK housing is staging a recovery. This is highly dependent on mortgage loan conditions and the BoE will be keen to keep rates low. We expect the Bank to lower the unemployment rate threshold on its forward guidance from 7.0% to 6.5% (and reduce the NAIRU from 6.5% to 6.0%). The hope medium-term, is that this housing-driven recovery will eventually become broader based with stronger confidence, consumption, exports, corporate investment and lower unemployment. Much will depend, however, on euro area recovery as of 2015. Longer-term, a possible UK referendum on EU membership remains a point of uncertainty.
Euro area still facing headwinds: Individual euro area economies are in very different stages on the leverage cycle. Germany is the most advanced, followed by France, Italy and Spain. For several euro area economies, financial fragmentation and fiscal austerity remain serious headwinds. 2014 will see the arrival of a Single Supervisory Mechanism. As we discuss in Box 10, progress on a Single Supervisory Mechanism continues to disappoint and our base line remains for only a gradual repair of credit channels. Moreover, structural reforms are also not progressing at the desired pace, albeit with significant variation from country to country. The danger for the euro area is to become trapped in a lost decade of very low growth and low inflation. The ECB still has options. The real game changer opportunities, however, reside with governments to deliver quantum leaps on reform – at both the euro area and national levels. For now, progress remains disappointingly slow.
Summing up our view, 2014 will thus be the first year post crisis when advanced economies make an increased contribution to global GDP growth.
Keep a close eye on China: it is on the cusp between the end of the leverage cycle (where as we reported over the past two days, it has been pumping bank assets at the ridiculous pace of $3.5 trillion per year) and on the verge of having its debt bubble bursting. What happens then is unclear.
Some thoughts on the above graphic from SocGen:
For the first time post-crisis, we expect advanced economies in 2014 to see a marked increase in their contribution to global growth. Emerging economies have over the past few years offered a welcome support to global growth, but this relied in part on a build-up of credit that now needs to be paid down. The hope is for advanced economies to take over the baton from the emerging economies as the main driver of global growth. The US is now poised for sustainable recovery and in Japan hopes remain that Abenomics will work. The euro area, however, continues to lag. As such the growth relay from emerging to advanced is likely to prove a bumpy process. Commodity markets will sit at the heart of this dynamic – our strategists look for range-bound markets in 2014.
This new rotation of the global leverage cycle is an integral part of our monetary policy outlook, which we discuss in greater detail in the following sections. Several features are worth noting:
Time for emerging economies to deleverage: Post crisis, emerging economies adopted accommodative economic policies to offset the collapse in demand for their output. Providing a further boost, accommodative monetary policies in advanced economies drove significant financial flows into the region. Combined, these fuelled credit expansion. With the turn in the US interest rate cycle back in the spring, external financing conditions tightened. Moreover, in a number of emerging economies, policymakers have become increasingly concerned by a build-up in leverage; this is not just a story of level, but also one of speed. As seen from our leverage cycle, we believe the emerging economies have now moved to a phase of deleveraging. Our emerging market theme, however, is not just one of a cyclical downturn. As we have highlighted on several occasions, we believe potential growth is structurally slowing and no more so than in China.
China must tame excess capacity: With NFC debt at over 150% of GDP and significant excess capacity, China is ripe for deleveraging. Already in 2013, a notable feature of our forecast has been that the Chinese authorities would resist market pressure to ease monetary policy and further fuel the credit bubble. Nonetheless, shadow bank credit has continued to expand and, with that, problems of excess capacity. China’s challenge now is to deleverage and reform. The two in many ways go hand in hand and we discuss these issues in Boxes 5 and 14. It is worth nothing here that reform in China is tantamount to removing the 100% implicit state guarantee. And looking ahead, even state-backed companies could be allowed to fail. Herein resides also a potential trigger for the risk scenario of a hard landing, should such a company failure be poorly managed and spin out of control.
Japan’s corporate sector to cut savings to invest: Investment and savings are two sides of the same coin and to secure sustainable recovery in Japan, corporations need to reduce savings and invest. The BoJ’s monetary policy is already working through the currency channel and our expectation is to see a pick-up in corporate investment next. This is not just a function of monetary policy, but also the two remaining arrows of Abenomics, namely fiscal stimulus and structural reform. We see significant opportunities medium-term from reform as discussed in Box 13. Short-term, the BoJ is poised to deliver further stimulus and we look for additional asset purchases to be announced early in the new fiscal year (commencing April 1).
US credit cycle is turning: Credit channels have been repaired, household balance sheets deleveraged and excess housing stock unwound. Combined, these lay the foundations for sustainable recovery. In 2013, fiscal tightening exerted a headwind to growth, but this is now easing allowing GDP growth to accelerate to 2.9% in 2014. For the Fed, setting the right monetary policy during this transition will be challenging. A glance at our leverage cycle suggests that the challenge as recovery gains traction over time is to avoid a build-up of excess leverage. This is not an immediate concern to our minds. Although we forecast household credit expansion, our forecast for household income growth is higher, entailing some further reduction of the household debt-to-income ratio.
UK housing credit has been boosted by government measures: Supported by policy initiatives, UK housing is staging a recovery. This is highly dependent on mortgage loan conditions and the BoE will be keen to keep rates low. We expect the Bank to lower the unemployment rate threshold on its forward guidance from 7.0% to 6.5% (and reduce the NAIRU from 6.5% to 6.0%). The hope medium-term, is that this housing-driven recovery will eventually become broader based with stronger confidence, consumption, exports, corporate investment and lower unemployment. Much will depend, however, on euro area recovery as of 2015. Longer-term, a possible UK referendum on EU membership remains a point of uncertainty.
Euro area still facing headwinds: Individual euro area economies are in very different stages on the leverage cycle. Germany is the most advanced, followed by France, Italy and Spain. For several euro area economies, financial fragmentation and fiscal austerity remain serious headwinds. 2014 will see the arrival of a Single Supervisory Mechanism. As we discuss in Box 10, progress on a Single Supervisory Mechanism continues to disappoint and our base line remains for only a gradual repair of credit channels. Moreover, structural reforms are also not progressing at the desired pace, albeit with significant variation from country to country. The danger for the euro area is to become trapped in a lost decade of very low growth and low inflation. The ECB still has options. The real game changer opportunities, however, reside with governments to deliver quantum leaps on reform – at both the euro area and national levels. For now, progress remains disappointingly slow.
Summing up our view, 2014 will thus be the first year post crisis when advanced economies make an increased contribution to global GDP growth.
磁电阻效应是指金属的电阻受磁场的影响而变化的一种现象，用变化的相对百分比（MR）来表征。一般金属导电时都有磁电阻效应，并具有如下三个基本特点：有磁场时的电阻比磁场为0时的电阻更大、MR的数值很小、各向异性
《硅火燎原》-15-巨磁电阻效应 
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http://blog.sciencenet.cn/blog-677221-712144.html
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15.巨磁电阻效应
人类对电现象和磁现象很早就有所认识，但将它们在本质上关联起来，却是1820年之后的事。
奧斯特(Orested,1777年-1851年)是丹麥物理學家，哥本哈根大学物理教授。1820年的春天，好几个月以来他都一直沉迷于有关电和磁方面的实验，想找出其中的联系。
他把磁針放在一个充满电的莱顿瓶旁边，磁针纹丝不动。
“莱顿瓶带的是静电，也许需要使用电流？”奥斯特一边想，一边用伏打电源接通电路，但磁针仍然毫无反应。那天傍晚，奧斯特带着满脑子的疑惑和各种改进实验的想法，走进了教室，为学生们上电学课。课程快结束时，奧斯特准备向学生演示他的电路实验，旁边还放着那个总是岿然不动而令他十分沮丧的小磁针。不过，这时奧斯特突然灵机一动，他把那个磁针的位置相对导线而言转了个90度。接下来，当奧斯特连通电源的那一霎那，他发现磁针明显地摆动了一下！磁针这个小小的动作让奥斯特惊喜若狂，也就从这一天开始，人们才逐渐认识了电和磁之间的紧密联系。
之后，法拉第发现电磁感应，特斯拉发明交流电，麦克斯韦创立了经典电磁理论，再后来，原子模型和量子理论的建立，使我们对电和磁的本质及相互联系有了更深刻的认识。
磁性的本质是什么？归根结底还是与电有关。物质的结构决定了物质的性质。磁性是来源于原子中电子的自旋运动和轨道运动。
在计算机和通讯技术的发展中，电磁学大展身手。硅半导体材料及其构成的大规模集成电路的研发，导致了微型计算机的出现和整个信息产业的飞跃，使电子技术迈进了一个全新的时代。我们都知道，计算机中的最重要部分是CPU和硬盘。CPU决定运算的速度，包含了异常复杂的电子线路，硬盘用作信息储存，其物理原理则是基于物质的磁性。无论是CPU还是硬盘，几十年来都是体积越来越小而容量则越来越大。
1956年9月，IBM向世界展示了第一台磁盘存储系统（RAMAC），可算是世界上第一个硬盘。它相当于两个冰箱那么大的体积，重量超过1吨，存储容量却只有5MB。而现在的存储容量是它的上千倍的微硬盘，体积却不过硬币大小。这种惊人的变化，要归功于科学和技术的力量，归功于‘磁电阻效应’的应用，特别是要归功于‘巨磁电阻效应’（GMR，Giant Magneto Resistance）的发现和应用。
什么是磁电阻效应？顾名思义，磁电阻效应是指金属的电阻受磁场的影响而变化的一种现象，用变化的相对百分比（MR）来表征。一般金属导电时都有磁电阻效应，并具有如下三个基本特点：有磁场时的电阻比磁场为0时的电阻更大、MR的数值很小、各向异性。
那么，什么又是巨磁电阻效应呢？
巨磁电阻效应是1988年由法国科学家阿尔贝·费尔和德国科学家彼得·格林贝格尔分别独立研究而发现的【1，2】。
两位科学家研究的是纳米级别的铁鉻相间（Fe/Cr）多层结构。这种结构的制备要归功于1960年代后期贝尔实验室的华人科学家卓以和（AlfredY. Cho）和他的同事亚瑟（J. R. Arthur）发明的分子束外延技术（MBE）。MBE是为单晶材料的生长而研究开发的，但受益的却远远不止芯片制造，它还促进了各种材料科学的发展。
纳米级别是多大呢？我们知道原子大小的数量级是‘埃’，1‘埃’等于10-10米，也就是0.1纳米。所以，纳米科学就是在原子的尺度上研究和操控原子的科学。
纳米技术的想法最早来自于著名的理论物理学家理查德·费曼。费曼对物理以及相关技术的远见卓识无与伦比，他1981年在波士顿MIT的报告，揭开了量子计算机研发的序幕，而早在1959年，他在美国物理学会年会上所作的着名演讲《在底部还有很大空间》中的天才预言【3】，便是如今热门的纳米技术的灵感来源。
费曼在报告中提出了一个新的想法，如果我们能够从单个的分子甚至原子开始进行组装和控制，以达到我们的要求的话，这将会极大地扩充我们获得物性的范围。他说：“至少在我看来，物理学的规律不排除一个原子一个原子地制造物品的可能性。”
这正是现在从事纳米研究的物理学家和工程师们所做的，纳米技术在许多领域中都取得了不凡的成就，从此节开始，我们将介绍几项与半导体材料相关的前沿研究。
刚才提到的分子束外延技术MBE以及1982年扫描隧道显微镜的诞生，为纳米研究人员扫除了障碍，克服了瓶颈，对纳米科技的发展，起了积极的推动作用。
第一个晶体管发明者之一的肖克利，把BJT设计成夹心饼干模样。从此后，材料科学家们喜欢上了这种结构，用各种不同的材料来做各种不同的多层夹心饼干。特别是他们有了MBE，夹心饼干做起来方便多了，一层一层地，相同材料或不同材料，加上去，又加上去，再加上去……固体的结构方式似乎可以随心所欲地人为生成。这种说法有点夸张，做起来绝对不是像我们现在说起来那么轻巧容易的。但无论如何，研究纳米材料的科学家们辛苦工作、乐此不疲。
巨磁电阻效应就是在这种‘铁鉻铁’的三明治（或多明治）结构中观察到的一种‘磁电阻效应’。不过，这种效应与原来通常磁电阻效应的三个基本特点完全相反：有磁场时电阻最小、MR的数值很大、各向同性。因而，人们才给它取了另外一个名字：巨磁电阻效应。
图15.1：巨磁电阻效应
如图15.1所示，巨磁电阻效应所用的材料，是在每两层铁磁性材料之间，夹上一层非磁性金属。当没有外加磁场时，相邻的铁磁材料的磁化方向互相相反（这点可以由调节中间‘鉻’层的厚度得到），这时候观察到的总电阻为最大。如果加上外磁场H，所有铁磁材料的磁化方向都变成与外磁场方向一致，这时候的总电阻最小。电阻变化的比值可达50%，是通常磁电阻效应的几十倍。
在元素周期表的100多种基本元素中，铁、钴、镍被称为铁磁性元素，因为它们能在外部磁場的作用下得以磁化，并能形成永久磁铁。这个性质可以从它们的原子结构，根据量子力学中的自旋及泡利不相容原理成功地解释。
图15.2：对巨磁电阻效应的发现和应用作出贡献的人
对于外磁场的变化，铁磁性元素较其它元素更为敏感，它的磁电阻效应也比其它金属更大，计算机硬盘早就利用铁磁性元素的这些性质来储存数据。法国和德国的物理学家在‘铁鉻铁’结构中发现了巨磁电阻效应之后，IBM的研究员斯图尔特·帕金（Stuart Parkin）1989年在其它材料上也发现了同样的效应。并且，帕金接着又研究自旋阀，造出自旋阀磁盘读头。1994年，帕金研制的新型读出磁头，将磁盘记录密度一下子提高了17倍，并且很快地成为行业技术标准，为IBM带来巨大的商业利益，当然也造福于人类文明。今天，几乎所有最新的磁头读出技术都是基于巨磁电阻原理而研制出来的。
2007年的诺贝尔物理奖颁发给了巨磁电阻效应的发现人阿尔贝·费尔和彼得·格林贝格。说到这儿，不得不为此次诺贝尔物理奖遗漏了IBM的帕金而感到遗憾，其实帕金对巨磁电阻原理的研究和应用，以及物理材料应用的其它领域，贡献之大是有目共睹的。不过，帕金得到了2006年的沃尔夫物理奖。
为什么会有巨磁电阻效应？它的物理原理是怎么样的？留待下一节中讲解。
参考资料：
【1】Fert, Albert; et al. (November 1988)."Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices".Physical Review Letters 61 (21): 2472–2475.
【2】G. Binasch, P. Grünberg, F. Saurenbach, and W. Zinn. “Enhancedmagnetoresistance in layered magnetic structures with antiferromagneticinterlayer exchange”, Phys. Rev. B 39, 4828(1989).
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该博文允许注册用户评论 请点击登录 评论 (11 个评论)
- [11]zhouyinbing88
- 电子自旋概念的引入，确实使量子论有了大的进展！但愚以为电子是光子的黑洞化得观念对自旋的产生可以提供物理学的支持！而且有助我们在相对论和光量子理论的基础上全面系统的统一的理解微观世界的真相！借助这种研究我得到了一些很好的物理结果请老师们读读窝在科学网的网文，请老师多多指教！谢谢！http://bbs.sciencenet.cn/thread-1219737-1-1.html
- [10]王云阳
- 强势关注
- [9]欧阳峰
- 嗯，你是对的。学习了！
- [8]欧阳峰
- “而现在的存储容量是它的上千倍的微硬盘，体积却不过硬币大小”
这是说的ｔｈｕｍｂ ｄｒｉｖｅ之类的器件吗？它可不是用磁性啊。或者真有那么小的磁性硬盘，是我孤陋寡闻了。 - 博主回复(2013-7-29 22:18)：我说的是“microdrive”，这方面我一定比你更孤陋寡闻，不知道你说的是哪一种？在写文章的时候，我到网上wiki了一下microdrive：http://en.wikipedia.org/wiki/Microdrive
microdrive的原理是和hard drive一样的，应该仍然使用磁性。
谢谢。
- [7]王勇
- 其实，关于GMR,一个中国科学家曾经跟Grunberg在一个组，他甚至比Grunberg早发现这个玩意，但是由于总总原因，Grunberg的成果先出来。
- 博主回复(2013-7-29 22:34)：这种令人遗憾的事情总是发生。
- [6]王国强
- 分子束外延技术（MBE）非常重要和普遍，其发明者为何没有获得NP？
- 博主回复(2013-7-29 22:40)：这种事说不清楚。卓以和得过美国国家科学奖。
- [5]张青天
- 张老师，您的科普文章有没有书买或者pdf下载啊？
- 博主回复(2013-7-29 22:43)：《蝴蝶效应之谜:走近分形与混沌》由清华出版社出版：
http://blog.sciencenet.cn/forum.php/blog-797010-706451.html
《世纪幽灵：走近量子纠缠》由科大出版社出版：
http://blog.sciencenet.cn/home.php?mod=space&uid=502977
- [4]田云川
- 好文！
- 博主回复(2013-7-29 22:44)：谢谢捧场。
- [3]HBG
- MBE/MOVPE/MOCVD deserve a NB prize just the same as diode laser, and IC
- 博主回复(2013-7-29 22:45)：I think so, but......
- [2]张弜
- 格林贝格 最早的报到似乎是专利。。。？
- 博主回复(2013-7-29 22:46)：不清楚这点
- [1]mirrorliwei
- GMR已经“老了”，如今都是TMR的技术了。
- 博主回复(2013-7-29 22:49)：You may be right. Thanks.
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